# Torque and PWM

I am using two SMCs on a Wild Thumper 6WD for a NASA Centennial Challenge. Here is a video

I just submitted to the challenge.

I need more torque at lower speeds. Will lowering the PWM frequency increase the torque? I can talk myself into both yes and no as the answer.

Hello.

Thanks for sharing your project; it looks cool! It is possible that lowering the PWM frequency could give you a slight increase in torque, but I think the effect will probably be insignificant. It should be easy for you to test that out for yourself, though.

The best way to get more torque at lower speed is to use motors with higher gear ratios (assuming you are willing to have a lower overall top speed). Does your Wild Thumper have 34:1 motors or 75:1 motors?

-Grant

I have the 34:1 motors.

Would increasing the voltage to 11.1 or 14.8 increase the torque? I am thinking that this will put more current in each pulse. I would have to limit the speed to a top limit 50% to minimize risk to the motors.

I would think there is a rise time for the voltage and current in the motors since they are big inductors. You will not get maximum torque until the full current is flowing.

I am asking in hopes of getting a clear answer before taking the time to do the experiment. There is a lot to do before June. I suspect there is not a clear answer or solution.

[quote]Would increasing the voltage to 11.1 or 14.8 increase the torque? I am thinking that this will put more current in each pulse. I would have to limit the speed to a top limit 50% to minimize risk to the motors.

I would think there is a rise time for the voltage and current in the motors since they are big inductors. You will not get maximum torque until the full current is flowing.[/quote]
I think this assessment isn’t really applicable for DC motors (at least not in the way it is for something like a stepper motor). Ultimately, the inertia of the rotor and inductance of the motor coil acts as a low-pass filter that leads to a relatively constant steady-state current through the motor. Doubling the supply voltage while restricting the duty cycle to 50% will essentially be functionally equivalent to using the original supply with no duty cycle restriction.

If you want more torque and speed, you need more power, which means you either need to use a higher-power motor or supply your motors with a higher voltage and risk wearing them out more quickly. If you just want to trade speed for torque, you can replace your motors with ones that have higher gear ratios.

Note that one important way to help ensure you have higher torque at low duty cycles is to make sure your driver is operating in drive-brake mode (slow decay) instead of drive-coast mode (fast decay). The Simple Motor Controller only offers drive-brake option (which is what you want).

I expect any effect of increased supply voltage (with corresponding decrease of maximum duty cycle) or PWM frequency change will be very subtle, which makes it hard to give you a clear answer. Note that the main reason for using a high PWM frequency is to eliminate audible noise in the motors. If this is not a concern for you, decreasing the frequency will at the very least lead to decreased switching-related power losses. However, at the point where the frequency gets too low for the motor to filter it into a smooth output, a torque ripple will show up on the output (e.g. imagine what would happen if you used a 50% duty cycle with a 1 Hz frequency).

-Grant

It’s hard to see from those videos, can you provide some close up pictures of what you’ve done? What does all that stuff do?

Also, how can I make a fork lift like that on the front of my thumper? Does Pololu sell the parts to make that? I can’t find anything in the catalog.

Hello.

Dagu makes a robot arm that could be used with the wild thumper. It also looks like the lift on the front could also be constructed using a tilt module for the movement. Something like this tilt module from ServoCity might work for you application.

- Grant

It doesn’t look like I can order that arm, because they aren’t located in the United States. I’ll try to find one some place else.

Do you know of any way to buy thumper parts? I would like to buy another top base for my thumper so I can have it stacked 2 high, and give me more room for other things I want to do. I can’t afford to buy an entirely new thumper just for that top piece.

I looked around but I haven’t found anyone selling just that piece.

Thanks

The only parts of the Wild Thumper that we carry separately are the wheels and motors; we do not carry just the metal plates. I do not know where you might find another one of those for the wild thumper, but you might try contacting Dagu directly. Alternatively, you could use our custom laser cutting service to make an expansion plate. You might also be able to use one of our RP5 expansion plates.

- Grant

I will try one of those plates. I sent an email yesterday to you about having a custom plow made for the front of my thumper, but I haven’t heard back from anyone yet.

Thanks,

I just followed up with the laser-cutting department, and they said they have already been in touch with you about your project.

- Grant

I have given some thought to speed control rather than voltage control. If you could measure your wheel speed you could use a PID to read wheel speed and control PWM output level to maintain speed. I think this is what is done on your car antilock brake system. How could you measure speed? A photocell watching marks on a tire or gear. My bicycle speedometer use a magnet on the wheel, a rotary encoder.

Note that heat and power are proportional to the square of the current (and thus also to the square of the voltage, assuming you have an Ohmic load.)
So, with a 50% PWM duty cycle, you may only get 1/4 the power.

When you are saying “torque at low speeds,” what does that mean? Does it mean that, when rolling at low linear speed above ground, and giving 100% PWM duty cycle, you do not have enough torque? If not, then why don’t you just increase the PWM duty cycle when you need more torque?