I need to control a screw jack with either a Stepper Motor or a DC motor via PWM. My RPM's are very low and range from 0.001 with a ramping load up to
265 oz-in to 100 rpm with virtually zero load returning to a home position. I am mainly conerned with maintaining a steady rpm rate within 5%. I do not have a lot of start and stopping. It will run 24 hours at a time at the low rpms.What would be the better way to go - Stepper or DC with PWM or alternative??
Thanks in advance
I am hardly an expert but it seems to me that your best route is to get a Permanent Magnet gear motor that's a little "big" for the job.
In such a case, the speed will be a function of input voltage and the actual load will not reduce the speed much.
The advantage of the stepper is that unless you really screwed up your load calculations you know EXACTLY how many revolutions per day/hour/minute/second will happen. The negative is that the controllers are expensive and the motors themselves aren't exactly cheap. Unless you have time on your hands you will want to buy the motors and the controllers from the same source.
With the gear motors you might revise you "specifications" and just accept an "off the shelf" speed that you can get from an AC powered gear motor.
Advantage? No complicated electronics. Just plug in the motor and your get so much lift/drop in 24 hours.
You don't mention how you want to measure position. Usually one can run the lead screw until some 'index' point is found. With a stepper you can simply count how many steps you make (assuming you don't slip because of excess torque). With a servo system you need some sort of encoder or sensor that you can reliably count revolutions.
Steppers tend to lose torque at higher speeds. As long as the wheel encoder/sensor can keep up, a servo system can usually be a lot faster.
Unless the DC motor drive is through some form of locking worm gear, the DC motor can't develop any 'holding torque' at zero or tiny RPM while a stepper can hold position in one place indefinitely (as long as power is applied).
Hope this helps...
daestrom
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