The Best Stepper Motors for Autonomous Navigation Races?

Hello,

I am in search of the best stepper motor for autonomous navigation races. I have used 3 different 9.6V sets of stepper motors on different versions of robots. However, the motors I have used seem to be very slow. All of the robots I have built are under 10 lbs and can only drive a few inches per second due to torque limitations at high stepping pulses. The motors I am currently using allow the robot to travel about 4 inches per second, and the information is included below. I am seeking some stepper motors that could drive the robot at about 2 feet per second. If anyone could let me know if these motors should produce faster speeds or if there are other motors more suited for a top speed of a 24 inches per second, it would be appreciated.

Thanks, Brian

MOTOR STEP,9.6VDC/75ohm

Jameco #172646 Mfg Ref # 42BYG205

NEMA Size 17 Stepper Motor

18" wire leads Bearing type: ball Insulation resistance: 100 MOhm @ 500VDC Dielectric strength: 500V 50Hz/minute Ambient temperature: -10°C to +55°C Step angle: 1.8° Voltage:9.6VDC Number of phases: 4 Phase resistance: 75Ohms Current 480mA Phase inductance: 15mH Detent torque:

Curious that the one specification missing is the maxiumum steps per second. All steppers have this specification, though most are about the same -- in the 200-400 cps range. For a 1.8 degree motor, that equates to 1-2 rps. That's on the slow side.

To increase the speed of a stepper, given a specific top cps speed:

Use larger diameter wheels. Given 1-2 rps, linear travel from a 4" wheel will be 12" at 200 cps; an 8" diameter wheel will be 25". However, doubling the size of the wheel halves the torque of the motor, so the motor must have sufficient torque. Many small steppers do not. You can try overvolting the steppers at 2X-4X normal voltages, but on a mobile robot, with 9.6v steppers, this would require 24-48 vdc power supply.

Always use ramping to achieve highest speeds. Accelerate to full cps. If you do this, and the motor can't produce the torque to drive the robot, then the motors are underpowered for this application.
Forget steppers, and go with a quality DC gear motor. They can be connected to an odometer to produce a closed loop feedback -- better than the open loop of a non-encoded stepper.

IMO, steppers are suitable for small "pocket bots" that travel over smooth surfaces. They're great as teaching tools for motion control, and the robots don't need to go fast. Only a few larger mobile robot bases use steppers. The Evolution ER-1 uses steppers, but it's very slow (steppers plus 4" wheels plus a gear-down using pulleys). Its makers didn't care to have it win raw speed races, and it can't.

-- Gordon Author of: Robot Builder's Bonanza & Robot Builder's Sourcebook

Brian wrote: