Building a motorized cable workout system

@tasan The motor you linked is a 3 phase 240VAC motor that likely weighs at least 50 lbs.

You will notice in the Norse system they are almost always standing on the motor assembly.

Very large stepper motors could probably be adapted to this activity. These are the kinds of motors seen on 3D printers but in larger sizes.

The code is not that difficult (it's 90+% library), the actual physical build is far more challenging.

Good luck.

Hi @tasan,

   Ron @zander has already suggested using stepper motors, which may indeed be a good idea, although broadly similar principles probably also apply to synchronous motors.

And I would agree, the mechanics are probably not trivial to build, although hopefully they are not too complex for you to be able to sketch out a reasonable plan.

I suspect the trickiest bit might be the electronics and associated software. I may be misunderstanding, but I think the power control system will be rather unusual.

In most cases, motors are supplied with 'more than ample' power to achieve their aim, such as drilling a hole or moving a certain distance. 

But in this case, the 'moving force' is muscle derived, and the electrical power must be carefully controlled to produce a restraining force, but not a moving force. This probably means controlling the currents through the motor windings to values, which are a function of the rotor position in relation to the motor poles.

Sorry, I don't know your expertise, etc., but if my understanding is on the correct lines, this might require a substantial amount of custom design.

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Because this does not look like a 'standard' solution, you might find it useful to start by building a small system with scaled down motors, etc. and develop the necessary control hardware and software.

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I know you said this was intended for you personally, but please remember the full sized system could become dangerous. I am not recommending that you take any risks ... such decisions and responsibility are totally yours.

This is also why building a small scale model first may be a good idea.

Please stay safe and good luck, Dave

Ah, yes, I think the actual controlling of the motor force would be hard. And the building of the physical system is something I am already confused about, haha.

I think I'll meet up with Norse Performance first and tell them my ideas for controlling the force and speed. The isokinetic mode is very close. The only thing it really needs is to be able to set the eccentric and concentric speed separately, I think 🙂

Hi @tasan,

  I am not too sure of your 'essential' aims. If they are to save a little money, compared to a commercial product, I suspect you will be disappointed ... this will clearly take a lot of your time, and probably at least as much money, possibly more, allowing for tools, materials and so on.

If, on the other hand, you are looking for a challenge and opportunity to learn some new skills, and accept that even if the final product does not achieve all you had hoped for, the 'journey' had been worthwhile, then you may wish to carry on.

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  I confess to not really understanding

my ideas for controlling the force and speed. The isokinetic mode is very close. The only thing it really needs is to be able to set the eccentric and concentric speed separately

My (very) limited experience of gym equipment suggests that most 'machines' rely on the user's muscles to define the pace of movement, whilst the user may preset the forces involved, by choosing weights, etc. The only obvious exception I can think of is the treadmill, in which the 'mill' defines the 'actual' running pace, albeit obviously one preselected by the user. Various machines may offer 'encouragement' in the form of screens, sound, etc., but in general the actual speed of lifting an object (say) on each occasion, is determined by the user's muscles.

I am not clear if your concept contradicts this 'rule' .. that is, whilst the machine defines the force needed to move the 'hand grip', which may be a constant or variable force as desired, it is still the user that determines the actual pace.

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If that is the case, then I suggest you clearly define which parameters the machine must provide and which the user is responsible for. Of course, there may also be a screen, sound prompts or similar to 'encourage' the user to behave in a particular way, but from a computing/electronics viewpoint, this may be an 'add-on', which probably needs some feedback from sensors, etc. for synchronisation.

I suggest you make some notes (for yourself), and map out the type of system you wish to achieve, being careful to segregate responsibilities to the correct parts.

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From there, you might wish to make a 'table-top' model system. Scale the forces by a factor of (say) 100, so 1000 N [100 kg (force)] becomes 10 N (1kg f), and maybe roughly quarter size in terms of linear dimensions, so 1 m becomes 25 cm, etc.

Then, perhaps use components, including motor, cords, aluminium extrusions, etc. which are easily available for the DIY 3D printer market, build a model of your intended system. With appropriate software and electronics, you should be able to develop the working properties that you desire, albeit you may only need 1 finger to provide the required force to operate the machine! You may wish to acquire a 3D printer to help you build the model, as a way of making the various mountings, etc.

If and when, you have a complete working model, then proceed to build a full-scale version. Although the motor, motor power source, motor driver and mechanics will clearly need to be much more substantial, most of the software may be directly transferable.

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In my view, we all have to decide what we wish to do in life. You will need to do a lot of research, reading, experiments, etc. Clearly forums like this one, can only provide a limited amount of support, and as with all research projects, there is a risk you will totally fail. But you may be one of those people who relish a challenge, and this particular challenge appeals to you.

