Download FIRST Electrical Design

FIRST Electrical Design
Michael Dessingue
College Mentor - Hudson Valley Community College
[email protected]
Team 250
Steve Shade
Controls and Simulation Engineer – Rolls-Royce
[email protected]
Teams 1111 & 7
Al Skierkiewicz
Broadcast Engineer - WTTW-TV
[email protected]
Team 111
Overview
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Electrical kit and IFI Hardware
Layout and Planning
Resistance and Ohm’s Law
Electrical Tools
Myth-Busting
Questions
IFI Hardware
Planning Your Electrical System
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Plan, create drawings just like mechanical
systems
Create a test bed early
Use test bed to test all systems before integrating
Communicate effectively with the mechanical
sub-teams early and often
Document everything
Documentation Example
Team 1111 Robot Controller Outputs
Outputs
PWM1
PWM2
Description
Drive Left
Drive Left
Color
Red
Blue
CB B/C
40-2
C
40-4
C
PWM3
PWM4
PWM5
PWM6
Drive Right
Drive Right
Lift
Lift
Green
Yellow
Orange
Brown
40-6
40-5
40-1
40-3
C
C
B
B
PWM7
Wrist
White
1-20
B
General Layout Tips
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Label and/or Color Code
Everything
Secure wire so a hit from another
robot doesn't stretch the wiring to
a breaking point or pull a terminal
out of a breaker, victor or spike
When in doubt, insulate
Secure the battery so it doesn't fall
out
Leave some slack in wire to allow
for swapping of parts
Be careful when running wiring
through frame members so that
mech heads don't drill into it at
some point down the road
FIRST Electrical Problem
How much voltage is lost in a typical
FIRST circuit?
Ohm’s Law
+
12 V
Battery
By Ohm’s Law:
V=I*R
_
12 V = I * 24 W
I = 0.5 A
24 W
Typical FIRST Circuit
120 A Circuit
Breaker
+
_
40 A Circuit
Breaker
12 V
Battery
Victor 884
Speed
Controller
Vout
Measured
Assuming the Victor 884 Speed Controller is given
an input signal of 254 from the Robot Controller,
how much voltage is output to the device?
Ideal Value
12 V
Actual Value
0V to ~11.63 V
Typical FIRST Circuit
120 A Circuit
Breaker
+
_
40 A Circuit
Breaker
12 V
Battery
Victor 884
Speed
Controller
Vout
Measured
Circuit consists of 8’ of #6, 4’ of #10, and 2’ of #10.
14 Connections in the circuit
Ideal Value
12 V
Actual Value
0V to ~11.63 V
More Wire Adds More Resistance
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“Standard Wire Foot” - A 10 gauge wire will
drop about 0.1 volt per foot at the stall current
of any of the drive motors.
There is resistance in every wire
.001 ohm/ft #10 wire
 .0004 ohm/ft #6 wire
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Typical FIRST Circuit
8’ #6 =
0.0032 W
6’ #10 = 0.0060 W
Rtotal = 0.0092 W
Steady State Current: 40 A
Voltage Drop = I*R = 40 A * 0.0092 W
= 0.368 V
Max Voltage at Device = 12V - 0.368V
= 11.632 V
Resistances:
Typical FIRST Circuit
8’ #6 =
0.0032 W
6’ #10 = 0.0060 W
Rtotal = 0.0092 W
CIM Motor Stall Current: 114 A
Voltage Drop = I*R = 114 A * 0.0092 W
= 1.05 V
Max Voltage at Device = 12V - 1.05V
= 10.95 V
Resistances:
At Stall Current of CIM, Max Voltage at the CIM
motor for the same current path is 10.95V!
