maintaining constant motor speed under varying loads


Thread Starter

Dusk Mixon

I am a mechanical engineering student working on a design project. We are designing a mechanism to drive a soil penetrometer into the soil at a constant speed of around 72in/min. The problem is that soil resistance forces will vary. I need to find a way to maintain this constant speed despite the variation in loading. I plan to use a DC motor to drive the mechanism. Any suggestions on an inexpensive way to accomplish this?

James Ingraham

Instead of a DC motor, you might try an AC servomotor. The servo drive controller will maintain a constant speed automatically. Personally, I recommend an Indramat Ecodrive 1.3, which can be run stand alone without needing an expensive motion control card. Indramat makes a wide range of servo motor sizes to hook up to the Ecodrive.

Sage Automation, Inc.

Alan Rimmington

A DC motor will maintain a constant speed up to its rated load therefore the speed variance
should not occur.
A common approach in industry for fixed speed variable load applications is to use a hydraulic
motor driven by a fixed displacement pump on an AC motor. If the pump HP is high enough to
overcome the maximum anticipated load (which it will have to be anyways no matter what method is
used) then the hydraulic motor's maximum speed will always be fixed by the pump size. For a
variable speed you can use a variable displacement hydraulic pump with a mechanical or electronic proportional volume control afixed to limit the maximum pump swash plate angle. By using an AC motor at 1750RPM you can get away with a smaller motor do to the mechanical advantage and speed reduction gained through the hydraulic motor. This approach is so common that hydraulic pump/motor units are available off the shelf, they are usually refered to as hydrostatic
transmissions or drives.

Cameron Anderson

DC motor with tach feedback into a DC controller should be fine for what your needing. Gear the DC motor to help velocity control and increase torque. Look at using a Roller Screw from Exlar Corporation. A roller screw will give you more force and smoother motion over a ballscrew. (Not sure what you plan to use for mechanics)

You mentioned low cost. Stick with DC motors. I assume this will run off a batery for vehicle, that's why you want DC?

The Indramat EcoDrive by far is not a good solution...that thing is huge and very expensive. DC motor should work just fine for you.

Cameron Anderson
Motion Control Specialist
Power/mation - St. Paul, MN
If you use a servo velocity loop, it will
automatically adjust the torque in order to maintain the velocity. Be sure the motor is capable of delivering the worst case torque. Also, if the drive has an internal torque loop, that will improve performance. Most drives do today. The motor vendor probably has software to help in sizing the motor.

Thomas B. Bullock, President
Bull's Eye Marketing Incorporated
Industrial Controls Consulting Division
104 S. Main Street, Suite 320
Fond du Lac, WI 54935
PH: 920: 929-6544
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What sort of DC motor controller are you going to use to control the speed? If you get one that supports encoder or tachometer feedback, you can close the loop back to the controller.

ie: The tachometer or encoder signal goes back to the DC motor controller. You set the controller up to run at 500 rpm (for example), and the controller uses the feedback from the tach/encoder to insure that the speed remains constant, supplying more or less current/voltage as needed to maintain speed.

Hope this helps...

--Joe Jansen
I already made a similar system like this to drill concrete. You also have steel bars, aggregate and so on. I use a dc-motor (no encoder!) and a programmable drive, with various digital and analog in/outputs (very small!). You just measure the amperage of your main circuit (by means of a coil) and keep that as constant as possible through programming (PID-control, I use only P-control) really very simple.