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Dagu Wild Thumper - motor conversion?

I got a Dagu Wild Thumper with 34:1 gearboxes. It has been a great learning opportunity,

But I have been thinking, I would like more gear reduction . . .

Has anyone ever swapped out the Wild Thumper motors with 99:1 or even 172:1 gearmotors from Pololu? What are the possible issues?

Thanks.

Hello.

In case you are not already aware, the Wild Thumper is available with 75:1 gearmotors if you want a robot with more torque without needing to replace the gearmotors.

However, it is possible to replace the motors on the Wild Thumper since you already have one. The only difficulty I can predict you might run into with assembly is that the length of our 25D gearmotors increases as the gear ratio increases. You can see the dimensions in our 25D gearmotor dimension diagram. There should be no problem with the 99:1 gearmotors since those are the same length as the 75:1 gearmotors. However, 172:1 gearmotor is a 2mm longer, so it is possible the motor bodies might stick out 2mm from the mounts.

Also, please keep in mind that using higher reduction gearboxes than the 75:1 may make it easier for your gearmotors to generate enough torque to damage themselves. For all of the 25D gearmotors, we recommend keeping applied loads well bellow 15 kg-cm (200 oz-in) to avoid damaging the gearboxes.

- Patrick

Hi, thanks very much! Just ordered some 99s. Can re-use the 34s in another project.

Is there a way to detect the torque that a motor shaft is experiencing, perhaps indirectly?

For example can I read the current draw and motor speed and say, hey, if the speed is 0 and current is 2 amps for a whole second duration of time, maybe I need to cut power or at least put the input signal back to neutral/0?

Thanks again.

You can estimate the torque the motor is exerting by measuring the current draw, which is linearly related to torque. (Although, keep in mind this relationship will gradually change over time as the motor is used.) You might consider using one of our current sensors for that.

- Patrick

Thanks Patrick, I am learning a lot! I do have some current sensors I can experiment with.

I am actually using, perhaps I forgot to say, the G2 Pololu motor controller. I noticed in the setup program on Windows, when I hook the USB cable to controller, that the software shows the instantaneous current draw on the motor. Am I interpreting this correctly and would I be able to pull that information out of the controller itself when the robot is not connected to Windows, for example possibly through the serial interface ?

I already have the current limiter set inside of that setup program, would that limit by itself be enough to prevent damage from overtorque??? so maybe it is not necessary to dynamically limit it while running?

The robot’s brain is a Raspberry Pi and I have been using the RC pulse width control, but I have been very interested by the possibility of using the serial interface.

Thanks

We have multiple products with G2 in the name, but it sounds like you are probably talking about one of our Simple Motor Controllers (SMCs) G2. You can get the board’s current sense measurement from its serial or I2C interface using the “Get variable” command. You can find more information about that in the “Using the serial and I2C interfaces” sections of the user’s guide.

However, if you configure the current limit setting when you set up your controller over USB, the SMC will actively use hardware current limiting to prevent the motor drawing more current on its own, so you may not necessarily need to monitor that with your Raspberry Pi. The SMCs current limiting feature can significantly reduce the chance of your motor damaging itself from trying to apply a greater torque than it can actually handle (if implemented correctly), but it is not entirely foolproof. If a high instantaneous load is externally applied on the output, there is a chance that damage could still occur before the current limit can trigger.

- Patrick

1 Like

Hello

Thanks very much for all this info. The 99:1 gearmotors are installed on the Wild Thumper and working perfectly. The slower speed is very helpful for this robot. The 172:1 would probably be good too.

The spec says stall torque is 15 kg⋅cm (210 oz⋅in), at 6.5 A.

To be honest I have 2 motor controllers each set at an 8 Amp limit, and each motor controller is running two motors (left side and right side). Now I am not 100% sure how to calculate the current situation when there are two motors on one controller, but it feels like I should set it lower. I just haven’t gotten around to it.

I am working on installing a current sensor to see what is actually going on, so I will definitely be able to tune the limits better when I see that data.

Thanks again.

I am glad to hear the your Wild Thumper is working well with the 99:1 gearmotors!

I do not have any specific suggestions for how you should approach setting the current limit with multiple motors being driven from a single channel, but installing another current sensor seems like a good way to get more information and start figuring out what to do.

By the way, just in case you have not thought of it already, another way you might consider protecting your motors and controllers is adding fuses.

- Patrick

oops! i was having too much fun and i broke a tooth on one of the 99:1 gears. not sure if it was from the robot falling from a height onto the floor too many times or if i pushed it too hard.

gonna order some new gears (or i guess the whole gearmotor if gears by themselves are not available) and redesign my robot to be more careful.

Unfortunately, we do not have the gears available separately, so you will probably need to get a new 99:1 gearmotor. In case it helps with the replacement process though, you can get the gearmotor, carefully remove its gearbox, and use that to replace the gearbox on the damaged unit. That way you will not have to do any soldering.

- Patrick

Thank you for all the help - I was able to replace the gearboxes without pulling the motors, just like you said. It is a lil’ bit tricky as it is possible to do it ‘backwards’ so the axle doesn’t spin after reassembly… but after flipping things around it worked.

I have now broken three of the 99:1 gearboxes, it seems to be correlated with the times my robot has fallen from a height (maybe about 20-30 inches off the floor). Since several of the gears inside the gearboxes are identical copies I was able to use the remains of 2 of the gearboxes to fix the third one. I reckon I will keep the robot closer to the ground in the future.

Oh no! I am sorry to hear about your new gearboxes being damaged. Unfortunately, falls like that are more than what I suspect the motors on the Wild Thumper chassis could handle repeatedly since all of the impact force would be transmitted radially through the motor output shafts. The Wild Thumper’s big rubber wheels and suspension probably help a little by slowing down the speed of the impact, but those factors would only go so far with a high enough drop, especially if your chassis has extra weight on it from your power supply, electronics, or other payloads.

- Patrick