Download A Mechatronic Spotting System that mimics Human Weight

NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
A Mechatronic Spotting System that mimics
Human Weight-training Assistance Behavior
Samuel N. Cubero
[email protected] www.samcubero.com
The Petroleum Institute, Abu Dhabi, United Arab Emirates (UAE)
Grant Wirth, Peter Kneale and Brett Nardi
Curtin University of Technology, Perth, Western Australia
about 340 of these injuries involved a loss of barbell
control. Also, within a 1 year period between March 1991
to April 1992, 11 deaths occurred due to asphyxia / anoxia
[3] (suffocation / lack of oxygen) or inability to breathe.
Training with excessively heavy weights can also cause
several non-fatal injuries such as damaged or torn muscle
bellies, torn or snapped ligaments, back or spinal injuries
and even broken ribs. Barbell drops can sometimes be
lethal [4] especially if the barbell falls out of the user’s
hands (using a ‘suicide grip’, where all fingers and the
thumb are all on one side of the bar) and the bar crushes
the rib cage at high speed due to a free-fall drop. Some
have been found dead at home or in their home gyms [3],
with their necks crushed, each suffocated by a heavy
barbell that could not be removed (due to muscle weakness
near the end of a set of repetitions).
Unfortunately, overconfident people and novice
weight trainers, including teenagers and children, may lack
knowledge and awareness of the risks involved with heavy
barbell training. Beginners usually do not consider the
possibility or consequences of ‘muscle failure’ (or the
inability to return the weight to its supports), especially
when performing the final few reps (repetitions) in a set of
lifts. Hence, some do not take adequate precautions to
protect themselves in the event of failure to return a heavy
barbell to the supports. For example, one easy way to
prevent barbell suffocation (for the bench press exercise)
is to place one chair on the left and another identical sized
chair on the right side of the flat bench so that the plates on
each side of the barbell can be stopped or supported by the
flat seat of each chair in the event of failure to return the
bar to the supports. This safety precaution, or similar
‘barbell catching’ safety equipment [5], may prevent the
bar from pinning down the user if the user struggles with
the weight and does not have enough strength to return the
bar to the top of the rack (or supports).
The main job of a human training partner or ‘spotter’
is to ‘spot’ or identify when the person exercising is failing
to make progress with a lift, and to provide low to high
levels of lifting assistance to lessen the effective load, in
order to help return the bar to the supports. Partial
assistance with a lift should be done in a gradual and
Summary
A leading cause of serious weight-training injuries and death
among bodybuilders and weight-lifters is training with heavy
barbells without the assistance of a human 'spotter' or gym
partner. A spotter can identify lack of progress or nearness of
muscle failure and provide additional force to assist with the lift,
so that the lift can progress slowly prior to complete muscle
failure, allowing the maximum benefit to be gained from the
exercise. This paper briefly describes the modeling, design and
performance of a computer-controlled mechatronic system that
automates the 'spotting' process for horizontal barbell exercises,
which includes exercises such as squats, bench press, overhead
shoulder press, barbell triceps extensions and bicep curls. The
system employs a unique pulley system attached to a strong
structural frame, a solenoid-activated locking brake, optical
position sensors and a low-cost embedded microcontroller
system that drives a display screen, control panel, a geared
electrical winch (motor) and cables attached to a standard barbell.
Known as the ‘Smart Gym’, the basic system design, its different
modes of operation and important safety features are described.
This system also has the potential to save the lives of many
people each year by preventing barbell suffocation (asphyxiation
due to neck compression) and fatal injuries caused by heavy,
free-falling barbells. Several fitness equipment manufacturers
have already shown interest in using concepts described in this
paper for implementation in current and future gym equipment.
Key words:
Gym, spotter,
mechatronic
spotting,
bench
press,
microcontroller,
1. Introduction
Many bodybuilders and weightlifters who prefer to train
alone, or who lift heavy weights without a training partner,
have been seriously injured or killed by barbells due to
lack of spotting assistance [1, 2]. For example, in the US,
from 1999 to 2002, 14 out of the 20 reported weighttraining-related deaths were due to asphyxiation / anoxia
[3], or inability to breathe due to one’s neck being trapped
under a heavy barbell and being too weak or too tired to
remove the barbell, while doing bench presses. Even
during the year 2002, in the US alone, approximately
3,820 weight-training-related injuries were reported and
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NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
controlled manner so that the most benefit can be derived
from the last few repetitions of the set. A skilled human
spotter does not remove the entire load immediately, but
taps up the bar or the lifter's elbows, or partially carries the
load, to make the last repetition progress slowly, or last as
long as possible, just before complete muscle failure
occurs. This ensures that the one exercising can still make
progress with the final lift and can return the barbell to the
supports (bar holder), or starting position. ‘Training to
failure’, accompanied with sufficient rest and a highprotein nutritious diet, promotes rapid muscle growth
based on muscle resistance training principles [6, 7].
Sometimes a training partner or human spotter is not
always available nor completely reliable at all times (e.g.
he or she might not be paying attention when help is
needed the most). Personal trainers or coaches can be very
expensive to hire to serve as spotters, and not all people
possess good spotting skills. A reliable and cost-effective
alternative to a human spotter is to use a 'smart machine' or
a mechatronic system that can automatically perform the
job of a human spotter. This paper describes a working
prototype ‘spotting system’ that mimics the behavior of a
human spotter by executing software on an embedded
AtmelTM microcontroller chip (8-bit computer or CPU).
Ball-screw linear actuators are comprised of several
high-precision components that need accurate machining
and assembly processes (such as the precision-machined
threaded shaft, ball-recirculating collar and rollerbearings), therefore, they are usually among the most
expensive of position-controllable linear actuators.
Typical ball-screw linear actuators (with a 1kW or larger
motor) usually cost between $3000-$7000 each depending
on stroke length, not including the controller and sensor
hardware.
The ExerboticsTM ECR (Chest Press / Row) machine
and ESP (Shoulder press / Lat pulldown) machine each
cost $14,995 USD [14]. The ExerboticsTM eLP (Leg
Press) machine, which allows a user to perform a ‘seated
squat’ exercise, costs $17,995 USD [14]. Because these
three different exercise machines use high-precision ballscrew linear actuators and expensive computer controllers,
they are very costly. In total, the cost of all three machines
adds up to $47,985 USD, which is far beyond the budget
of most home-users, amateur weight-trainers, and most
small gyms (also, conventional weight-stack machines cost
much less). In fact, all the same muscle groups (chest,
back, shoulders, biceps, triceps and legs) can be exercised
safely, and with full spotting assistance, for a small
fraction of the cost of all three aforementioned
ExerboticsTM machines.
2. Current ‘state of the art’ in gym equipment
3. ‘Smart Gym’ prototype spotting system
Many different types of computer-controlled exercise
machines have been developed by a large number of
researchers and product developers. Examples of such
equipment are described by [8-13]. They fall into two
main categories: (1) Systems that are added onto existing
‘passive’ force resistance exercise equipment; and (2)
Systems that generate ‘active’ force resistance using
position, speed and force controlled actuators. Many of
these systems are basically ‘add-on’ devices that monitor
existing forms of gym equipment [12] (which produce
‘passive’ or constant force resistance that can be manually
adjusted, i.e. those that rely on dead weights or mechanical
friction). Others employ variable-resistance methods and
even force-controlled actuators (which involves ‘active’ or
‘closed-loop’ force-controlled or torque-controlled
actuators), which are generally more expensive to develop
because they require more components and sensors and a
programmable computer-based controller.
Perhaps the most technically advanced computer
controlled exercise machines on the market which offer
variable resistance forces (i.e. Type ‘2’ machines) are
manufactured by ExerboticsTM [14]. ExerboticsTM gym
machines use high-precision force-controlled (electric
motor-driven) ball-screw linear actuators. They employ
in-line load cells to provide force-sensing to achieve
closed-loop variable force-control in the ‘forward’ and
‘reverse’ directions of movement.
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The ‘Smart Gym’ spotting system described in this paper
is a ‘Type 1’ exercise machine that uses a conventional
barbell (with passive weights), carried by cables. It
provides real-time ‘human-like’ spotting assistance, it can
quickly detect and halt barbell free-falls, plus perform
workout data logging / monitoring. The first prototype of
the ‘Smart Gym’ costs approximately $2000 AUD in
materials and parts only, including the power supply and
PC, without labor. If labor costs and profit margins were
to be included, the ‘Smart Gym’ could easily sell at a retail
price much less than the price of one ExerboticsTM
machine, offering excellent value for money while being
able to train most of the major muscle groups.
This paper describes the ‘Smart Gym’ [15]
mechatronic engineering project that was supervised by
the first author at Curtin University, Perth, Western
Australia.
Three mechatronic engineering students,
namely, Grant Wirth, Brett Nardi and Peter Kneale,
worked under the first author’s direct supervision and
guidance to design, develop, build and test all the
hardware and software for the 'Smart Gym' prototype.
Others who deserve credit for providing technical
assistance with the electronics manufacturing and
workshop fabrication efforts include Jeff Pickles, David
Collier and Russell Wilkinson.
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The main objectives for developing this ‘Smart Gym’
(mechatronic spotting machine) project are listed below:
1. Allow a wide variety of free weight exercises using a
conventional barbell loaded with plates (e.g. bench
press, squats, arm curls, shoulder press)
2. Automate and replace the safety function performed
by a human 'gym spotter' by carrying the entire
barbell load when necessary. This includes weightlifting assistance when very low speed or ‘stalling’ is
detected, and stopping the barbell from falling if it is
accidentally released or dropped.
3. Maximize the benefits of weight training exercises
(by providing varying levels of spotting assistance
and customizing a variety of ‘spotting’ parameters).
4. Monitor the results of weight-training performance
and record workout information over time (e.g. show
number of full reps, estimate calories burned, etc.)
5. Keep materials and component costs to a minimum in
order to purchase all the components and build a
working prototype within 1 year.
6. Ensure high user safety and predictable performance
of the system at all times, even during a power failure.
Also, ensure that all materials and components are
'strong enough' and will not fail catastrophically.
The main subsystems of the ‘Smart Gym’, as shown
in Figures 1 to 4, are listed below:

