VEX V5 motors are much more powerful than previous VEX motors, but even they are not infinitely powerful. They are, however, able to give programmers a lot of information on what is going on in the motor. We wanted to test the hypothesis that adding rubber bands to an arm can significantly reduce the work done by a motor, and both reduce power consumption and increase motor life. We also wanted to demonstrate one way to use V5 motor properties to measure the performance your robot mechanisms. Here is a picture of our test robot, with two #64 rubber bands installed on a simple 4-bar link arm on our V5 Clawbot: 


 

There is only one motor on the arm lift.

We used the "watts" and "arm rotation in degrees" properties of the V5 motor and wrote this simple program in Python: 


import vex

#region config
brain = vex.Brain();
motor_arm = vex.Motor(vex.Ports.PORT5, vex.GearSetting.RATIO18_1, False)
#endregion config

# main thread
motor_arm.set_velocity(75, vex.VelocityUnits.RPM)
while not motor_arm.rotation(vex.RotationUnits.DEG) > 670:
    motor_arm.spin(vex.DirectionType.FWD)
    print (''.join([str(x) for x in [motor_arm.rotation(vex.RotationUnits.DEG), ';', motor_arm.power(vex.PowerUnits.WATT), ';']]))
motor_arm.stop(vex.BrakeType.HOLD)


The most important part is that print command, which writes information to the Program Status Area (or "console") in Robot Mesh Studio. We then used the RM Studio "save as a file" option to write this data to a text file, which we then imported to Excel, where we graphed it.


We ran four tests:


Arm with no weight on the claw, and no rubber bands

Arm with no weight on the claw, and 2 rubber bands

Arm with a 482g (about one pound) weight on the claw, and no rubber bands

with a 482g (about one pound) weight on the claw, and 2 rubber bands


The results were interesting, and proved that rubber bands are a cheap effective way to help motors in lifting.

In the first test with no weight and 2 rubber bands, the motor drew an average of 1.5 watts. We plotted watts used vs. the current angle of the arm like this:

 


With the arm with no additional weight, peak power used was about 3.7 watts (note that this is near the maximum power of the older Model 393 motors). Average power used was 1.5 watts.



With no additional weight, but with two rubber bands, the maximum power usage was around 2.2 watts, and average power used was only .83 watts. Power usage decreased 45% with the addition of the rubber bands.



Using our test weight, we put some real strain on the V5 motor, drawing a maximum of 8 watts, and an average of 3.83 watts. The older 393 motor would not have been able to lift this. 



Again using our test weight with the addition of two rubber bands, our test motor maxed out at 6.2 watts, with an average consumption of 2.71 watts. This is a 29% reduction in power used, even with a signficantly heavy weight.


Conclusion

Unsurprisingly, rubber bands make lifts work better, even with bigger motors. Also, the V5 motors give you a lot more data on what is going on, and offer interesting opportunities to measure real performance.