Robodrive pcb

RoboDrive PCB

Dual brushed motor drive with wireless (BLE/Wifi) connectivity. 12V input, 2x 2Amp motor outputs, 2x servo PWM signals, and LED output

System Diagram

Basic

The basic robodrive setup has a 12V battery and 2 brushed motors. This is enough to build a differential-drive robot setup.

Brushless Motor and Servo

Combat robots often use a brushless motor to power a spinning weapon. Many brushless electronics speed controllers (ESC), have a battery elimination circuit (BEC). This BEC often produces a high-current 5V output that can be used to power a servo. The robodrive ties the 5V pin of the servo connectors together allowing the ESC to pass through 5V power to the servo.

Servo and Voltage regulator

While the robodrive does have an internal 5V regulator, it cannot provide even close enough current to power a servo. One easy way to provide 5V power to servo is to use a 5V regulator. Connector the regulators input to the battery and its output to the unused servo connector on the robodrive. The robodrive ties the 5V pin of the servo connectors together allowing the 5V power pass between regulator the servo.

Internal Servo Connections

The servo 5V pins are tied together. Internally the robodrive uses separate 5V regulator to power the microcontroller.

Parts List

Filter Capacitors

Used to optionally low-pass filter and battery voltage signals, may not be needed.

Female Socket

Optional allows motor and power input wires to be connected to right angle headers more easily.

Voltage Regulator Heating

30mA * (12.6V - 5V) = 0.228W

• Junction to Ambient = 207C/W = +47C

LED

LED output is used for indicating device power and mode of operation A high candela output LED should be used so only a few mA of current produces enough brightness

• XL-302SURD : 600mcd @ 20mA
• C503B-RAN-CZ0C0AA1 : 15000mcd @ 20mA 15degrees, 2.1Vf
  • (3.3V - 2.1V) / 1kOhm = 1.2mA --> 900 mcd
• C503B-RCN-CW0Z0AA1 : 5100mcd @ 20mA 30degrees, 2.1Vf
  • (3.3V - 2.1V) / 1kOhm = 1.2mA --> 306 mcd

Current ILIM

Motor (PZ22GR9120R-084BBH) max stall current 1.3Amp. This is probably continuous stall current, so peak is probably larger. Pick a current trip limit of 1.5Amp

Itrip = 1.5 Amp : RLim (kOhm) = 64 / ITrip = 42kOhm Itrip = 1.64 Amp : RLip (kOhm) = 64 / ITrip = 39kOhm Itrip = 2.0 Amp : RLim (kOhm) = 64 / ITrip = 36kOhm

Firmware

TODO : Link to Firmware

Revisions

• Rev A :
  • Used DRV8424PPWPR dual motor drive
    • Part was difficult to hand solder
    • Required a number of extra passives
  • Used single 6-POS R/A heaeder
    • Female connectors did not fit properly
  • Part libraries
    • Used some proprietary libraries
• Rev B :
  • 22 x 44mm
  • Uses 2x DRV8871DDAR single motor drivers
    • 1.27mm pitch should be easier to solder
    • Requires fewer passives
  • Power input move to "front"
    • Easier to wire to typical brushless ESC
  • LED output added
    • Makesit
  • Spacing for R/A header
    • Female connectors requires extra spacing to fit
  • Part Libraries
    • All open source / custom