Friends:
At what point does one specify a motor rather than a servo for wheeled robot? What are the conditions (criteria) which lead one to select the motor instead?
Thanks, -Migs
A servo IS a motor!
Servo motors generally cost more, and provide better control. a feedback device (encoder) is generally used on a servo. An encoder could also be used on most any motor.
A motor controller appropriate for the servo and an encoder are needed for a closed loop system.
Alan KM6VV
Hi Alan:
What is a “closed loop system?”
Migs
(I’m CP1PG, but I have been out of ham radio for a few years)
It’s necessary to move up from servos modified for continuous rotation, to DC gearhead motors when the weight of the robot excedes about 2lbs. In addition to being more powerful the DC gear head motors can be much faster. A modified servo is roughly 60rpm, but dc gear head motors can be had at rpm’s of 200 or more, and still be strong enough to move the extra weight.
Thanks Jim!
I notice that those DC gearhead motors tend to be 12VDC. Are they still BS2 controllable? Or do they require another board in between? If so which?
Thanks,
Migs
Yes, a board will be required between the BS2 and the motor. Motors draw significant more current than the BS2 can handle. You can use an H-bridge between the motor and the BS2.
try here for some choices
lynxmotion.com/Category.aspx?CategoryID=10
Chris
The voltage isn’t necessarily the problem, it’s the current. We have DC gear head motors with 7.2vdc and 12vdc windings. Controlling them from a stamp requires a motor controller. For most rovers the Sabertooth 2 x 5 or 2 x 10 will work great. The simplest method of controlling the motors is in RC mode where they respond to 1mS to 2mS pulses just like a servo. These are the RC version of the motor controllers.
You can use any MCU (Micro Controller Unit) to run the motors, but you can’t directly run the motors from the MCU pins. You will need a motor controller such as the Scorpion motor controller that is controlled via the MCU. The stamp would send commands to the Scorpion to turn “on” or “off” the DC motors. A seperate battery is used to power the Scorpion controller board that supplies power to the DC motors.
Hope this helps.
You guys are very helpful.
In just a few brief messages I’ve figured out how to do some things. Do DC motors such as these hold position when the power is stopped? Or do they gradually wind down in speed?
Migs
I’m not sure if PWM will provide any kind of holding power or not. If a DC motor has its polarity swapped fast enough I guess it could try to hold a position but would not be very accurate. Good question.
Normally when the power is removed it wines down like you said.
This is a property of the motor controller. Good motor controllers short out the motor leads when not driving the motor to act as a brake. It’s pretty effective. This ensures the motor doesn’t freewheel when the speed is reduced. I believe the Sabertooth motor controllers do this.
As would be necessary in some type of Sumo Bot… Migs
Hi Migs,
A closed loop system is one in which location of the end effector or other device is returned to the driver device. Any off-position generates an error signal of the proper polarity to drive the end effector to the required location. Servo systems are quite good at this, using special control algorithms (PID) to provide proportional-integral-derivative control. Our R/C servos are closed loop in them selves, but not back to the requesting controller. CNC systems and other motion control systems using “servo motors” use either stepper motors (open loop) or servos (closed loop) in their design.
I’m KM6VV, still active, west coast of CA, USA Glad to meet you!
Alan
If you are interested, here is the definitive introduction to PID control.
embedded.com/2000/0010/0010feat3.htm
The magic word here is “feedback loop”. I should have seen it. Thanks for the additional information. I’ll look at it right away. -Migs