My Vex Controller/motor torque tests

Here are a few measurements I made in the kitchen to get an idea of how much the controller dead zone actually is and the static torque loss due to the installation on my bbot referenced in See attached file for plots and pictures.

Note: Measurement made with Fluke 77 meter. Voltage measured directly across motor output terminals on the controller board. Motor had no load.
Motor appears to have a total dead zone of 6 pwm cnts and center at 133 rather than the expected 127

Pendulum Robot Motor Controller Measurements
Motor Motor
pwm voltage
0 7.26
10 7.26
20 7.26
35 7.26
40 7.23
50 6.71
60 6.46
70 6.13
80 5.63
90 4.95
100 3.9
110 2.3
120 0.23
125 0.04 fluke noisy here …small pulses
128 0.002 fluke noisy here
130 0
136 0
137 0.015 fluke noisy here
140 0.055 fluke noisy here
150 1.5
160 3.35
170 4.6
180 5.4
190 5.9
200 6.37
210 6.67
220 7.05
230 7.35
240 7.35
255 7.35

Static Torque Measurements made in the kitchen

axle backlash total dead zone = 13.8 deg

measured static torque
no weight on wheels…left in_lb (%max) =2.6(44%)…right in_lb (%max) =1.7(28%)
weight on wheels…=3.4(56%)…=2.1(34%)
stall…right in_lb =7.4
stall less static (no weight)…right in_lb =5.7
spec value …in_lb =6.0

Note: Motors were installed on robot. A yard stick was clamped to
wheels and a known weight moved along yard stick until motor moved.
See jpg for setup.

There is about 1 in_lb difference in the left and right motor installations.
Static friction torque ranges from 28% to 56% of spec torque.
Spec motor stall torque is 6 in_lb… measured is 5.7 so very close.
Torque Measurement Weight Applied (Small).JPG
Controller voltage plot vs pwm.JPG

You just randomly selected one of your Motors for this test?? I would guess that you would find variance of Center on both sides of 127. I find that even when my Transmitter is “trimmed” to 127, some Motors are still getting that “little whine” indicating that there is some sort of tendency of movement.

sure, I just want to get some data points up. Take some data and post it here and we can get a good sampling. Im still not sure how they measure the spec torque. Dynamic and static measurements would be quite different.