A computer-controlled motor-driven winch and cable
braking system. (The cable braking system allows
reliable switching between 2 different modes of
operation)

A programmable controller (a modern 8-bit AtmelTM
AVR microcontroller) and cable position sensor

A strong mounting frame with cables and pulleys
(maximum barbell load = 200 kg)

A user interface (LCD screen with several pushbuttons for setting parameters and operating modes)
Fig. 2. Assisted shoulder press
Fig. 3. Assisted squats
Fig. 4. Assisted bicep curls
4. Mechanical design and operation
The ‘Smart Gym’ was designed to allow humanpowered lifting of a horizontal barbell (State 1, no motor
assistance) and to provide spotting assistance (State 2,
partial or full motor assistance) only when necessary. The
two states of operation are described in Figs. 5 and 6. In
Fig. 5, the motor (or winch) should remain stationary and
the spring reel cable should be free to move (brake is ‘off’
or in the unclamped state) to allow the user to perform free
lifting of the barbell. The spring reel only provides a small
positive tension force to ensure the cables do not go slack
during the upstroke of the barbell, which could cause the
cables to come off the pulley wheels. A position sensor
(optical encoder wheel) is attached to ‘Pulley C’ to
monitor cable position and speed.
Fig. 1. Structural framework of the ‘Smart Gym’ [15]
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prevents any ‘back-driving’ or movement of the cable.
Hence, the ‘Smart Gym’ is able to ‘fail safe’ in the event
of power failures. Another useful feature of the C-cleat is
that if the cable is locked in the cams, the cams are very
hard to open unless the tension on the cable is dropped to
very low levels (or if the barbell load is lifted up slightly
by the user). This is an ideal safety feature for a ‘Smart
Gym’ because the cable cannot be released or unlocked
unless the user lifts the weight slightly upwards to remove
tension in the cable, thus ensuring that the barbell will not
fall if the brake happens to turn ‘off’.
Fig. 5. Cable brake in ‘off’ state allows user to lift
(while motor stationary)
Fig. 7. (a) Solenoid is on and brake is ‘off’ (left); (b) Solenoid is off and
brake is ‘on’ (right). [15]
The C-cleat also behaves like a one-way brake when
activated, allowing the cable to still run into the spring reel
to maintain constant tension for the cable, but it will not
release the cable unless the cam fingers are opened by
activating the solenoid (which causes the solenoid piston
to retract). Therefore, even if the user pushes the barbell
upwards faster than the winding rate of the winch (when
the motor is lifting the barbell load upwards), the spring
reel will keep trying to pull in more cable and maintain
constant positive tension on the cable, without allowing
the cable to go ‘slack’ and possibly come off the pulley
wheels.
Statics and yield failure analysis were performed on
all load-bearing components to select safe sizes for all
parts. Vector force analysis (combined loading statics
analysis) [17], ‘Von Mises’ (or equivalent stress) yield
testing and failure analysis [18] were performed on
custom-designed load-bearing beams, columns and tubes
of the structural framework, cables, pulley shafts and
component mountings to ensure that materials will not fail
by yielding or buckling. Safety factors were kept above 2
(i.e. materials and dimensions for parts were selected or
designed to handle up to 2 times the maximum expected
‘worst-case’ Von Mises stress, even under combined
loading, without yield failure.) One or more of the
components or materials could fail by yielding if a person
weighing more than 200 kg suspends his / her entire
weight on top of a fully loaded 200 kg barbell suspended
by the 2 supporting cables, as shown in Fig. 6, however, it
is likely that the C-cleat (cable brake) will allow the cable
to slip first, or fail earlier, before any metal materials yield.
Fig. 6. Cable brake in ‘on’ state allows motor to lift entire barbell
(spotting)
When the spring reel cable is locked (solenoid is
deactivated), the cable brake is ‘on’ (see Fig. 6), the DC
motor can be activated and take over the lifting of the
entire barbell load (or perform spotting) at a given speed.
The ‘2 cam fingers’ used for the brake system shown
in Figs. 5 and 6 are actually part of the RonstanTM C-cleat
product [16], a high-strength clamping mechanism that
only allows rope or cable movement in one direction. This
product is typically used in many marine applications,
such as locking ropes on sailing boats or for holding the
positions of booms or horizontal spars or poles, which
hold the feet of sails. The C-cleat chosen has a maximum
holding strength of 200 kg and testing had proven that it
could successfully hold over 160 kg (all the weight
available at the time). When all power is suddenly turned
off, the solenoid deactivates, the ‘spring-return’
mechanism inside each C-cleat ‘finger’ automatically
locks against the rope / cable (i.e. cable brake turns ‘on’),
and the barbell cannot fall or move, because the motorpowered winch is also switched off, and its high-gear-ratio
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NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
because it can provide sufficiently accurate position
measurements and very long life performance. It is a noncontact sensor that does not suffer physical material wear,
unlike multi-turn rotary potentiometers (rheostats or
mechanical voltage dividers) which eventually wear out
and fail due to contact friction. The custom-designed and
built slotted disc (wheel) shown in Fig. 9 is attached to a
pulley (‘C’ in Fig. 5) to measure cable displacements to
3.84 mm resolution.
5. Electrical power and control hardware
5.1 Optical position sensor
With the aid of a cable position sensor, a controller can
calculate the velocity of the cable, detect a struggling user
(who is lifting the barbell too slowly or who is unable to
lift any further), and hence, take appropriate spotting
action. A barbell position sensor (optical encoder) was
fitted on pulley C (Fig. 5) to monitor the barbell’s moves.
Fig. 9. Custom-made 2-channel Optical position sensor (45 slots) [15]
5.2 Electric motor selection
Several different kinds of motors could be used for raising
and lowering the barbell. Mechanical actuators like
pneumatic and hydraulic motors are not suitable for this
application because they produce a great deal of noise and
require bulky and costly pumps / compressors, storage
tanks, hoses, fittings and support hardware, such as filters,
manifolds, pressure regulators and valves. Hence, electric
motors provide a lower cost solution, since 240V AC
power is widely available in most countries. A designer is
also faced with a wide range of choice for selecting an
appropriate or ideal electric motor to best suit this
application. The following types of electric motors can be
used since they operate quietly and are readily available:

AC induction machines

Brushless DC motors (BLDC or ‘stepper motors’)

