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Lab 4
Stepper Motor Control
Spring 2001
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Purpose
In this lab you will do the following tasks:
 Learn how to use a R/C servo
 Learn how to use a stepper motor
Discussions
RC - Servos
In control terms, a servo implies a “smart” plant, i.e. one that measures it’s own feedback and controls itself.
Instead of sending an open-loop voltage, the signal into a servo is a setpoint for the servo to go to (often referred
to as “servoing” to a position). All servos have built-in controllers that move a motor or other device to a given
setpoint.
R/C servos are use by hobbyists to fly scale aircraft, cars, boats, etc. Each servo has 3 lines. The signal line
(usually white or yellow) receives a setpoint via a pulse. Two other lines (red and black) are used for power
(one at 5-10 volts, and one at ground). The signal pulse is sent every 20 ms, and the duration of the pulse high
(called the duty cycle) sets the position of the servo. Read you STAMP book if you want to know more on the
signal and how a servo works.
A pulse of approximately 2 ms moves the servo full counterclockwise (about 90 degrees), and a pulse of approx.
1 ms moves the servo full clockwise (about –90 degrees). The servo range of motion is usually around 180 –
270 degrees.
Stepper Motors
Stepper motors are motors that have a fixed number of positions that they can move to, and these position
correspond to coils and teeth within the motor. Energizing a coil holds a stepper motor fixed at a given position.
A stepper motor moves by continuously activating adjacent stator coils. When the coils are energized in a
programmed manner, a stepper motor moves between these positions. We can count the number of positions
(called steps) that the motor needs to take to move a set distance. The rate of the input pulses determines the
velocity of the motor. The inputs to the driver circuit are typically digital pulses, with each pulse corresponding
to one step of incremental motion. Another input to the driver is the direction of the stepping motion. Each
pulse input to the driver advances the rotor shaft one step toward the commanded direction, at which point the
motor latches to the new equilibrium position. If properly controlled, the number of steps that the motor moves
is equal to the number of input command pulses. Thus, no angular position feedback is needed for many stepper
motor applications; stepper motors are mostly used in open loop fashion.
Stepper Motor Performance Characteristics:
(1) Resolution: This is number of steps per revolution. Typical resolutions are full step resolution: 200 steps,
half step resolution: 400 steps, and micro stepping: over 10,000 steps.
(2) Accuracy: This is the dimensional tolerance with respect to the nominal position.
(3) Holding (Static) Torque: This is the restoring torque versus the rotor position. This torque forces the rotor
back to the equilibrium positions where the torque returns to zero.
(4) Dynamic Torque (Pull-Out Torque): This is generally the torque-speed curve, which indicates the maximum
static torque load at corresponding speeds (or pulse rate) without pulling the motor out of synchronization
with the input pulse rate. This curve shows the torque envelope that the motor can be used. When inertial
loads are attached to the motor shaft, the effective maximum torque is reduced. The particular speed-torque
curve associated with a given inertial load is called pull-in torque.
IN
Internal
Software
Gate
Internal
Software
Trigger
Counter 1
GATE
OUT
Internal
Internal 2.5MHz
Software Clock
Gate
Clock
IN
Counter 2
GATE
Out to
Stepper
Motor
Driver
OUT
AND gate with one
line inverted
TRIGGER
Internal
Software
Trigger
TRIGGER
Figure 1: Counter #1 and #2 setup for controlling a single stepper motor.
To command/control the stepper motor at your bench, you will be using the CIO-INT32 board installed
in the PC. The CIO-INT32 is a very versatile digital interface board. The heart of the CIO-INT32 is a pair of
Zilog Z8536 programmable digital I/O chips. Each chip is equipped with 20 bits of Digital I/O that can be
independently set for input/output and 3 counter/timers that can be set up independently or chained together
internally.
