Motor driver boards

The motor driver boards

The steering motor and brake actuator are both driven by a separate motor driver board.

The boards can supply up to 10A at 36V (or 20A if the fuse is replaced with a larger one) and are controlled via two PWM signals A and B. These signal can be switched from 10 to 500 KHz though around 100 KHz is recommended. The heatsinks on these devices were chosen assuming use that rarely approaches 10A and should be replaced if the application changes.

The motor driver board plugs into the main board and sits on top of it. Note that the board plugs in so that it covers the sam7 on the main board as is shown by the outline on the main board. Plugging the boards together the wrong way around will burn out the motor driver chip.

The motor driver boards contain a DRV8402 dual motor driver chip. This chip can supply up to 24A of current to the motors and has internal protection from short circuits and overheating. The circuit was originally designed to use the DRV8432, an almost identical chip, but due to problems with supply the DRV8402 was used instead.

The same motor driver was used for both boards despite their differing requirements to cut down on the additional time and cost of creating and troubleshooting a second board. Several commercial motor driver boards were looked at for driving the motor. Problems were found with all of them however as most that could supply the required peak current of ten amps at 36V were large and significantly more costly than making our own. To save money and allow the motor boards to fit the same enclosure as the others we created our own motor driver.

##Heat sink sizing

###Motor heatsink

The motor driver chips we intended to use (DRV8432) are 97% efficient however the due to supply issues we are using the DRV8402 that is only 95% efficient. The motor at peak can draw 10 Amps however its continuous current is 2.85 Amps

Heat to dissipate = 0.05 * 2.85 Amps * 24 Volts = 3.42 Watts

The chip can run at up to 100 degrees before overheating thus assuming 20 degrees air temp maximum thermal resistance = 29.2 C/W Total

Thermal paste thermal resistance < 0.01 C/W DRV8432 Chip heat slug to pad thermal resistance 1.1 C/W

Maximum heat sink thermal resistance =28.1 C/W

Using an old Pentium 3 heatsink that has been cut down to size 15mm by 35 mm by 50 mm we calculated the thermal resistance using this method of estimation.

Assuming airflow of 0.1 m/s thermal resistance of heatsink = 24.4 C/W

The boards with motor drivers will require ventilation to provide some air flow as a sealed container's thermal resistance will far exceed the thermal resistance left

####Linear actuator heatsink

The linear actuator can draw up to 1.5 Amps with a max duty cycle of 25% without overheating.

Heat to dissipate = 0.05 * 1.5 Amps * 24 Volts * 25% = 0.45 Watts

For the same reasons as above

Maximum heat sink thermal resistance = 175.1 C/W

The heatsink used was 70% of the size of the one used on the motor and thus exceed the specification. The heat will therefore not be an issue.

Troubleshooting

All the components of the motor board were chosen to handle up to 20A and 36V. This is with the exception of the 10A fuse on the boards. It is white and located next to the big capacitor.

The motor driver outputs two status signals OTW and FAULT

FAULT   OTW    DESCRIPTION
  0      0     Overtemperature shut down or overcurrent shut down or undervoltage protection
  0      1     Overcurrent shut-down or GVDD undervoltage protection occurred
  1      0     Overtemperature warning
  1      1     Device under normal operation

The motor driver does not behave correctly when switched slowly (less then around 10KHz) and will break the output PWM up with a large number of spikes.

The motor driver chip requires 12V to switch the gates but uses 3.3V for all its logic. This means that there are several pins that can only tolerate 3.3V next to pins that require 12V so a short here may cause the motor driver chip to burn out.

If the motor driver chip has burnt out there will probably be an internal short between vreg (pin 10) and ground (pin 8).