Reducing Resistance
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Check every crimp to make sure the wires do
not move or turn when pulled
Use the correct tool for the job
Solder all critical joints
Shorten the length of your wires (also helps in
keeping things neat and traceable)
Crimp Connections
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Buy a good crimper for about $20
Home Depot, Lowes Electrical Sections
 Many Auto Parts Stores also stock crimpers
 Look for crimper with good handles and can used
with wire gauges 10 to 24
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Soldering
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Good Practice to solder all connections where high
currents exist
Use Appropriate Size iron for the job
Use a Rosin Core Solder for all electronics
Wire Size
6
10
16
24
Min Power (W)
60
40
30
20
Max Power (W)
100+
80
60
30
Other Required Tools
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Multimeter (DMM)
Voltage
 Resistance
 Continuity
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Wire Strippers
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Re-strip any wires where copper strands are lost
Myth-Busting
The RC, OI, Victors and Spikes need
external components to run (i.e.
capacitors, voltage regulators, etc.)
IFI has done a good job of designing the power and
internal circuitry of all the kit electronic components.
There is no additional circuitry required for reliable
operation. The fan that is mounted on the speed
controller is required though and most teams will
wire this fan to the controller power input. The fan
then becomes an indication of good input power to
the controller.
The controllers can’t go from
forward to reverse quickly.
The speed controllers do exactly what you tell them
to do. Your robot cannot make the sudden changes
you are demanding for other reasons related to
mechanical design and physics. You cannot hope
that the control system will overcome all other losses.
It does not have the power resources to overcome
the momentum of a charging 130 lb. robot and
change it’s direction.
The controllers and motors are
not matched, the switching is all
wrong.
This may seem to be the case, but the
components work very well together. All teams
use the same motors and drive components so
there is no disadvantage to any team using the
supplied parts.
The OI says my battery voltage is
10.5 but my voltmeter reads 12 at
the battery. It must be broken.
Your RC voltage monitor accurately reads the voltage
that is present at it’s input. If your RC reads 10.5
volts, there is considerable loss in the wiring and
connections. Check that you have connected the RC
to the #1 or #2 positions on the breaker panel and
check that your connectors are well crimped and are
tight and fully engaged on the push on connections.
The battery is too small.
The battery is actually very powerful. Most
teams have no problem driving a 130 pound
robot for more than two matches with the kit
battery. If your robot drains a charged battery
by the end of a match, the mechanical design is
inefficient or you are using some form of tank
drive. (treads or four or more non-steering drive
wheels)
TYPE ES18-12
CAPACITY
5HR 3.06A 15.3 AH
1HR 10.80A 10.8 AH
1C 18.0A 9.0 AH
INTERNAL RESISTANCE
APPROX. 15m
MAX. DISCHARGE CURRENT
230 A (5 SEC.)
MAX. CHARGE CURRENT 5.34
The main battery cannot be used
lying down.
The main battery can be used in any
orientation, including upside down. It can be
charged in any direction except upside down.
Battery terminals must be protected at all times
and the battery must be secured in the robot.
You can’t play when your battery is lying on
the field.
A sparking motor is defective
Sparks are normal in DC brush type motors.
The magnetic fields in a motor generate high
voltages that spark across gaps in the brush
assembly. Motors that are working hard or
have worn brushes produce more sparking.
I can only get 11 volts at my
motor running. The breaker
panel is defective.
This actually is an effect of the some of the principles discussed
earlier. High currents in the wires we use result in some voltage
drop. Measuring at the motor, is in effect, compensating for this
loss. Remember the wire foot, every foot of #10 at stall drops
0.1 volts. A one volt drop is an indication you have 10 wire feet
of loss on the robot between the battery and the motor. This
could be two 10 gauge wires, five feet long, or four feet and a
speed controller or three feet, a speed controller and a breaker
and some connectors.
The backup battery is
disconnected when you power
off.
According to the RC manual, Team LEDs (and the backup
circuit in the RC) will go out after four seconds if the RC has
not established contact with an OI connected to the arena
controller. If an arena controller is connected and a link has
been established, the RC will shut down about four minutes
after main power has been removed. The backup battery
supplies current to the RC, modem, servos and team LEDs
when the main battery has fallen below about 7.2 volts. You
must hit reset to save the backup.
My Chalupa is only running a light load
but it keeps tripping the breaker, the
breaker must be defective.
A current monitor would verify what the motor current
actually is. Many manufacturers make clamp on probes
that will monitor current for use with you multimeter.
If the motor current is high check that there isn’t a
problem in the drive system by running the robot with
the wheels off the ground. If motor current is normal,
suspect bearing side loads, misaligned wheels, etc. If it
is high, remove the motor from the transmission and
try again, if it is high suspect a defective motor, if low,
suspect a problem in the transmission.
I don’t need to insulate the black
wire
The black wire carries the same current as the red wire
it is paired with. By insulating both wires, you are
“backing up the backup”. If the insulation on a wire
fails, the insulation on the other wire keeps the
electrical system safe. (backup the backup is a common
method used by NASA and others to insure safety,
reliability.) The black wire on the motors are not
connected to battery negative all the time.
Main circuit breaker is
vibration sensitive it needs
to be shock mounted.
This was true of the old panel type breaker, but it is
not true if the breaker supplied in your kit this year or
last. These breakers were designed to be used in
vehicles and boats. A few have turned up this season
that were sensitive to light tapping on the red
disconnect button. These are defective breakers and
should be replaced.
Protect the radio by putting it
down inside the robot.
It is important to protect the radio modem and
the rubber antenna that sticks out the top. To
mount it down inside all of the metallic parts,
motors and transmissions, is reducing is ability
to communicate with the OI modem. The
robot modem needs to be mounted in a
protected area with the antenna vertical and as
far from metallic structures as possible.
The antenna on the robot can be
anywhere in any orientation,
same with the OI.
Antenna coupling is greatest when the
antennas are mounted in the same orientation.
Coupling is minimum when the antennas are
mounted 90 degrees apart. The radios still
appear to work but the margin of good signal
is vastly reduced.
The IFI control system is awful,
my robot keeps cutting out.
A robot that cuts out on the field is most often a
result of input power to the RC falling below 7 volts.
A high current draw when running will take the
battery voltage down temporarily. The RC will go to
backup and shut down while the input voltage is low.
When it returns, the RC will act normally.
Occasionally a modem problem may occur on the
field, the IFI reps are monitoring every robot and can
tell most problems from their monitoring station.
#4 wire is way better than #6.
This is partly true. If you are running a long
distance with the primary wiring and you can
stand the extra weight, then 4 gauge may be a
good choice. Mating 4 gauge to the Anderson
connector is a problem for most teams. For
short runs and the best weight savings, 6 gauge
is perfectly fine.
Soldering is better than crimping.
Manufacturers crimp contacts all the time and
the military requires crimping only. The big
difference is the crimp is made with a very
expensive crimp tool or by machine. For our
purposes, a soldered connection adds a little
insurance to the connection. A good soldered
joint is one that is mechanically sound to start
with. Crimp first, then solder, then insulate.
A motor will run at free speed if
you connect it to a battery.
The motor specifications are recorded under
very strict testing guidelines and using
equipment that takes away any variables in
testing. The motor may get you close to tested
specifications but don’t expect to duplicate
results in your shop with a battery.
The electrical rules don’t meet
electrical practice. (NEC)
The electrical rules attempt to follow NEC
guidelines if you check the “open air” tables.
This allows a #12 wire to be used in open air
where a #10 for the same current must be
used in conduit. I personally prefer to use #10
for all high current wiring on the robot, and
#18 for the lower current valves and RC.
The top ten robot
myths of all time!
10. The motors do not have
large enough wire.
The wire supplied with all motors is designed by the
manufacturer for use with these motors. The wires
although undersized, are fairly short and will add very
little loss to the system. Put the controller close to the
motor if you think you need to reduce the loss. If you
shorten the leads and add connectors to get a larger
gauge wire, you won’t gain an advantage. Note: The
Chalupa motor may be damaged if you open it.