Brushed DC motors (series, shunt and compound)
AC motors and brushless DC motors (BLDC) are
commonly used in industry, however, both types require
fairly complex controllers to control motor speed and / or
commutation. AC motors are best used in situations that
require constant speed and torque. BLDC motors require
an expensive (dedicated) controller to control speed.
Brushed DC motors, however, operate on simple
principles and can be easily controlled by an embedded
controller without requiring too much development effort.
Fig. 8. Schematic of the control system for the ‘Smart Gym’
Figure 8 shows the main subsystems of the electrical
power and control system for the ‘Smart Gym’ prototype.
An AtmelTM AVR ATmega8535 microcontroller [19] or
CPU (Central Processing Unit) was chosen to execute
software (control algorithms), send text to the LCD screen,
activate or deactivate the cable brake (C-cleat), and
generate appropriate drive signals (control signals for
upward or downward movement of the barbell at a known
speed) for the geared DC motor / winch. The actual
position of the cable is detected by the position sensor
system (a HCTL 2022 [20] ‘up/down’ counter or
‘quadrature decoder’ IC or ‘Integrated Circuit’ scans two
photo-diode sensors on the slotted wheel in Fig. 9 at a
frequency of 29 kHz generated by a ‘555 timer’ IC [21].
This frequency is high enough to ensure accurate position
sensing even at the highest expected speeds for the optical
encoder wheel.) The custom-built position sensor outputs
a 16-bit absolute position value which is read by the AVR
microcontroller (as 2 separate bytes). An optical encoder
position sensor was chosen to monitor cable position
5.3 System modeling
A simple model of the ‘Smart gym’ was made to
determine the motor criteria suitable for this application.
An equivalent system can be modeled in order to
determine the maximum required torque of the motor, and
hence determine a set of motors suitable for the application.
m is the motor torque, L is the torque produced by the
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barbell load, 1 is the torque exerted on gear 1 by gear 2,
and 2 is the torque exerted on gear 2 by gear 1.
The winch motor selected was the SD11 shunt wound
motor from ParvaluxTM Corporation. The motor provides
approximately 40 Nm of torque due to an externally fitted
200:1 ratio gearbox (LWS). Figure 11 shows the nearly
linear ‘Voltage vs. No load speed’ characteristics of this
DC motor, operating at a steady state speed. Using Eq. 7,
the ‘Smart gym’ requires a theoretical maximum torque of
13.5 Nm from a motor, using the values: ( n1 / n2 ) =
(1/200) = 0.005; JL = 0.168 kg.m2 ; Jm = 0.006 kg.m2 ;
2
1 = 2200 rad/s ; m = 200 kg ; and r2 = 0.029 m.
Since the maximum torque output of the SD11
motor-driven gearbox is 40 Nm, this provides a ‘safety
factor’ of 2.96 for a 200 kg mass. The fastest lifting speed
seems slow due to the 200:1 gear ratio, so the motor needs
to be run at full speed to achieve a respectable lifting rate.
1 n1
Gear 1
Jm
2 2
m 1
L
JL
Gear 2
n2
Fig. 10. Equivalent system (showing side view of gears)
Jm and JL are the mass moments of inertia for the motor
and the load respectively. We can also model viscous
friction on shafts 1 and 2 with f1 and f2 . The gear ratio is
( n1 / n2 ) where ‘n’ is the number of teeth on a gear. Also,
 is angular speed, or the same as  . Therefore:

 J 
(1)
 m   1  f 1 1  J m 1
(2)
 L   2  f 2  2  J L 2
(3)
n 
Fig. 11. Voltage vs. Speed (unloaded) steady state for SD11 motor [15]
5.4 Motor modeling in ‘Armature controlled mode’
(4)
 2   1  2 
 n1 
There are three main types of wound DC motors,
namely, the series motor, the shunt motor and the
compound motor. Each motor has different torque speed
characteristics due to the arrangement between the stator
magnetic field and the field armature. The motor which
best suits the requirements of the ‘Smart Gym’ is the SD11,
however the shunt layout must be disconnected in order to
operate in a mode known as ‘armature controlled mode’.
It is impossible to drive the motor in both directions if the
series or shunt layout is kept. This is because the polarity
on both the armature and field will remain the same.
In this mode of operation the field winding is
maintained at a constant voltage, which is usually the
highest voltage available in this system. The voltage in the
armature can be modulated to control the speed of the
motor. This section describes the control theory behind
this mode of operation. It can also be seen that if the field
and armature voltage supplies are exchanged, it is possible
to operate the motor in field current controlled mode,
where the armature is held at a constant potential and the
field voltage is varied. However in most cases, armature
controlled mode is the preferred option. There are
numerous advantages to operating in ‘armature controlled
By substitution, the equation for motor torque is:
 n 
2

 n 

2
 n 
 m    1  J L  J m  1    1  f 2  f 1  1   1  L
  n 2 

  n 2 

 n2 
(5)
 L  mgr
(6)
2
Given m = total mass, g = gravity (9.81 m.s-2), r2 = pulley
radius for wheel 2, and neglecting viscous friction [15]:
 n 
2

 n 
 m    1  J L  J m  1   1  mgr
  n 2 

 n2 
(7)
2
The motor and gearbox needs to be selected with a
maximum torque output greater than m from Eq. 7 in
order to be able to achieve the desired maximum angular
acceleration 1 for shaft 1 and the entire system.
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mode’ as opposed to ‘field controlled mode’. The most
important reason for operating in ‘armature controlled
mode’ is due to the much lower inductance that exists in
the armature than in the field. The field has an inductance
of 119H, whereas the armature has a much lower
inductance of 162mH and would be much more responsive
to voltage changes. Inductance in electrical circuits resists
the change of current due to the property of magnetic
storage for inductors. High inductance is not desirable if a
voltage switching scheme is used. For example, if the
motor needs to be stopped in the quickest possible time
from a running state, the inductance in the driving winding
should be as small as possible otherwise the system will be
slower and much less responsive, due to the inductor’s
ability to store energy.
1
 (s)
B
s
J
Using ‘Kirchoff’s voltage law’ and Fig. 13:
Va  La
di a
dt
(12)
 R a i a  V BEMF
The above equation contains the term for the ‘back
electromotive force’ ( VBEMF ) of the system. This is
generated as the armature rotates and cuts through the
magnetic field produced by the field windings. The back
EMF produced is proportional to the velocity of the
armature (where Kb is a constant).
V BEMF  K b 
Tm
B
(viscous friction)
Km
 (s)
Fig. 12. Free body diagram of the load on a motor
Va (s)
The torque of the motor Tm is proportional to the current Ia
in the armature winding. Using Fig. 12,
Tm  K m I a
(13)
Substitution leads to the following transfer function of
input voltage to rotational speed [15].
J
T

