# Motor Driven Clamp Help

#### benha

Joined Jan 4, 2011
49
Hi!

I'm in the early stages of noodling a project to build a wood working clamp that's electrically driven. For this, I'd like to be able to set the clamping force I'd like to exert, and then have the motor drive the clamp until that force is achieved, at which point the clamp is locked until things are dry.

The motor would presumably be a brake motor so after the clamp is set the motor brake engages and power to the drive circuit would be removed. The motor brake would hold the clamp shut.

What I'm not sure how to do is to drive the motor until a given clamping force is achieved. Presumably this is a function of voltage or amperage consumed by the motor, and the specific numbers can be empirically measured with a scale when commissioning the machine. But the specifics of what I'd control and how are a bit beyond me. Anyone have suggestions?

Thanks,
-Ben

#### AlbertHall

Joined Jun 4, 2014
8,789
Using a motor with worm gearing eliminates the need for the brake as it cannot be driven backwards by the load.

#### Sensacell

Joined Jun 19, 2012
2,518
You are going to need a force sensing element, separate from the drive motor, and a complex mechanism and control system.

Trying to measure clamping force via the motor current is not practical, there are too many vagaries that make this idea unworkable.
(Just ask yourself - what devices have you seen in the real world that utilize this principle?)

A geared motor clamping something down would also have energy stored in the form of rotational momentum, when you reach a certain motor current resulting from the reflected load, you try to stop the motor- but the spinning mass continues to dump energy into the system- over tightening beyond what the motor could ever un-jam, it's now stuck. Another problem is that dynamic and static friction are not the same.
The dynamic friction of the system while moving is always less, this works to make the above situation worse, the torque required to un-jam it is much higher still.

Joined Mar 10, 2018
3,696
Load cells of course good for force measurement. Or resistive sensors -

https://www.mouser.com/ProductDetail/SparkFun/SEN-09376?qs=WyAARYrbSnZm3k2OB4XwnA==&gclid=EAIaIQobChMItvyPj8jy3wIVBrbICh2UbAzMEAYYAyABEgLlRPD_BwE

There is a method of sensorless motor speed control, but not sure if this could

http://www.cypress.com/documentation/application-notes/an93637-psoc-4-sensorless-field-oriented-control-foc

Might be possible, with a shaft encoder, to detect motor stall as it applies force
and using the shaft encoder and a knowledge of incremental position and a
model of shaft position vs compression in worm gear to measure force. Of
course this suffers from material differences under compression. But just a thought.....

Regards, Dana.

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#### Bernard

Joined Aug 7, 2008
5,335

#### drc_567

Joined Dec 29, 2008
751
.. for a purpose that you describe ... clamping wood ... precision force measurement may be unnecessary. ... Suggest designing an incremental system using a stepper motor and a gear system ... as mentioned above a worm driven gear should resist slipping and reversing ... For force calibration, get a scale and observe the number of motor steps required for a useful range of the scale increments, the steps being incremented subsequent to the initial contact of the clamp surface to the wood surface.
This design would not be particularly accurate, but would involve a less complicated design procedure, and would produce an approximate degree of force control.

Joined Jul 18, 2013
19,421
There was no mention of what you intend using for the controller, μprocessor etc.
As mention a few times, the worm and pinion method allows a low torque motor to be used without back feeding, first find out the force/torque required to clamp, if the ratio of the worm & pinion is known then the continuous torque required for a suitable motor can be used.
If using the motor to continuously clamp then the stall current of the motor has to fall into the continuous torque rating of the motor, i.e. not the peak torque.
This way if the motor were to be fed from a PWM signal, the degree of PWM output could include a limit, which is within the continuous stall torque of the motor.
Power could be removed from the motor without any brake due to the Worm/pinion effect.
This way no feedback element would be required or brake.
Max.

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#### benha

Joined Jan 4, 2011
49
Interesting. Thank you all for the thoughtful input!

This is clearly a lot more complicated than I had hoped...

Seems like the right way to do this "electrically" may be to power a little air compressor and then just put an adjustable regulator in line to a pneumatic cylinder.

#### drc_567

Joined Dec 29, 2008
751
... 1 in. diameter cylinder at 40 psig = ~ 31 lb. force
... 2 in. diameter cylinder at 40 psig = ~ 125 lb. force
... 4 in. diameter cylinder at 40 psig = ~ 500 lb. force
... 6 in diameter cylinder at 40 psig = ~ 1130 lb. force

... You might be able to get by with a 6" diameter pneumatic cylinder and a bicycle pump.

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#### Sensacell

Joined Jun 19, 2012
2,518
Air powered idea is much more reasonable.

Simple physics
Cheap materials
Shop air is a reasonable assumption