I am testing the dual-motor parallel connection in V5,which are coupled using gears. After a period of rotation, the two motors will compete with each other, consuming more and more power.
What is the situation? How to correctly parallel the motors?
I suspect that both motors are speed closed-loop controlled, but due to small errors in the encoder, although they are coupled together by gears, the measured rotational speeds are not exactly the same. After a period of accumulation, the errors become larger and larger, and the slow one wants to speed up, while the fast one wants to slow down. The two motors compete with each other, and eventually the power is completely lost.
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I use motor Group to program the 2 motors and one of them is reversed.
I’m wondering if it’s possible to set voltage of the motor group instead of a speed, which means perform an open-loop control.
Your description suggests to me that you might be accidentally using motors with different ratio gearing cartridges installed. You’ll see a red, green or blue colour in the arc-shaped window next to the socket for the axle on each motor. For example, the motor on the product page has a green cartridge installed. V5 Smart Motor & Gear Cartridges - VEX Robotics Click through the thumbnails underneath to see the details of the three different cartridge colours and speeds. If your code doesn’t correctly account for the cartridge ratio of each motor - as would be the case if you though they were the same, but they’re not - then they will fight to go at their own speed as you describe. There’s discussion to be had around what kind of stop critierion you set in each motor if you’re getting a problem despite getting the cartridges right, but the healthiest option depends on what the duty and rest cycles look like in the application.
To spin a motorgroup with voltage use motor.spin(fwd, voltage amount, voltage units). An example is motor.spin(fwd,12,volt); this will spin the motor at maximum voltage.
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I’m agreeing here somewhat.
We’ve used parallel motors for a LONG time w/o these problems. This leads me to think the problem’s in the user code and not the vex internal code/sensors/etc.
I’m sure the cartridges gear ratio are same and the two motors rotate at almost the same speed when drived independently.
That’s it. The motorgroup works well now.
You didn’t mention actually checking the colour, so in geek mode, I have to push for that detail 
If the cartridges are the same colour, then the ratios are the same. If they’re different and you drive with voltage, the two motors will not be fighting only because you’re telling them to put in a certain amount of effort regardless of rotation speed. In that situation you would OK, I guess, but it’s a shame not to be absolutely certain of what you’re using. Out here in the forum, we don’t know what’s going on in your head - you might have checked the colour, but haven’t said so. Omitted details can bite pretty hard further on in a project when you need to change your approach.
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