The choice is yours.

Best wishes and good luck with whatever you decide, Dave

@tasan @davee As Dave said, there are terms and concepts I don't understand. You will need to change your wording to fundamental units related to resistance.

One thing I just realized is that all circular motors that can be used as generators (i.e. you pulling the rotor around) will feel jerky as the electro magnetic force is proportional to the distance between poles. This 'jerky' motion is extremely undesirable in your application. There are such things as linear motors but although I have no familiarity with them I think they may be too bulky. This is exactly the reason why gravity is normally used since at human scales it is constant. Very cheap toy like spring devices are inappropriate for your aims.

I can't see how this concept can work unless you are ok with jerky motion.

Thank you for the insights 🙂

I spoke to them on the phone earlier and they said that this product was not commercially viable before EV-motors came along that could provide a very smooth resistance and very high torque at low RPMs.

There have been research machines used for physiology research for many years that can create what is called isokinetic resistance. That is, your movement will be isokinetic, the joint angle will move with a fixed speed regardless of how hard you try to move the "weight" or resistance. 60 degrees per second is a very common value.

I think an example is warranted:

No matter how hard this man tries to extend his foot, the machine will only move at a fixed speed. He can give 50% of max or 100% of max, but the change of joint angle of his knee will happen at the same speed. Here it is set that the machine resists while he extends and then stops resisting until he "resets" by relaxing his leg.

But, importantly, it can be configured so that he starts pushing at 100% force and continues doing so even when getting to the end of the range. The machine would then, at the end of the range when his leg is fully extended, automatically increase the resistance until his leg would start moving in the opposite direction at the same given speed. As we are stronger at resisting a movement than creating the movement, this could mean a 50 - 60% increase in force.

They can also be configured to resist in both directions, but that's not something I want in my application. Here is a more in-depth video for those so inclined:

Norse Performance has taken this concept and created four modes on their cable machine:

Standard mode
You just set a given weight and the machine will resist "with that weight"

Eccentric mode
You set a given weight and a given increase in weight. When you hit the end range, the machine will increase the resistance by the increase you have set. In the case of a bicep curl, you will essentially lift a weight of say 15 kg on the way up that suddenly will become heavier and weigh say 20 kg on the way down, before becoming lighter again. This is huge for strength training and training for muscle size because you can get more work done using fewer reps, less time, accumulating less need for recovery between sessions, etc. But I digress.

Isokinetic mode
Here you set a given movement speed. Since this is a cable machine, you define it in meters per second rather than a given change of angle. You can set everything from 0.1 to 1 meter per second. If you set it to 0.1 you will only be able to pull 10 cm of cable out from the machine per second. Never more. You can pull as hard as you want (well, depending on the motor), but the cable speed will only be 10 cm per second. Importantly, as you get more and more fatigued, the motor will start resisting less and less to keep that speed. The same is true if you just start pulling with much less force. So in this mode, you don't need to know how much "weight" you want to have on the machine. You just start pulling as hard as you can and the machine keeps a constant speed. When you get to the end of the range, the motor reverses direction and starts pulling on the cable at the given speed set. If you stop resisting, the machine stops pulling.

Concentric mode
Opposite of eccentric mode, not really that interesting for me.

What I am trying to achieve is kind of a combination of isokinetic mode and eccentric mode. Or rather, an isokinetic mode that has a different speed for feeding cable out of the machine and pulling cable into the machine.

If we take the example of a bicep curl again: When I start pulling by bending my elbow, I want the machine to resist so that the cable moves at about 70 cm per second. But when I reach a given point, my pre-determined end range, I want the machine to start pulling on the cable while I resist as hard as I can. This time I want it to pull with a speed of say 20 cm per second.

So the concentric part of my bicep curl will be faster than the eccentric part, the part where I resist the machine pulling my arm down. And all this will just happen by itself while my only focus is pulling as hard as I can, always fighting to get my arm up and to have it stay up, while the machine does the rest.

Writing this all out like this makes me realize that it would probably be easier just convincing Norse Performance to add that mode to their machine 😅

Hi @tasan,

  Certainly starting with a machine designed for the purpose is likely to be easier than starting from scratch, and if you can persuade someone else to do the task, that will be easier still. 😀 😀 

From a quick glance at your explanation, then I suspect either BLDC (brushless dc) or stepper motors, possibly with gears, could be 'persuaded' to do the tasks, albeit in both cases with electronics to sense the position of the rotor, with respect to the poles, combined with current control that emulates a sine wave to determine the position of the rotor, with respect to the motor poles. If you look up how 'microstepping' for a stepper motor, works, some of the principles may become clearer! However, this is only part of the control algorithm/mechanism required. (I think electric vehicles use BLDC motors, but they are also being used in many other applications, including battery powered drills, 4 rotor style drone helicopters and battery powered vacuum cleaners.)

