# How to create an electronically controllable resistance to a physical movement?

What is the best way to create an electronically controllable resistance to a physical movement?

For example, at the gym you control the resistance to movement by putting more load, more weight. Is there a way to control that resistance electronically?

I was thinking of a motor, it's possible to use it? Or through electromagnetism?

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Linear or rotational movement? –  Andy aka Jun 9 '14 at 11:31
It's rotational movement :) –  Nihkorb Jun 9 '14 at 11:42
What power and how do you dissipate that power? Energy reclamation or heat loss? –  Andy aka Jun 9 '14 at 11:43
Sorry @Andyaka I'm not understanding your questions :/ I'm talking about a physical resistance to a movement not a ohmic resistance.. I want something that I can control the force against my movement electronically.. I was thoughting in use motor, but can also be some brakes I don't know what is possible to use, that I can then control.. –  Nihkorb Jun 9 '14 at 11:51
Force x distance is energy whether it's rotational or linear. If you are trying to restrict movement/speed then you are going to dissipate power - conservation of energy. How much power could be involved and how would you dissipate it? –  Andy aka Jun 9 '14 at 11:55

A permanent magnet DC generator (or a generator with constant field excitation) can be modeled as a voltage source proportional to velocity (angular velocity in the case of a rotary generator) in series with some coil resistance.

Since viscous damping requires a force proportional to velocity, simply loading the output of the generator with a variable resistance RL will create a variable damping factor proportional to RL+RG (where RG is the winding resistance). We want the torque to be -c$v$ where $v$ is the motor rotation speed to simulate viscous damping, so in this case c = $k(R_L+R_G)$, where k is a constant that depends on the generator construction.

An electronically-variable resistance can be created by literally switching resistances in and out, or it can be done with a MOSFET or BJT electronic load that simulates a resistance. The maximum damping with a passive load is limited by the internal resistance of the generator. With an active load and external power supply it should be possible to simulate a negative resistance externally to further reduce the total equivalent resistance.

For linear motion, a linear PM motor could be used, or a rack and pinion used to convert linear to rotational motion.

A second method, most useful if motion is guaranteed, would be to use coulomb friction (for example as a caliper and brake pads dragging on a rotor) and control the force applied to the brake pads using a torque motor or other method (for example change the position of an actuator that is spring loaded, so use Hook's law to determine the force). This works because dry friction is proportional to the normal force applied, and the proportionality factor $\mu$ is a function of the materials involved

However, this method will have stiction (nonlinear behavior) before motion starts.

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Hum thank you for the answer. I also thought about using some kind of brakes, this also seems a good solution for me, I'll do a deeper search about it :) –  Nihkorb Jun 9 '14 at 13:50

I think you can find the answer of your question via search about the mechanical and electronically mechanism of Stationary bicycle. look at this figure:

Is it necessary to explain about the mechanical mechanism of it?

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That kinda looks like a solenoid pulling on a brake shoe, similar to my second suggestion. What do the 'magnets' contribute? –  Spehro Pefhany Jun 9 '14 at 12:22
@SpehroPefhany The magnets are for creating tension. when you increase the pedal resistance, the magnets move closer to the flywheel, creating tension. When you decrease the pedal resistance, the magnets move farther from the flywheel, making it easier to pedal. you can see the patent of it in this page: freepatentsonline.com/7077789.html –  Roh Jun 9 '14 at 12:58
Presumably this is from eddy currents induced in the flywheel ( effectively a generator with a shorted output) but it would be nice to see a description of how it really works and a formula. –  Spehro Pefhany Jun 9 '14 at 13:09

Pump a fluid through a closed loop that includes an electrically operated variable valve. Depending on how much fluid is in the reservoir, how long it must run, how long it rests before you run it again, and how hard you'll be driving it, you might need a radiator in the system as well.

Edit: Look for indoor bicycle training stands. They use either fans, generators, or pumps (CycleOps makes one) for resistance, and the resistance units are pretty compact. Some feature a remote (push-pull cable) resistance adjuster; some just depend on the bike's own gear to adjust it. But something along that line should get you started.

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I also thought in something like that, you know if there is already an existing solution like that? :) –  Nihkorb Jun 9 '14 at 13:37

What you are looking for is a damper mechanism. The previously shown system (the stationary bike) is an adjustable magnetic damper. This patent explains its operation.

If you want to control the damping level electronically you might want to look at eddy current brakes, which produce the magnetic field from a winding, so can be controlled rapidly without moving parts.

You can of course attach a conventional DC motor to the apparatus, and emulate a damper or spring-mass-damper system electronically, however this has limitations over the above methods (due to the limited update rate of the software implementation when dealing with high frequency input signals, and the difficulty of estimating velocity at low speeds).

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Well on most servomotors and motors the torque depends on the power supply current.

However, It's probably not best practice and I don't think there actually is specific actuators allowing precise force control.

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You can use a control system to do this for you.

In essence you have a motor with feedback for the force you are trying to provide. For instance, you might attach a motor to a bicycle crank set, and put a strain gauge on the crank arms. This would allow you to use the motor to spin the crankset at a speed faster or slower than the rider is trying to maintain in order to produce the strain - or replicate the force - of either wind resistance, a hill, or freewheeling down a hill.

Another option is to trade the strain gauge for a rotary encoder, then model the physical system in the digital world. You can model springs, or mass, or anything else you can imagine in terms of where they move the cranks to and how the physical world would really respond. So you might have a high mass you're modeling, as though they are really riding a bike and they weigh 180lb, so when they aren't moving the crank, the motor puts no force on the crank. But as they attempt the move the crank, the rotary encoder senses a few pulses in a given direction, and the motor will push back with nearly equivalant force, as it models the rider gaining momentum. As the digital model of the system gains momentum, the motor will reduce the push-back, and eventually it will take very little real energy to maintain a given speed. You can then model hills up and down, wind resistance, etc in the digital model, and have the motor present those forces to the crankset.

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