Author Topic: Four wheel differential drive  (Read 1588 times)

0 Members and 1 Guest are viewing this topic.

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Four wheel differential drive
« on: July 27, 2013, 10:57:42 AM »
I did a search here to try and find out if anyone has done a four wheel differential drive robot with four servo motors.  I am wanting to build a robot lawnmower that will go up and down and sideways across steep slopes and over rough terrain.  Knowing this terrain when dealing with human driven vehicles, four wheel drive is a necessity.  I am not sure if this translates to the device I want to build but if it does my thinking is four motors .... with each side's motors wired in parallel for control and to perform skid steer type of operations.  Or I could possibly use two motors in differential drive and use a belt or chain transfer to the other wheels on the same side of this machine.  Besides power and extra battery (and weight)  needs for the extra two motors ..... what other drawbacks am I looking at?  If someone has done this ...is the alternative of using a belt or chain situation better for four wheel drive?  Thanks for any input you can give on this.

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #1 on: July 27, 2013, 01:30:28 PM »
The Dagu Wild Thumper rovers do four-wheel and six-wheel differential drive steering, one motor per wheel.
They are a bit smaller than real lawn mowers, though -- it may be that differential steering something heavy enough to mow the lawn would actually rip up that same lawn when turning...

Offline waltr

  • Supreme Robot
  • *****
  • Posts: 1,944
  • Helpful? 98
Re: Four wheel differential drive
« Reply #2 on: July 27, 2013, 05:26:36 PM »
Also, DC motors with a gear reduction would be a better choice than Steppers.

Four wheel drive, differential steering has been discussed on the forums here. Try widening your search and searching for discussion of DC motors verses stepper motors separately.

Offline MrWizard

  • Full Member
  • ***
  • Posts: 116
  • Helpful? 0
  • My cylon friend told me a killing joke......
Re: Four wheel differential drive
« Reply #3 on: July 28, 2013, 02:16:59 AM »
Why not make a hexabot type lawn mower wouldn't that be great (and almost impossible ) :-)

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Re: Four wheel differential drive
« Reply #4 on: July 28, 2013, 11:59:35 PM »
My plan is to use DC motors with gear reduction.  Just trying to decide if four motors versus two motors with belt or chain transfer to the other wheels on each side of the machine is a better idea.  Tracks would be ideal but add significantly to the cost.  The Dagu is neat ...but I have a feeling it may not be able to what I want to do.  If you have ever bush hogged tall brambles and small saplings I think you will know what I mean.  This things has to be like a little tractor ...or ATV ..able to deal with hauling or pulling around some type of self powered mowing attachment and that weight is going to be at least a hundred pounds.

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #5 on: July 29, 2013, 11:01:06 AM »
Right -- I wasn't suggesting that you use the thumper for your mower, just that it's an existence proof of differential steering for terrain navigation.

Also, the smaller, "bobcat" style loaders used on construction sites use differential steering; they don't actually turn their wheels to steer. Again, the do tear up the turf a bit.

Your project seems like it will need a lot of steel framing and welding, and really strong motors. Please post pictures as you assemble it :-)

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Re: Four wheel differential drive
« Reply #6 on: July 30, 2013, 07:14:53 PM »
Sure ...if I can pull this off I will post what I do.  Been doing a lot of thinking on how to get four wheel drive.  I keep coming back to possibly four motors but that means more battery demand too I would imagine.  Double the motors equals double the batteries?  Have thought about ways to transfer drive from two motors to four tires ...belts, chains and even tracks ....wishing tracks were not so dang expensive.  Any further feedback on this would be great ...especially if someone has done 4 wheel drive with these size of motors.  Thanks again...

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #7 on: July 30, 2013, 08:36:25 PM »
With four motors, each motor can be half the power of what two motors would require, so power draw would be roughly equal.
Also, motor controllers for weaker motors are cheaper than motor controllers for stronger motors (although you need more of them, of course.)

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Re: Four wheel differential drive
« Reply #8 on: July 30, 2013, 09:09:03 PM »
I believe I would all the motors of equal power.  If I wire them in parallel on each side ....then I don't really need more motor controllers ...just possibly one that will handle more current draw?  Am I thinking right here?

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #9 on: July 30, 2013, 11:03:36 PM »
Yes, if you don't need individual control, you only need two channels of motor control.

I don't understand why you'd add more motors that are as strong. In your case, I would be looking at something like either two motors with a power of half a horsepower each, or four motors with a power of a quarter horsepower each. In either case, you deliver one horsepower to the ground. Also, if you need suspension, four wheel independent drive may be simpler to build than a transmission/chain that allows individual wheel movement.

