NATCar Hints and Tricks

1
NATCar Hints and Tricks

• Steve VanDeBogart
• Gabe Nell
• Jay Smith

2
Physical Construction

• Sturdy
• Two foot drop test
• Bumper to protect front wheels
• Ruler on the bumper
• Metal (bolts) conduct!
• Nuts and bolts will loosen
• Lock-Tight ?
• Make it easy to fix things
• Use plugs to connect wires
• Make sure they only go on one way
• Avoid a bendy sensor platform

3
Motor Driver

• Use big wires for this circuit
• A full H-Bridge is not necessary
• We used one transistor with regenerative braking
• Drive the transistor all the way
• We used a Max 621 to accomplish this
• (Internal charge pump 5V -gt 16V )
• Pulldown resistors
• Cutoff switch

4
Power Supply

• We used the Linear Technologies 1370 DC-DC
  converter
• 500 kHz, 6A switching regulator
• For low noise performance
• Use a PCB with a ground plane
• Follow layout guidelines
• Used low drop out regulators for each component
  (CPU, Sensors, etc)
• Leave room for noise absorbing capacitors

5
Speed Sensors

• We mounted a rubber disk on the rear axle
• Every 1/8th of the disk was removed
• An optical sensor triggered an interrupt on each
  transition
• Use time since last interrupt, desired velocity,
  and PI control with a leaky bucket Integrator

6
Position Sensors

• We used four magnetic sensors
• Left, Right, Center (Horizontal Orientation)
• Wide (Vertical Orientation)
• Centimeter accurate detection out to 20cm in each
  direction
• Gain Bandwidth product is important
• PCB construction may reduce noise

7
Sensor Readings
8
Perceived Location
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Sensor Tips

• Make it easy to generate the previous two graphs
• The ruler on the front bumper helps
• Have enough sensors so that you can
  un-ambiguously know where you are relative to the
  track
• Keep in mind how far from the track is legal
• If you have enough CPU, convert your readings to
  common units (cm, in, etc)
• Generate graphs often
• We had several sensors (coils) go 'bad'
• Had to repeat work

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More Sensor Tips

• Don't hardcode specific sensor values
• Do calibration phase
• Determine needed values during calibration
• Sensor Height can be an important variable
• Don't neglect to play with it.

11
Steering Control

• PD Steering control
• Proportional gain 5.14 radians/meter
• Derivative gain 0.514 radians/meter
• Make sure you have the right sign on your
  derivative gain
• New_PWM Center_PWM Max_PWM_Offset Pgain
  error Dgain (error-old_error)
• old_error is a circular buffer with a 10ms delay

12
Software

• Soft real time application
• Set servo value every 10ms (cheat from 20ms)
• Read sensors (takes time)
• Calculate new steering angle (may be complicated)
• Be aware of when you need to change a value for
  it to take effect
• Speed Control
• We did asynchronous
• Synchronous can work
• Want fine granularity on servo control
• What to do on reset?
• Development -- Stop dead
• Production -- Maybe go as best you can

13
User Interface

• You're the user, make it easy for yourself
• Label your LEDs, so you know what they mean when
  you're under time pressure
• Label your switches
• Have hardware controls for the settings you want
  to tweak
• There's not time to reload or recompile software
  when it's your turn at the competition.

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• The End