Tm ( s )
(11)
J

(14)
La J

B   K bK m
R 
 s  a  s 
  


J   La J
L
a 





Using MATLABTM and experimentally gathered data, an
approximate model of the DC motor was found. The plot
in Fig. 14 shows a simulated ‘step response’ of the system.
(8)
 J 
J   T m  B   J   T m  B 
Variable armature voltage Va
Ra
(9)
Constant field Vf
Rf
La
+
Lf
Va_
+
Vf _
ia
Fig. 13. Armature control mode for the SD11 DC motor
Taking the Laplace Transform gives the transfer function
for input torque to output speed.
 ( s )( Js  B )  T m ( s )
Fig. 14. Simulated ‘step response’ of the DC motor [15]
The step response shown above gives an approximation of
time delay expected in the system. Because some of the
motor parameters were found experimentally, the response
can only be used as an estimate. The derived model can be
(10)
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NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
used as a tool for developing a controller if the speed of
response is not fast enough in the physical system. The
simulation results show that this kind of motor would
respond quickly enough for the ‘Smart Gym’ application.
control electronics level. For such dissipation levels it is
necessary to use a power electronic switching circuit that
can meet the requirements of the controller.
5.5 H-bridge motor driver circuitry
Diagrams Fig. 15(b) and 15(c) show the switching states
of the H-Bridge circuit for controlling the mean armature
voltage Va (using ‘Pulse Width Modulation’, or PWM) and
direction of rotation. The four switches (labeled S1, S2,
S3 & S4) are electronically controlled to divert the current
path flowing across the load of the bridge.
The
microcontroller sets the PWM ‘duty’ and the state of the
switches via the control software, which will determine the
speed and direction of rotation of the DC motor shaft.
Fig. 16. Schematic for H-bridge controlled by Intersil HIP4081 chip [15]
An 'H-bridge' circuit, constructed from 4 N-channel
MOSFETs (high current switches for S1, S2, S3 and S4)
and an IntersilTM ‘HIP4081 [22] MOSFET gate driver
chip’, was designed and built to drive the SD11 motor
using PWM signals from the CPU (Fig. 8). The HIP4081
gate driver chip prevents destructive conditions such as
‘shoot through’, while controlling 4 external N-channel
MOSFETs to perform the first four rows of (useful)
switching functions shown in Table 1. The SD11 motor
requires a supply voltage of 80V (DC) for its field and
armature to meet the needs of the ‘Smart Gym’ prototype.
Figure 16 shows a detailed schematic showing how an
AtmelTM ATmega8535 microcontroller is connected to the
HIP4081 gate driver chip, to drive the 4 MOSFETs
(switches S1, S2, S3 and S4) of the H-bridge. Figure 17
shows the artwork for the final H-bridge PCB. Figures 16
and 17 were created using ‘Eagle PCB’ CAD software.
Fig. 15. (a) ‘Shoot through’ (short); (b) Forward; (c) Reverse [15]
Table 1: H-bridge states (1 = switch closed; 0 = switch open)
S1
1
0
1
0
0
1
S2
0
1
1
0
1
0
S3
0
1
0
1
0
1
S4
1
0
0
1
1
0
State
Forward
Reverse
Brake
Brake
Shoot
through
It can be seen from Table 1 that only four valid states of
switching are useful for the ‘Smart Gym’ application
(‘Forward’, ‘Reverse’ and two ways to ‘Brake’). The rest
are meaningless and would result in a destructive
condition known as ‘shoot through’. ‘Shoot through’
occurs when either side of the bridge is shorted between
the high voltage rail and the common ground. Suitable
circuitry protection must be used to make sure that this
condition does not occur to avoid overheating wires and
damaging circuit board components.
The H-bridge circuit is needed to control mean
armature current or mean armature voltage. The H-Bridge
load will be the inductor winding of the armature. In this
mode, the field winding is held at constant potential and
reversing the direction of the current through the armature
winding will allow change of rotational direction. Whilst
the motor is running at full power, it is consuming
approximately 200W of power, which is significant
compared to the low levels of power dissipation at the
© N&N Global Technology 2015
DOI : 02.IJES.2015.1.4
Fig. 17. Double-sided PCB (Printed Circuit Board) for H-bridge [15]
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NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
4.
The user cannot lift the weight for a specified time.
(2 seconds for a ‘Beginner’, 3 seconds for an
‘Intermediate user’ and 4 seconds for an ‘Advanced
user’)
‘Partial assistance’ (intermittent spotting support) is
required when any of the following states are detected:
1. The barbell has stopped for too long or is moving
backwards before the top position has been reached.
The user struggles (velocity stalls) on an upward
repetition for a specified time (2 seconds for a
‘Beginner’, 3 seconds for an ‘Intermediate user’ and
4 seconds for an ‘Advanced user’).
2. The position is close to the user-defined minimum
position.
3. Downward acceleration is significant after the barbell
has been released (in case the user let go of the
barbell as it was being lifted, or if the barbell slipped
out of both hands, its fall will be stopped as quickly
as possible to prevent risk of injury).
4. When the barbell has stopped near the top position on
the final rep (end of the set).
Fig. 18. (a) Top view of H-bridge circuit (left); (b) heat sinks (right) [15]
5.6 Cable brake drive circuit
The 24V DC solenoid for the ‘Cable brake’ shown in Fig.
7 is activated by a simple SPST (Single Pole Single
Throw) power relay (switch), which is powered by a
transistor connected directly to an output pin of the AVR.
Alternatively, the solenoid can also be switched ‘on’ or
‘off’ just as easily using an N-channel MOSFET (switch),
where software sets the state of its driving output pin.
5.7 ‘Smart Gym’ controller functions
There are two possible ways in which the system will act
in order to aid the user. These are categorized as ‘Full
assistance’ and ‘Partial assistance’ and are defined as
follows:
Full assistance: The user is considered to be in an
unsafe situation, the entire barbell load must be carried by
the motor and the set must end. When this is required, the
system will take full control and lift the barbell to the userdefined (or start) rack position.
Partial assistance: This is when the user is deemed
