Best way to accomplish variable resistance to linear motion?

[Triode]

I'm trying to figure out the best way to offer controlled variable resistance to human muscular motion. I know that in exercise machines this is done with a motor, but that might not have the range of resistance I need, ranging from about 150 lbs, to almost none and it must switch quickly. This is for a medical device to diagnose muscular/nerve disorders.

Ideally, it should let them move as freely as if they are moving through air and in a moment switch to resist. The resistance could vary from a hard stop like a wall to a slight resistance, like they hit a movable object.

I can think of several ways:
-An air shock with a very large fast servo valve; expensive but very effective with almost no resistance when on and a high maximum
-A rack and pinion attached to a motor which can actively resist, or use coil shorting; cheap, but minimum resistance is probably not that low, and maximum is probably not that high, will resist fast movement more even in low resistance mode
-Electro magnetic clutch - very fast, medium expense, very low off resistance, poor precision in resistance when on

Since there are so many devices out there I thought I'd see what people know of, anything could work even if it's very expensive (if it works well enough) or low quality (if its cheap enough) so let me know what you've seen.

(as usual I will explain why I need to ask questions if I am qualified to be doing something this advanced, I make systems for researchers, so I make mechatronic tools but I don't always know all about whatever I'm doing)

 

[ronv]

The motor may be the best one.
They make disk brake kits for golf carts that might work.

 

[Triode]

I just don't know if that would have low enough minimum force resistance or high enough max force resistance. Maybe a specialized linear motor could, it only has to move about 0.7 meters.

 

[cachehiker]

hysteresis brakes

We have one for horsepower testing and were considering them for life testing of PMDC motors. In the end though, we used an alternator out of a '69 Chevy Malibu for life testing. The alternators have no lead time, are only $40 with core exchange, and can be purchased near anywhere. Anybody with an IQ above a kumquat should therefore be able to repair the fixture when it fails every 20,000 hours or so.

 

[ronv]

What is the motion? Is it like a tread mill that only goes in one direction or does it need to return to some position?

 

[dr pepper]

You can reduce the minimum resistance of a motor by using a small amount of power assist.

 

[unclejed613]

an aluminum disk and some magnets might work here. aluminum is paramagnetic, and i've seen aluminum vanes and magnets used for various braking devices where motion damping without mechanical friction is required.

 

[Pommie]

How will the force be transmitted to the subject? Via a handle with a cable attached? A leg raiser? Lifting? Rowing action? etc.

I envisage a long flat steel bar with brake pads either side being pushed by a solenoid.

Mike.

 

[alec_t]

Band brake applied by solenoid?

 

[Mr RB]

I would go high tech and use the motor torque as both resistance and recovery motion (if needed). Applying a constant current to the DC motor will give a pretty fair constant torque (which would be constant "resistance" to the persons muscle).

So safety you could put a optical rotary encoder on the motor and limit any motor speed so if something breaks or the person lets go it will only move at a limited speed when the motor pulls it back.

 

[Mikebits]

Air cylinder

 

[Triode]

To answer some questions the motion will be limb extension, like a stair stepper motion but with one leg or a chest press motion but with one arm, it can also be a pulling motion. I like the motor current control idea, as the motions will already need encoders. If it is creating a virtual block I can see how it would let go as they pushed away from that point, but I'm not sure how it would know to help them in the right direction when they are supposed to be allowed to move freely.

 

[More Coffee]

I havent a clue about what your doing ..but id putting something into motion,controlling it then stopping it is what you need ..have a look at this place.

https://www.sdp-si.com/


By the jest of what ive read ..an electromagnetic clucth ..might come in handy....

When the electromagnetic clutch is used in automobiles, there may be a clutch release switch inside the gear lever. The driver operates the switch by holding the gear lever to change the gear, thus cutting off current to the electromagnet and disengaging the clutch. With this mechanism, there is no need to depress the clutch pedal. Alternatively, the switch may be replaced by a touch sensor or proximity sensor which senses the presence of the hand near the lever and cuts off the current. The advantages of using this type of clutch for automobiles are that complicated linkages are not required to actuate the clutch, and the driver needs to apply a considerably reduced force to operate the clutch. It is a type of semi-automatic transmission.

Electromagnetic clutches are also often found in AWD systems, and are used to vary the amount of power sent to individual wheels or axles.

A smaller electromagnetic clutch connects the air conditioning compressor to a pulley driven by the crankshaft, allowing the compressor to cycle on only when needed

Its not quite what I had in mind ...but a hub filled with silicon grease/iron dust the two shafts would hook up when current is applied to an electromagnet on the out side turning the grease almost to a solid providing power transmission..a slight current would be like slipping the clutch ,only no wear or heat.

 

[i_build_stuff]

The first thing that comes to mind for me is to use a regular old servo system and put a force sensor in the feedback loop, so it's "regulating" the pressure that it applies. Then you have a controller (PC, microcontroller, etc.) that tells it what its "target" force will be based on position, speed, or whatever else you want.

The electronics / motors / sensors are all available as off-the-shelf parts too.

 

[ronv]

The clutches seem like the best solution. Low mass when free running and variable. Just like brakes but easy to control.