differeential steering complications

[momin]

I'm building an autonomous mobile robot for my FYP. differential steering is the technique I'm using to steer the unit. wheels configuration is as follow: two drive wheels in the rear each one has its own stepper motor and one caster wheel in the front. the robot is supposed to navigate on tiles covered floor and wheels are made of plastic not rubber. i have no localization method for my robot. so its important to me to know whether I'm going to face complications due to wheels slippage and skidding or not. and how to reduce these effects.

 

[blueroomelectronics]

Wheel encoders will help, not much you can do about slippage. My Mongoose kit uses differential drive and optical rotation encoders on each drive wheel.

 

[momin]

thanx blueroom for replying
rotary encoders will only give the no. of revolutions (or steps) the wheels have spin. they dont give any data about slippage.

 

[blueroomelectronics]

A drag / caster wheel with encoder may help but if your robot can't sense it's surroundings it'll never be perfectly accurate.

 

[dknguyen]

Accelerating and decelerating slowly will also help.

 

[blueroomelectronics]

Oh yea the plastic wheels will probably slip like crazy. Why not use rubber?

PS since it's a stepper you won't need the encoders.

 

[duffy]

Stepper motor drives tend to increase the slippage. They have a series of high-acceleration impulses that can break free of the static friction force between the wheels and the floor.

 

[momin]

i tried differential steering using rubber wheels and a small caster wheel in front. slippage and skidding weren't the problem!. the problem is that some times (once every 30-50 steps) the left wheel motor misses some steps even that the unit is supposed to move forward. i think this is because the two motors (left-wheel & right-wheel motors) differ in their coil resistance. one motor has 13Ω/coil and the other has 20Ω/coil. this difference may cause the torque to differ in the wheels which causes one wheel to lag behind the other. am i right in my analysis??. if so i need suggestions to solve this complication.

 

[Sceadwian]

This is why attaching rotary encoders to steppers results in high precision driver circuits =) You don't have to worry about slippage if you actually KNOW where the drive shaft is located.

 

[duffy]

That's a pretty big difference in resistance. Are they two completely different steppers, or rated for different voltages or something?

You may want to try swapping motor drives and see if the problem follows the motor or the drive. I've seen many software problems present themselves as electrical problems.

 

[momin]

the motors are from the same manufacturer and they both have the same prefix in thier part no. (PM55L-048-.....). from outside they look the same but when i measured the coil resistance, thier is a difference.
i already did swap the connectors of the motors and the problem still occur at the left wheel motor.