to be in a safe situation, but some assistance is required in
order to complete the single rep or set. This assistance
provides small additional lifting forces (or intermittent
spotting) on the barbell to help the user complete a
repetition. This kind of assistance generates just enough
momentum for the user to barely finish the end of the
repetition (where the barbell is lifted at lowest discernible
speed), to ensure that the loaded muscles of the lifter reach
the verge of failure, or are almost completely fatigued.
‘Full assistance’ is required when any of the
following states are detected:
1. Emergency stop button is pressed.
2. Downward velocity or acceleration is too large.
(Acceleration exceeds 5 m/s2 towards the user in the
lower 40% of the barbell range; or Velocity exceeds
0.7 m/s towards the user in the lower 40% of the
range of motion; i.e. if the barbell is dropped by
accident.)
3. Partial assistance has already been used 3 times
consecutively (indicating muscle failure).
© N&N Global Technology 2015
DOI : 02.IJES.2015.1.4
Fig. 19. (a) ‘Smart Gym’ control box; (b) LCD screen text display [15]
When the ‘Smart Gym’ controller is first switched on, the
AtmelTM AVR ATmega8535 microcontroller initializes its
operating variables. If the ‘Smart Gym’ is used for the
first time, the user must manually input (or record) the
start and end positions for the barbell limits. This
calibrates the position sensor so the controller knows
exactly where the ‘bottom’ and ‘top’ positions for a type
of exercise are located. These ‘top’ and ‘bottom’ positions
for repetitions are stored in non-volatile EEPROM
memory for each type of exercise, however, they can be
reprogrammed to suit different users.
During this
calibration procedure, the barbell can be manually raised
or lowered using the ‘Up’ and ‘Down’ keys on the control
panel shown in Fig. 19. The ‘bottom’ or ‘top’ position is
recorded using the ‘Enter’ (or ‘Select’  ) button each time
the barbell is in the correct position for a particular
exercise. The 4 buttons on the control box allow the user
to navigate through different ‘menus’ of the control
program (CPU software). The ‘x’ button is like a cancel
or ‘Escape’ button that returns to the previous menu.
The controller interface panel allows the user to
control all settings and functions of the ‘Smart Gym’, from
manually controlling the system components to setting
exercise requirements and starting the workout. A list of
all the options available to the user is shown below:
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NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
 Manual control of the system
 Setting desired number of reps (repetitions) and sets
(groups of repetitions)
 Setting difficulty level (Beginner, Intermediate or
Advanced user)
 Defining barbell limits (setting ‘bottom’ and ‘top’
positions for each kind of exercise)
 Selecting an exercise and beginning the workout
6. Software design and development
High-level control software was written quickly and easily
using the BASIC language BASCOM-AVRTM [23]
(similar to MicrosoftTM QBasic). BASIC was selected
because it is much simpler to learn and use than the C
language, yet it is functionally equivalent to C and allows
complete control of almost any AVR chip. Functions
written in BASIC usually take less than half the time and
effort to develop compared to developing equivalent
functions with C. High-level code is first compiled to
native AVR machine code before being downloaded to a
chip’s ‘Flash’ memory using ‘AVR Studio’ or ‘BASCOMAVR’. BASCOM-AVR allows monitoring of serial
communications between the PC and AVR using a built-in
‘Terminal emulator’ application that displays all ASCII
character data that is sent and received via a standard RS232 serial communications port (or a virtual COM port).
Fig. 20. Example of position, velocity & acceleration vs. time plots [15]
Figure 20 shows example plots vs. time for cable position,
velocity and acceleration. Point A is at the ‘bottom’ point
of a repetition, and this corresponds to a low or zero
velocity at point B. The vertical line C in Fig. 20 shows
the effect of rapid braking of the cable just after the barbell
is deliberately dropped (released for free-fall). After line
D, starting from the top position, the barbell is dropped
again and the controller quickly applies the cable lock and
stops the motor to stop the barbell. At point E, the cable
lock is activated, full assistance is activated and the barbell
is returned to the rack or start position by the motor.
Fig. 21. Main flowchart (refer to Figs. 22, 23 and 24 for details) [15]
© N&N Global Technology 2015
DOI : 02.IJES.2015.1.4
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NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
Figure 21 is the highest level flowchart, or the ‘main’
function. It includes every sub-function and loops
continuously until the user requests an ‘exit’. After
initialization and the menu, each loop will update the
system, pass once through the assistance algorithm, then
inputs are checked and data is sent to the LCD screen.
Fig. 24. ‘Assistance algorithm’ module flowchart [15]
Table 2: Examples of states that can trigger ‘full assistance’ [15]
Full assistance required for Rep #
1 2 3 4
Position Critical: 0-2% of range
% of
Middle: 20-80%
range
Top: 80-100%
X X
Velocity Stall for >2 seconds
Values
Or Stall > N seconds
in
Level 1: <-0.05, > 0.03
m/s
Level 2: <-0.5, > 0.5
down -
Level 3: < -2, > 1 m/s
up +
Accele-
Critical: < -7 m/s2
7 8 9 10
↓
X X
Bottom: 2-20%
Fig. 22. ‘Initialize’ module flowchart (refer to Fig. 21); from [15]
5 6
↓
X↓ X
↓
X X
↓
X
X X X X X ↓X X X X X
↓
X
↓
X
↓
X
X
X
2
ration
Medium: < -2 m/s
m/s2
Low: < 0
Other
Barbell just released
X
X
X X X X X X X X X X
Final rep
X
Partial assistance x3
X
Table 3: Example of states that can trigger partial assistance [15]
Partial assistance required for Rep #
1
Position
X
Critical: 0-2% of range
Bottom: 2-20%
% of range
2
3
4
5
X
X
Middle: 20-80%
X
Top: 80-100%
Fig. 23. ‘User menu’ module flowchart to set exercise parameters [15]
Velocity
X
X
X
Stall
The user menu will be controlled by the user with the ‘up’,
‘down’, ‘enter’ and ‘exit’ buttons. The default setting is 3
sets of 10 reps, intermediate assistance and 30 kg of
weight. If the user wishes to change any of these settings,
he/she simply needs to navigate to the required menu
