Hey guys, Martin here. I competed in the distant past of 2014 and 2015 VRC seasons. I have been mentoring an FRC team for the past few years, and with that some basic knowledge of motor performance parameters. As I was making spreadsheets of the various motors available to FRC it struck me to look back at VRC and see what the latest V5 motor’s parameters were.
So the V5 smart motor is definitely an excellently designed package. It comes with current capping and speed capping. Big difference from the 393 motor and building a robot around the motor’s PTC days. But for the technical nerds I find it surprising that more of the V5’s performance parameters were not published. By performance parameters I mean free RPM, free current, true stall current and true stall torque.
What we have to work with is this data plot published by VEX:
We observe that due to the current capping, true stall current and stall torque are not observable on this plot. And we know that the V5 motor is speed capped because if 100RPM (with torque cartridge) were the free speed then the torque at that RPM would be zero.
To get an estimate of free RPM, free current, true stall current and true stall torque, we can sample points on the linear uncapped portions of the current and torque curve, and then perform a linear interpolation. The torque linear interpolation y intercept is true stall torque, x intercept is free RPM. The current linear interpolation y intercept is true stall current, and the interpolated line’s y value at free RPM is free current. With these information and the nominal 12.8V of VEX V5 battery, full motor performance curves can be recovered.
To pull some data off of the current and torque curves we overlap the VEX data plot with an excel plot, plot some data points and manually adjust them until they accurately reflect the data.
current estimate
rpm Current (A)
60 2.44
70 2.05
80 1.68
90 1.27
100 0.95
Interpolation result: m = -0.0376 b = 4.686
torque estimate
rpm torque
60 2.01
70 1.69
80 1.35
90 0.99
100 0.71
Interpolation result: m = -0.0330 b = 3.990
Therefore:
True stall current: 4.686A
True stall torque: 3.990Nm
Free RPM = -(3.990/-0.0330) = 120.909RPM
Free current = -0.0376*120.909+4.686 = 0.13982A
Estimate Kv = 120.909/12.8 = 9.44602 RPM/V
Putting all of these metrics through some standard motor curve calculation and we get the following recovered theoretical V5 smart motor performance:
I threw in the efficiency too and plotted it on the current/torque axis.
Observations:
• The 2.5A current limiting is designed right at the peak power. This is so that the peak performance of the motor is not impacted. It saves the motor from burning up without hurting its performance, if the loading is designed properly.
• Speed limit appears to be useful for applications like flywheels. As long as the sustained power demand is lower than the max power output at 100RPM, the speed limit acts like a speed controller. Not that doing speed feedback control is anything difficult in VEX V5, but for starting teams this is a simpler way to just get things working.
This little project is mostly done to satisfy my curiosity. But the performance parameters I estimated here could aid some more tech savvy teams in designing their gearing and mechanism to have the motors sit perfectly at their desired spot on the curve.