pwm not changing duty cycle

Using Arduino Motors, Mechanics, Power and CNC
#1

I'm using a skid steer design for my two motor robot with wheels. I got some new motors , but changing the pwm does not seem to change the shaft speed of the motors; therefore the robot does not turn. Actually I should say that it turns very slowly. Almost so slowly that you can't tell it's turning. I'm using a mosfet for each wheel, and the motors have a separate 5v power supply (rechargeable aa batteries).

int motorPinL = 5;
int motorPinR = 6;

void setup() {
  pinMode(motorPinL, OUTPUT);
  pinMode(motorPinR, OUTPUT);
}

void loop() {
  analogWrite (motorPinR, 240);
  analogWrite (motorPinL, 200);
}

The only way to make it turn is to do this:

  analogWrite (motorPinR, 240);
  analogWrite (motorPinL, 1);

Any ideas?

#2

You are probably using a decay mode, which coasts rather than braking. Which H-bridges and how are
they wired?

#3

I'm not using H bridges. Just using a mosfet for each motor so it goes forward only. Also, using pwm to change direction worked fine with the old motors. The new motors are slightly different.

#4

There's your problem. You have no braking at all.

I suspect your new motors are much better (less friction), thus they work worse for this!

#5

yes, that makes sense for a lot of things now! Is there a work around from a coding standpoint without the H-bridges? Unfortunately I already had a few PCB's made.

#6

The MOSFET can make the motor turn but cannot prevent external forces from turning it, it's either being driven or free wheeling.

#7

TeslaIaint:
yes, that makes sense for a lot of things now! Is there a work around from a coding standpoint without the H-bridges? Unfortunately I already had a few PCB's made.

No, you have no braking in the circuit. You need at least 1/2 H-bridge circuit for active braking.

#8