option and change it with the up/down buttons, pressing
‘enter’ to confirm each selection. A ‘monitor exercise’
option can also be activated to transmit exercise data for
each set (like in Fig. 20) to a PC.
m/s
Level 1: < -.05, > 0.03
Level 2: < -0.5, > 0.5
Level 3: < -2, > 1 m/s
Acceleration
Critical: < -7 m/s2
X
Medium: < -2 m/s2
© N&N Global Technology 2015
DOI : 02.IJES.2015.1.4
Stall for > 2 seconds
11
m/s2
Low: < 0
Other
Barbell just released
X
NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
In order to realize a ‘professional’ or commercial
quality product, many improvements are needed to
produce a more robust and feature-rich ‘Smart Gym’. For
example, many professional bodybuilders and competitive
weight-lifters tend to squat or bench-press barbells that
weigh over 500 lbs (over 227 kg), or sometimes up to
1100 lbs (500 kg), which requires much stronger
components (bigger structural framework, cables, bolts
and pulleys, plus a much bigger motor and cable brake). A
force sensor or load cell could be placed in-line with the
cable to monitor cable tension and this can be used to
implement more features, such as providing accurate
‘closed-loop’ force control for precise spotting assistance
and even calculating the amount of work or energy
consumed (Work = Force x Distance). Also, an attractive
touch-screen monitor or display can replace the 16x2-line
LCD display and the control box shown in Fig. 19. A
high-resolution graphical interface could be created to
provide the user with a rich GUI (Graphical User
Interface) that provides more features, such as detailed
operating instructions (with pictures, illustrations, or even
video tutorials with audio), coaching advice on how to
execute exercises effectively and properly, and perhaps
even a personalized digital ‘training log’ or a database
feature that records workout achievements. Software can
record, monitor, analyze and report strength and
performance gains for each user, providing information
such as total energy expended for a workout, average
power output (Power = Work / time), etc. as long as the
user is willing to input accurate information about the
barbell weights that were used for each set.
Using force-controlled linear actuators, such as
pneumatic double-acting air cylinders or ball-screw linear
actuators, it is possible for resistance forces to be
automatically set by a user so that plates do not need to be
manually loaded and unloaded on each side of a barbell, or
weight-stack pins do not need to be moved, and load
information does not have to be manually entered by a
user. ExerboticsTM gym equipment allows users to
electronically set the resistance load via a computer screen
without having to manually lift any plates or change a
weight-stack pin position, therefore, despite being very
expensive, they demonstrate excellent levels of
convenience and ‘ease of use’ for the end-user. ‘Active’
force-controlled linear actuators, which employ pressurecontrolled pneumatic cylinders or ‘air muscles’, will be the
subject of future research work, because they can be built
at much lower cost than precision machined ball-screw
linear actuators, the kind used on ExerboticsTM gym
equipment [14].
Another major improvement to the existing ‘Smart
Gym’ design would be to use brushless DC motors (BLDC
or stepper motors) to reduce long term maintenance costs.
‘Brushless’ DC motors require much less maintenance
compared to ‘brushed’ DC motors (which employ
Two ‘lookup tables’ were designed to determine the states
or exact conditions when ‘full assistance’ or ‘partial
assistance’ is required. These conditions are continually
checked during each iteration of the ‘main loop’ in Fig. 21.
Tables 2 and 3 show examples of conditions that could
trigger ‘full’ or ‘partial’ assistance for each repetition in a
set. Each of these states can be checked continuously via
software (The ‘X’ can be placed in any desired cell where
assistance is required). Such spotting preferences can also
be ‘saved’ for each individual user, just like any of the
other major exercise parameters, like the preferred number
of sets and number of reps.
If the keypad ‘E-stop’ (or ‘Emergency stop’) button is
pressed at any time, the entire ‘Smart Gym’ will return to
the main menu and the cable brake will be engaged,
stopping all movement of the barbell almost immediately.
7. Discussion
A demonstration video showing the most important
features and functions of the ‘Smart Gym’ prototype can
be viewed online at [24]. It is interesting to note that the
barbell weight ‘feels’ very natural for loads above 20 kg.
When lifting weights below 20 kg, the influence of the
spring-reel (cable tensioning device) becomes noticeable,
especially with small weights. The spring reel tension
force could be reduced a lot further without risk of any
cable coming off a pulley.
Perhaps a gearbox ratio of 100:1 or even 67:1, for the
same size motor, would have been more desirable for
producing much faster lifting speeds (double or triple the
existing speed), because the maximum motor torque was
already almost 3 times higher than necessary for lifting
200 kg. At these gear ratios, variable speed control may
have been much more noticeable, rather than always
running the winch motor at maximum speed, to produce
only very low lifting speeds due to the 200:1 gear ratio.
The only major disadvantage of a much lower gear ratio is
the greater tendency for a large barbell load to ‘back-drive’
the actual drive motor or winch if it is kept in a ‘neutral
state’ (with the H-bridge ‘brake’ mode not activated).
Although the 200:1 gearbox requires that the SD11 motor
runs at full speed almost all of the time during spotting, the
same functionality and performance of the H-bridge motor
driver circuitry described in this paper can be achieved
with a simple DPDT (Double Pole Double Throw) relay to
drive the DC motor in the ‘forward’ and ‘reverse’
directions at top speed from an 80V DC power supply.
Hence, for a low-cost version of the ‘Smart Gym’, it may
be possible to avoid all the effort, complexities and costs
involved in implementing and manufacturing a
complicated H-bridge circuit using the complex
components and control methods described in Section 5.5.
© N&N Global Technology 2015
DOI : 02.IJES.2015.1.4