That being said, if you want simple, just drive on two of the four wheels (rear left and rear right, say) and call it good :-)

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Re: Four wheel differential drive
« Reply #10 on: July 30, 2013, 11:57:18 PM »
I hear you ...but I really am feeling like I need four wheel drive.  As far as the motors ....I already have two of them ....out the Jazzy power chair ...so if I am to do four motors ....I gues I feel like they should be matched as close as possible.  I don't think I will need suspension for this thing.  Basically I am thinking of it like a little four wheel drive tractor that I can hook different attachments to for mowing, weedeating  and spraying ...on rough tough inclines .....places that I hate to deal with!  But I believe it will have to be a powerful little beast to do it.

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #11 on: July 31, 2013, 10:14:08 AM »
Gotcha. Looking forward to pictures when you start building :-)

Offline Billy

  • Robot Overlord
  • ****
  • Posts: 122
  • Helpful? 2
Re: Four wheel differential drive
« Reply #12 on: August 12, 2013, 02:22:50 PM »
I have a automated lawn cart that uses two wheel chair motors (left and right) and dual chain drive to make it 4 wheel drive.  Chain drive is probably better, as a loss of traction on one wheel doesn't result in much drama. Wiring motors in parallel will allow a low traction wheel to spin freely which will dig holes, burn the grass, throw rocks, etc.  With chain drive, each wheel on a side have to spin together.

If your yard is anything like mine, you need 4 wheel drive, but you can start with 2 wheel drive, and then add a chain drive later.

Of course you'll want to consider what tools you have available. Chain drive requires more mechanical work than wiring motors in parallel.

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #13 on: August 12, 2013, 02:43:39 PM »
Quote
Wiring motors in parallel will allow a low traction wheel to spin freely which will dig holes, burn the grass, throw rocks, etc.

If you have good encoders and controllers, that won't happen, as the speed will be limited.

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Re: Four wheel differential drive
« Reply #14 on: August 12, 2013, 06:19:57 PM »
Some interesting input again ....still thinking through how I am going to do this so thank you.  Had to do the weedeating again today ...can't wait to have this thing do this rather than me!

Offline Billy

  • Robot Overlord
  • ****
  • Posts: 122
  • Helpful? 2
Re: Four wheel differential drive
« Reply #15 on: August 14, 2013, 01:02:22 AM »
Quote
Wiring motors in parallel will allow a low traction wheel to spin freely which will dig holes, burn the grass, throw rocks, etc.

If you have good encoders and controllers, that won't happen, as the speed will be limited.

I can't think of a way to use encoders and feedback with motors in parallel, so I assumed he was using an open loop control like RC.  If he is attempting closed loop, then he either needs 4 motors or chain drive.

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #16 on: August 14, 2013, 06:17:16 PM »
Quote
I can't think of a way to use encoders and feedback with motors in parallel

He needs one encoder per motor, with knowledge of which encoders are tied to what motors.
Decide what speed you want the wheels to turn. Give PWM to motors. If the wheels turn faster than desired speed, turn down PWM. If wheels turn slower than desired, turn up PWM.
This works no matter how many control loops you're using; you'll run the same loop (with possibly different outputs for each) for each physical motor.

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Re: Four wheel differential drive
« Reply #17 on: August 14, 2013, 08:00:39 PM »
So if I simply wire the motors on each side in parallel and connect them to a Sabretooth 2x60 motor controller ...I am heading for trouble? 

Offline jwatte

  • Supreme Robot
  • *****
  • Posts: 1,343
  • Helpful? 81
Re: Four wheel differential drive
« Reply #18 on: August 14, 2013, 11:04:28 PM »
If the motors have the same rating, it'll probably work out OK from a power point of view.
But, from a controller/encoder point of view, you won't be able to detect that one motor is free-wheeling and the other is stalling, for example.
Two Sabretooths? :-)
« Last Edit: August 14, 2013, 11:06:12 PM by jwatte »

Offline headrcTopic starter

  • Jr. Member
  • **
  • Posts: 16
  • Helpful? 0
Re: Four wheel differential drive
« Reply #19 on: August 14, 2013, 11:53:14 PM »
MM...starting to wonder if I should just use two motors with transmission system for the two wheels on both sides ...although you know ...I found another gentleman on another forum doing exactly what I am talking about ...4 motors ...two on each side wired in parallel and using the sabretooth 2x60.  Stated it worked great ...but he was just puttering around a yard ...not doing what I am going to do with this thing.

 


Get Your Ad Here

data_list