To drive the stepper motor you will be using the CMD-50 stepper motor driver from American Precision
Industries. This small stepper motor driver requires a voltage supply in the range 15-40 VDC and can produce
3.5 amps/phase maximum. This is more than enough power to drive the small stepper motor at your bench. We
will be powering the CMD-50 with 20 VDC and applying approximately 0.3-0.4 amps/phase. There are four
important digital input lines into the CMD-50 (they are labeled on the top of the CMD-50 box as):
RUN/(RESET), HALF/(FULL), STEP IN, and CW/(CCW). The purpose of each of these input lines is as
follows:
STEP IN
This line receives the square wave voltage pulses that command the stepper motor to step
to its next position. With each pulse (when the input square wave changes from 5V to
0V), the driver activates the adjacent motor coil. The frequency of the pulses then
determines the speed the motor spins.
RUN/(RESET) When this input line is high (5 volt), the CMD-50 is in run mode (which we will call
RUN). When this input line is low (0 volts), the CMD-50 is in RESET mode. In this
mode, the driver ignores the input into the STEP IN port above and holds the stepper
motor at it current position (Note: the use of a signal between 0 and 5 volts is called a
TTL logic signal, or just TTL for short).
HALF/(FULL) When this input line is high, the driver excites adjacent coils or poles in order to step the
motor half way (HALF) between the poles. In the low state, the CMD-50 uses single pole
stepping the motor a full step (FULL). The half-step method produces more torque
compared to the full-step. In most applications, the half-step method is used simply to
obtain more torque.
CW/(CCW)
When this line is high, the CMD-50 commands the motor to step in the clockwise (CW)
direction. In the low state, the CMD-50 commands the motor to step in the counter
clockwise direction (CCW).
To command the stepper motor to perform both velocity commands (spin at a certain speed) and position
commands (step x number of steps from current position), we used two timer/counter channels and 3 digital I/O
outputs. The three digital outputs are used to command 3 of the 4 lines described above: HALF/(FULL),
RUN/(RESET), and CW/(CCW). The output of timer/counter #2 drives the STEP IN line.
Study Figure 1 to see how the Zilog 8536 chip is configured to accomplish the stepper motor control. The
timer/counter #2 is used as a square wave generator. The frequency of this square wave corresponds to the
speed you would like the stepper motor to spin (think pulses/second). Timer/counter #1 is used to make the
stepper motor only move a certain number of steps. The stepper motor will then stop after this position is
achieved (position command). Timer/counter #1 counts the number of steps that are sent to the stepper motor
and when the desired number of steps has been reached, timer/counter #1’s output goes HI. This gates off
timer/counter #2 and thus stops the square wave to the stepper driver. If you would like to just command the
stepper motor to spin at a certain speed you can pretty much ignore timer/counter #1 and just program
timer/counter #2 with the correct frequency.
The source code uses the following functions. Read through the comments to better understand how the stepper
control is accomplished. If you wish, you can read through the Z8536 manual to better understand that chip.
The manual is located in one of the bookshelves of the Mechatronics Lab.