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NNGT Int.J. on Embedded Systems, Vol. 2, Feb 2015
conductive brushes that may eventually wear out and need
replacement after several months of continuous operation).
One of the big disadvantages of using BLDC motors is the
need for a separate motor controller system, because
controlling stepping or switching states for such a motor
typically requires a great deal of computational power
(using up a lot of chip resources and control time) for a
single microcontroller like an AtmelTM AVR ATmega8535.
assist or even protect people in their fitness workouts and
in their daily training sessions.
It is hoped that this paper will encourage and
motivate product developers, engineers and researchers to
create new useful machines and high-tech devices to
further advance the fields of strength and fitness training
and to find innovative ways to raise health and safety
standards worldwide.
8. Conclusion
Acknowledgments
All of the original objectives for this project listed in
the introduction were achieved and demonstrated
successfully using this prototype. The ‘Smart Gym’
described in this paper can successfully detect and respond
quickly to different kinds of states that indicate the need
for ‘full’ and ‘partial’ assistance, as well as ‘free-fall’ of
the barbell. The ‘C-cleat’ (cable brake) design is effective
at quickly arresting any cable movement and allowing the
motor to lift the total barbell load.
The design principles and mechanisms used in simple
‘add-on’ mechatronic systems like the ‘Smart Gym’ could
be implemented or combined with most existing gym
equipment that use weight-stacks, pulleys and cables.
Such additions to standard gym machines would ensure
higher workout safety, improve overall training quality
and allow the recording of workout activity for monitoring
a user’s progress and strength gains over time. ‘Passive’
resistance systems like the ‘Smart Gym’ are much simpler
and cheaper to manufacture than ‘active’ force-controlled
gym equipment. They also have the potential to become
more popular for home users and could capture a larger
market share than much more expensive products which
employ ‘active’ force-controlled linear ball-screw
actuators.
In the not-too-distant future, ‘computer controlled’
gym equipment and their force-controlled actuators could
eventually become very popular worldwide and may be
adopted by many gyms and chains of fitness centers. Such
technologies could find practical uses in many other
motion control, automation and robotics applications as
their controllers, actuators and sensor components become
cheaper, faster, easier to control and easier to use.
Growing in popularity are devices like pedometers
(step counters), heart rate monitors, wireless ad-hoc ‘Smart
sensor’ networks and GPS-enabled embedded systems
used in team sports like ‘Paint-ball’ and football, mobile
phone fitness 'apps', interactive 3D games and workout
programs (e.g. for XboxTM Kinect and WiiTM consoles)
and other high-tech health-related products. Strength and
fitness training is undoubtedly going to be a rapidly
growing area for mechatronics, robotics and embedded
systems application development in the near future, since
there are many opportunities to use such technologies to
The first author wishes to thank former Curtin University
Engineering students, Grant Wirth, Peter Kneale and Brett
Nardi for allowing several figures from their final-year
project thesis [15] to be used in this paper. As the project
supervisor and adviser of the ‘Smart Gym’ final-year
undergraduate project, the first author gives full credit to
these three co-authors (now degree-qualified graduate
Mechatronic engineers) for coming up with the original
ideas and designs shown in most of the figures in this
paper (marked with Reference [15]). All the ideas,
methods and designs described in this paper are the
culmination of hundreds of hours of shared ideas, research
and diligent hard work on the part of all team members.
The money to purchase materials and parts for this project
was kindly provided by the Department of Mechanical
Engineering, Curtin University of Technology, Australia.
© N&N Global Technology 2015
DOI : 02.IJES.2015.1.4
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Dr. Sam Cubero was born in 1972 and was
raised and educated entirely in Australia since the age of 2 after
his Filipino parents immigrated from the Philippines to Australia
in 1974. He completed his B.Eng (Bachelor of Engineering)
degree, with Honours, in 1993, at the University of Queensland,
Brisbane, specializing in mechanical engineering. He completed
his Ph.D in Mechatronic Engineering at the University of
Southern Queensland (USQ) in 1998, specializing in embedded
controller development, robotic walking vehicles, simulation
programming and variable force, speed and position controlled
actuators. From 1998 to 2007, he lectured and developed the
teaching and lab materials for several new subjects at Curtin
University of Technology, Western Australia, in the areas of 2D
and 3D CAD (Engineering Graphics), practical mechatronics
(designing, building and controlling custom-made mobile robots,
robot arms and embedded systems), automation and motion
control, machine design and manufacturing, microcontroller and
PC programming, and electronics (for designing and building
mobile robots and motion controllers for mechatronic and
remote-controlled systems). From 2007 to 2010, he worked at
the University of Southern Queensland (USQ), teaching several
different engineering subjects, including robotics and machine
vision labs, CAD for engineering graphics and surveying, and
PBL (project based) engineering design subjects. Dr. Sam
Cubero currently works at the Petroleum Institute, Abu Dhabi,
United Arab Emirates (UAE), teaching several different
engineering subjects and supervising research projects relating to
product manufacturing, design, CAD, walking vehicles,
exoskeleton robot suits and several different mobile robots. One
of Dr. Sam Cubero’s part-time hobbies is amateur bodybuilding
(regular weight-training and sports nutrition). You can view his
detailed CV and sample movies of many of his successful
robotics and machine-vision research and development projects
at www.samcubero.com
© N&N Global Technology 2015
DOI : 02.IJES.2015.1.4
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