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Motor input voltage drop

Hi everyone. We are designing a three-wheeled omnidirectional robot which should transport the weights about 10 kg. We are using pololu 18.75:1 gearbox with 64 CPR encoder. We supply 12V power to these motors with STMicrolelectronics L6206 drivers. When i connect 12V power supply directly to the motors they exhibit a good performance but if i use drivers in between motor and power supply and apply 100% PWM duty cycle, i see about 7-8V at the input of the motor(which is also output of the driver). So this causes the robot not to be able to carry the weights we want to bear. I’m new to the motor world so if i’m doing something wrong or if these motors actuallty can not handle this weights please let me know and upgrade my knowledge or motor.
Thank you.

Hello.

I am concerned about your choice of motors and motor drivers for this application. You have selected the versions with the lowest gear ratio, which will probably struggle to generate enough torque to move a robot that heavy, especially in a three-wheeled omnidirectional setup. Our 37D gearmotors with higher gear ratios are probably more appropriate for what you are trying to do.

Your motor drivers seem underpowered for these motors and for an application like this in general (they are only rated for a maximum of 2.8 A per channel). Are you somehow paralleling the outputs to use two channels per motor, and if so, are you confident you are doing that right? How did you choose this particular driver?

As far as why you are seeing such a big voltage drop between your supply and the motor driver outputs, I suggest you use an oscilloscope to look at what is going on both at your supply and at the driver outputs, with and without the motors connected. Can you post some screenshots? Also, what is your power supply.

By the way, please note that a reasonable upper limit for the radial load on these gearmotor output shafts is approximately 1 to 1.5 kgf, so if you have all the weight of your robot supported by the shafts, that is not going to work out well. For an application like this, you should be using some kind of linkage between the motor and the wheel, such as belts or external gears, so that the weight of the robot can be supported by the chassis itself.

-Patrick

Hi Mr @PatrickM, thanks for your kind suggestions and enlightenment. After your reply I started to measure voltage step by step starting from power supply output to input of the motors. What I observe was that voltage drops gradually when I proceed towards motor input. So we decided to measure the resistance of our cables used from power supply to motor drivers. This path was long and thin enough to have high resistance. We measured the total resistance of the cable and there it is! It was about 10 ohms!
This means for every 100mA current we have 1V voltage drop. We reconstucted our cabling with proper cables and it started to work. We have about 11.6 V at inputs of the motors.
For your paralelling question, yes we are parallelling the outputs of 2 H-bridges in these drivers. They contain 2 bridges per chip so we can draw 5.6A from a single chip.
For now, it seems it can carry our load but we will do more load tests with these motors and if we observe the weaknesses you indicated we will replace our motors with the higher gear ratios.
Again thank you very much for your reply.
Regards
Nazım