9. Wire is wire is wire.
Not really. There are various styles of wire, based
on your need for flexibility. These wires have
distinctly different reactions to connectors.
Superflex wire(137 strands) in an SLU70 type
connector will pull out making a high resistance
connection and it is difficult to strip without cutting
or breaking strands. A low strand (9 or 17) wire
may be too hard to terminate with a manual
crimper. It may also break with the repeated flexing
encountered on our robots. The wire sizing rules
this year allow you to make decisions on weight but
in most case will be a disadvantage electrically. The
drawing in the guide is misleading.
This is misleading. The wire does not go under
the screw. It goes between the collar and the tab.
Fine strands would be pulled up along the sides of
the screw and very few would be held in place.
The connector supplied last year was terminated
as shown but could not terminate fine strand wire.
8. The battery could electrocute
you, why do we use it?
The voltage generated by this battery is not
capable hurting anyone due to direct contact.
The battery can still cause harm if misused.
Shorts across the terminals can generate high
amounts of heat capable of burning anyone
who touches it. I have seen fires on the field
and in the pit due to shorting out the battery.
Keep battery terminals insulated at all times.
7. Battery connectors are too
small, underrated.
Although rated at 50 amps, the overall heat generated
in the connector during a two minute match is low
enough that teams do not need to worry. If the
connector is improperly crimped, damaged or
misaligned, or the robot design is significantly
inefficient, some heating of the connector is possible
and damage could be the result. I have seen little
evidence of connector damage. Using alligator clips
on the charger to connect to the Anderson battery
connector will damage the surface.
6. The main battery cannot be
used lying down.
The main battery can be used in any orientation,
including upside down. It can be charged in any
direction except upside down. Battery terminals
must be protected at all times and the battery
must be secured in the robot. You can’t play
when your battery is lying on the field. Also
don’t pick up the battery by the wires, as internal
damage will result.
5. My four wheel drive robot eats
batteries, there is something
wrong with control system.
Four wheel or tank drive systems use incredible
amounts of current when turning using high friction
drive surfaces like belting or knobby tires. When a
robot turns it must drag the wheels or treads sideways
across the carpet. In a tight or fast turn, this high
friction translates into near stall conditions for all the
drive motors. The result is temporary current draw
above 200 amps in a four motor drive. That may be
enough to draw the voltage in the battery below RC
minimum.
4. The backup battery is not used
when the main power is shut off.
According to the RC manual, Team LEDs (and the
backup circuit in the RC) will shut down after four
seconds if the RC has not established contact with an
OI connected to the arena controller. If an arena
controller is connected and a link has been
established, the backup battery will still be connected
to everything on the robot for up to four minutes
after main power has been removed. With servos and
LEDs, a lot of power is watsted. Press reset after
every power off.
3. The battery has memory and
needs to be discharged to zero.
This is one of the most common myths. It
arises from a particular type of NiCad battery
behavior. Gel cell batteries do not have a
memory that needs any special handling. Charge
them normally with the supplied chargers and
never at more than 6 amps.
2. The battery is too small.
The battery is actually very powerful. Most teams
have no problem driving a 130 pound robot for more
than two matches with the kit battery. If your robot
drains a charged battery by the end of a match, the
mechanical design is inefficient or you are using some
form of tank drive. (treads or four or more nonsteering drive wheels). In a hard match or a restart,
you may not have enough battery reserve to last two
minutes.
1. You don’t need to calibrate
speed controllers.
The biggest myth of all. Do not believe this
one. The speed controller has the ability to
adapt to each joystick you use. Since each
speed controller is not matched to the joystick
you were shipped, they must be calibrated.
Calibration gives the controller the ability to
match the maximum travel on the joystick to
the maximum output on the controller.