Routines:
init_step
- Sets up ports and timers
go_position - Go to a target position (relative)
go_velocity - Run at a target velocity
stop
- Stop the pulses in position or velocity mode
Counter control routines:
set_frequency - Set the frequency for the steps (timer 2)
set_position - Set the number of steps to go
disable_ctrs - Inhibit counter activity
enable_ctrs - Allow counter activity
pause_ctr
- Pause counting
continue_ctr - Continue counting
tigger_ctr
- Start a counter running
count_in_prog - Indicates whether a counter is running
clear_IP
- Clear the interrupt flag (set when ctr finishes)
Digital I/O routines:
set_RUN
- Set driver to "RUN"
set_HOLD
- Set driver to "HOLD"
set_CW
- Set clockwise direction
set_CCW
- Set counter-clockwise direction
set_HALF
- Step by half-steps
set_FULL
- Step by full-steps
The line connections to the computer are shown below in Figure 2:
STEPPER MOTOR
TIMER
HALF/FULL
CW/CCW
STEP IN RETURN
C1 Gate
C1 In
C2 Gate
C2 In
C3 Gate
C3 In
HALF/FULL
CW/CCW
STEP IN RETURN
C1 Gate
C1 In
C2 Gate
C2 In
C3 Gate
C3 In
I/O
C2A7
C2A5
C2A3
C2A1
C2B7
C2B5
C2B3
C2B1
CH2 INT OUT
5MHz OUT
C2C3
C2C1
C1A7
C1A5
C1A3
C1A1
C1B7
C1B5
C1B3
C1B1
NC
INT INPUT
C1C3
C1C1
+5V
PIN
1 
3 
5 
7 
9 
11 
13 
15 
17 
19 
21 
23 
25 
27 
29 
31 
33 
35 
37 
39 
41 
43 
45 
47 
49 
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PIN
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
I/O
TIMER
C2A6
C2A4
C2A2
C2A0
C2B6 C1 Trig
C2B4 C1 Out
C2B2 C2 Trig
C2B0 C2 Out
CH1 INT OUT
2.5 MHz OUT
C2C2 C3 Trig
C2C0 C3 Out
C1A6
C1A4
C1A2
C1A0
C1B6 C1 Trig
C1B4 C1 Out
C1B2 C2 Trig
C1B0 C2 Out
NC
INT ENABLE
C1C2 C3 Trig
C1C0 C3 Out
GND
STEPPER MOTOR
RUN/RST
STEP IN
RUN/RST
STEP IN
Figure 2: The pin-out of the CIO INT-32 board.
In Lab Assignment:
RC Servos (15 min)
1. Using the code supplied to you, determine approximately how many degrees the servos can move through.
2. Scope a signal going to the servo. Sketch what it looks like. Very often hobbyists use IC chips to test their
servos. What chip would they most likely use?
3. Experiment with the servos. Do the servos move faster or slower than the stepper or DC motors? Is there a
minimum angular resolution (approximate)? For their size, how would you rate the torque output of the
servos?
4. If you only need to command 1 stepper motor with the Int32 board, how many R/C servos can you command
(this does NOT mean to code this)? If you are not using stepper motors, how many servos then?
Stepper Motors (remainder of lab)
5. Play with the steppers. Can you stop them with your hand at low velocity? At high velocity?
6. Command the motor to start moving with a certain velocity. Find out the highest attainable speed for your
stepper motor if the motor is started at rest.
7. Incrementally ramp up the speed of the motor (i.e. command it to 2000, then 3000, then 4000…). Find out
the highest attainable speed by using this method. Is this highest speed less or more than the speed from 1
above? Why would this be?
8. We want to implement a capability to go to a high velocity without doing the ramp up procedure manually.
We will set up another RTSS timer to run a task that commands the motor to ramp up to a speed at a given
acceleration. The starting speed, final speed, and acceleration should be downloaded from VB. To do this,
you will need to use components developed in previous labs and make major modifications.
A. Start by creating a new RTSS timer by copying all variables and function calls used to create the servo
timer.
B. When you call your new “RtCreateTimer” function, leave the priority of the timer the same as the
servo’s timer (Lab 6 will study priorities).
C. In the call to the new “RtSetTimerRelative” function, set the timer to a 20 ms sample rate.
D. Within the RTSS timer, make the speed change at each time sample using a linear ramp (fit a line to the
first velocity using the acceleration as the slope). Shut off the ramp once the final speed is achieved.
Note: you will have to make a global variable that keeps track of time. It is best to save the global time
value as an integer to prevent round-off error, and then multiply it by the time step when the “actual”
time is needed. Otherwise, the round off error would slowly (to a computer, quickly to humans) drift. In
addition, the “shut off” of the ramp should be done using a flag that is high when the final velocity is
reached or when the user hits a “stop ramp” button (this allows the user to stop the ramp if it is not
working or incredibly slow).