Need to reduce bot speed

[MikeK]

I think I'm already up against a wall, but I thought I'd ask anyway...

I've got an R/C car that I'm using as a line follower bot (outdoor, semi-rugged course).  It's a pretty good platform, but unfortunately this car was designed to go really fast.  I've taken the PWM frequency down to 500Hz and gotten a little more torque and lower stall speed, but I still have no ability to go slow.  Aside from modifying the gearbox to get a better ratio, is there anything else I could try?

[Kylepowers]

You could increase the wheel size. Other wise when you controle the motor are you doing so through the built in chip on the platform or are you useing a motor driver/ h-bridge?

[Soeren]

Hi,

You could increase the wheel size.

How would that help?
Decreasing wheel size sounds more helpful

[Soeren]

Hi,

I've taken the PWM frequency down to 500Hz and gotten a little more torque and lower stall speed,

"Lower stall speed"??
I assume the stall speed was zero to start with, so does this mean going backwards or something 

but I still have no ability to go slow.  Aside from modifying the gearbox to get a better ratio, is there anything else I could try?

If you feed it say 1% duty cycle PWM (eg. at the 500Hz) it doesn't go slow?  (or whatever low duty cycle it needs to roll).

You could try adding wheel encoders to the equation. increase duty cycle 'till it moves and reduce it if the encoder tells you you're above wanted speed.

Since it probably has got a single drive motor, you could most likely just mod it somewhere in the present gearbox, by changing a gear or two and it would be so much easier to get a good result that way.

[MikeK]

when you controle the motor are you doing so through the built in chip on the platform or are you useing a motor driver/ h-bridge?

I'm using the on-board H-bridge circuitry: Four high-current transistors controlled by two signal-level transistors that I pulse from the PIC's PWM.  Works well.

If you feed it say 1% duty cycle PWM (eg. at the 500Hz) it doesn't go slow?

Correct.  It won't move until the duty cycle is about 17%.  If I set the duty cycle to 15%, the car won't move, but I can pick it up and the wheels zoom.  It's a torque issue.

You could try adding wheel encoders to the equation. increase duty cycle 'till it moves and reduce it if the encoder tells you you're above wanted speed.

I should have mentioned, I already have a wheel encoder.  Duty cycle won't move the bot until it gets high enough, and then it goes fast.

Since it probably has got a single drive motor, you could most likely just mod it somewhere in the present gearbox, by changing a gear or two and it would be so much easier to get a good result that way.

I don't think that's going to be easy.  Here's the gearbox:


The Hi/Lo switch is on Lo to get the most torque (not that it makes much difference, I think that was added for "coolness").  And the gearbox is pretty compact.  I'm not sure what I can modify, but I'll open it up for a look.

[rbtying]

If you've got wheel encoders, perhaps you ought to try a velocity PID control loop?  It would increase duty cycle to get the wheels moving (as in the beginning), and once the desired velocity is reached, it would cut back on the duty cycle and try to maintain the same velocity.  It seems to me as though you're having trouble generating enough torque to overcome static friction, but once it overcomes the static friction barrier, the same duty cycle sets the wheel to move too quickly.  A control loop could solve this problem easily enough, I would think.

[Soeren]

Hi,

I should have mentioned, I already have a wheel encoder.  Duty cycle won't move the bot until it gets high enough, and then it goes fast.

Not if you slow it down when the encoders tell you it's speeding. As soon as it has negotiated the inertia, it will run at a lower duty cycle PWM and with the encoders, it shouldn't bee too hard getting it to comply.

I don't think that's going to be easy.  Here's the gearbox:
[snip photo]
The Hi/Lo switch is on Lo to get the most torque (not that it makes much difference, I think that was added for "coolness").  And the gearbox is pretty compact.  I'm not sure what I can modify, but I'll open it up for a look.

The easiest way would be to add a dual wheel like this right after the gear wheel on the motor shaft.
It will take a bit of hack 'n' slash to the plastic encapsulating of the gear box, moving the motor a bit to make room for the additional gear, installing a blind axle for this gearl and finally bonding some plastic or other material over it to close it up.
But it would easily give you the best balance between speed and torque - just measure the top speed, calculate how much reduction you can afford and select a gear based on that.

[corrado33]

You could always make the bot heavier.  That'd slow it's acceleration at least.  Not the best idea, as it'd wear out your expensive bits too.  Just a thought.

Or you could remount the rear wheels somewhere else and use the "axels" to mount a pulley to, then mount a bigger pulley wherever you mounted the wheel.  Again, not the best idea I'm just throwing them out.