MIT Junior Cheetah

Overview:

The goal of this project is to build a robotic quadruped similar to the MIT Cheetah, but on a much smaller scale and at significantly lower cost.  The robot’s actuators will be  inexpensive brushed DC motors with 18.75:1 gearboxes.  The motors are driven with hobby-grade H-bridge ICs, and controlled by an mbed microcontroller.

Progress:

The following components are being used:




Component

Vendor

Price

mbed – LPC1768

Sparkfun

$49.95

19:1 Metal Gearmotor 37Dx52L mm with 64 CPR Encoder

Pololu

$39.95

MC33926 Motor Driver Carrier

Pololu

17.95

VNH5019 Motor Driver Carrier (for higher current version)

Pololu

24.95

The first step was to develop a PD controller for the motor, using an mbed microcontroller for processing, and small H-bridge IC to drive the motor, and a quadrature encoder for position feedback.  The position control loop samples the encoder at 2.5 kHz.  Inside the PD position controller runs a 11 kHz PI current control loop, to command the appropriate voltage to the motor.  The code the mbed runs can be found here.

Because the motors used are hobby-grade, the provided specification sheet gives relatively few details.  For the purpose of simulating the robot, some of the missing specifications were experimentally determined.  A Kt of .16 N*m was measured by stalling the motor against a torque sensor for a series of 50 ms pulses, at a number of different current levels.  Viscous damping of the gearbox was measured to be b = .000598 N*m*s/rad   by running the motor at constant speeds, and measuring both the angular velocity and current draw.  The current draw was then converted into a torque by use of the previously found Kt.  Finally, the inertia of the motor and gearbox system was calculated by recording the angular velocity of the motor at a constant speed, and then opening the motor circuit.  The time constant of the angular velocity was used with damping coefficient b to calculate an inertia J = 0.000341 kg*m^2.  Data collected can be found here.

A simple single-motor leg was designed and 3D printed to test the jumping ability of the motors.  For simplicity and ease of construction, a four bar linkage was designed to constrain the foot to motion in an approximately straight line normal to the ground.

left: linkage mechanism and motion path.  right: leg built using the linkage

At or near stall conditions, the motor saturates the current limit of the motor driver circuit used.  A driver with a much higher peak current limit (30 A vs 5 A) will be used on the next version of the leg.

 

Second version, with a higher current motor driver and modified geometry: