Third-Scale Vehicle

Drive-by-Wire Retrofit

Force measurements

The following measurements were taken to determine design specifications for the motors used for steer and brake by-wire. These were performed with the incredibly sophisticated equipment of a luggage scale.

  • 7kg measured to turn the steering wheel. Measured perpendicular to the radius of the steering wheel at a distance of 3in from the center. This results in (7kg)(9.8m/s/s)(0.08m) = (68.6N)(0.08m) = 5.5 Nm of torque.
  • 35kg measured to rotate (apply) the brake. Measured at the small metal plate at a radius of 1.5in. This results in (35kg)(9.8m/s/s)(0.038m) = 13 Nm of torque.
  • 14kg measured to rotate (apply) the brake. Measured on the metal tubing extending from the brake pedal, at a radius of 4in. This results in (14kg)(9.8m/s/s)(0.1m) = 13.7 Nm of torque. Since the last two measurements corroborate each other, the torque specification for the motor for brake-by-wire is set to 14 Nm.

Motor Selection

Motor Mounts

Power and Electrical

Electronics Voltage Needs

Power Requirements

  • The stock motor pulls 16A at 24V, leading to 16*24= 384W
  • The Docyne motor (throttle) is 5A at 18V –> 5*18 = 90W
  • The Docyne motor (brake) is also 90W (although used much less frequently)
  • The On Board Computer (OBC) uses 65W
  • The NVIDIA Orin + ZED cameras use roughly 100W (?check this empirically?)

These power needs sum to 729W (Watts). This informs the capacity of the battery we select (in Amp-Hours, Ah) based on the length of time we wish to operate the vehicle.

Battery Selection

There is a benefit to selecting a higher voltage battery largely because it enables faster speeds, by driving the stock drive motor at a higher voltage. There are also DC-DC step down converters that can be used to provide lower voltages for the electronics onboard. Our battery selection considers the following criteria:

  • Voltage greater than 24V
  • Hobbyists have demonstrated that the stock motor can handle at least up to 75V and maybe 92.5V. So we aim for a voltage less than 75V.
  • The vehicle should be able to run continuously for 2 hours.

We found cost effective batteries typically used for golf carts at 48V. Li Time sells a 48V 30Ah LiFePO Battery that has bluetooth connectivity for diagnostics. The nominal voltage is actually 51.2V, so a step-down converter may be needed for the OBC.

Given the 48V, assuming no losses in the DC-DC conversion, the current draw would be 729W/48V = 15.2A (amps). Rounding this up to 16A to include losses, a 16Ah = 16000mAh battery would last 1 hour of operation and a 32Ah battery would last 2 hours. Thus the 30Ah battery should last roughly 2 hours.

Sensor Mounts