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I suspect you will also need a method of measuring the force being applied to wire, and some 'magic' software to convert the incoming sensor data into motor winding current waveforms to dynamically provide the required mechanical resistance. The constant speed mode might be a fairly simple algorithm, whilst other variants may be more complex.

Ron @zander, please note that although motors do indeed, also act as generators, this is probably not the relevant mode in this case. I suspect this is more like microstepping a stepper motor, except instead of being commanded to do a certain number of steps at a certain pace, the microstepping depends upon the forces exerted on the spindle, combined with a suitable algorithm to dynamically adjust the current levels and step rates, to achieve a simulated resistance force. With suitable choice of components and algorithms, I think it should be possible to produce a smooth motion effect. Of course, I am not saying that this is an easy task, merely that I think it is possible. If the hardware is suitably designed, it might be possible to emulate the different modes by choosing different software algorithms.

Best wishes, and good luck, Dave

@tasan If you are just building this for yourself, then that is one thing, if you are contemplating a product for sale then that is very different.

In the latter, you will need a lot of legal and liability coverage for the inevitable lawsuits. Even if just for yourself, there are still issues and all of that is on top of a mistake in code causing you personal injury.

EV motors are not a new kind of motor, they are existing types that are best suited to the requirements of EV racing. The best ($$$$$) of them claim to 'eliminate' the torque ripple that I alluded to earlier so that is a plus.

If you just want the machine for yourself, then as you said, convince Norse to add the mode you want. I am surprised they don't already have it.

Let us know how it goes.

@davee Check out what an EV motor is that @tasan mentioned. As far as motor vs generator, he needs both and it doesn't matter since torque is torque. If he is pulling the cable out, then it is in generator mode, if he is only resisting then it's in motor mode. In the case of the bicep curl, lifting is the generator mode and returning to the start position is the motor mode. I have spent enough time in a gym to know at least that much. Those machines used cables, weights, and eccentric cams in some cases that I think @tasan could explain.

I think he now knows this is not a DIY project, the product exists and he is just looking for an additional mode. Personally, they should give @tasan a free machine for suggesting that new mode.

Steppers are not useful for this, too much torque ripple. The EV racing community even uses modified induction motors as well as several other kinds that I have never encountered. Of course, they have millions of dollars in their budget to create those EV racing motors, I don't know if Norse has that kind of an R&D budget.

Sorry Ron @zander,

   Not easy for me to prove without building a system, but I think there is a controlled motor "mode", albeit not one you, or I have ever used, that could be useful in this application. Of course, I am not insisting any commercial product uses it, although they might. Just suggesting it as a possibility.

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Essentially, imagine a stepper motor with microstepping controller. Using the 'conventional' software that a robot like Inq's bots, or a 3D printer, has, then it is possible to select a particular rotor angle. Then provided the motor has enough holding torque capability, and the current through the windings is high enough, then the motor will resist any 'reasonable' attempt to rotate its rotor. Of course, if the external force on the spindle is great enough, then the spindle will move, but for now just assume the motor and current always 'win' over the external forces.

Now, include some kind of force sensor that measures the external force being applied to the spindle, and feed its data to a 'magic' algorithm that controls the microstepping driver.

Thus, if say the handle and cable wire (of a machine similar to shown in the photos), was wound around a cylinder, attached to the spindle of the motor, then when the handle was pulled with sufficient force, it would tend to unwind the cable, but the motor could exert sufficient holding force to prevent any significant motion.

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However, the 'magic' algorithm is able to start the motor microstepping in an "unwind" direction at a constant speed, whilst the external force (detected by the sensor), exceeded a certain value.

I think this could be the starting basis of the 'Isokinetic Mode'. That is, the machine is static when no force is applied to the cable, but when a force is applied, that exceeds a minimum threshold, it would begin to unwind at a constant speed.

Increasing the force on the cable would not increase the speed of the unwinding, as the motor holding torque has been designed to exceed any force the user could reasonably apply.

Microstepping allows the motor to move in steps of a small fraction of degree, so that individual steps would be imperceptible to user, and providing the microstepping occurred at a constant rate, the unwinding would appear to be at a fixed speed.

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This is obviously only a small part of the different cases the machine would need to cope with, but I think it is plausible that the other cases can be provided by extending the 'magic' algorithm with more options, etc.

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As for whether @tasan wishes or would be wise to try developing such a machine, is a totally different question, which I cannot answer, as it depends upon personal choices and capabilities. I also offer no guarantee that my suggestion can be made to work ... that is for the R&D phase to determine.

I have already agreed, it is unlikely to be easy or 'economically justified', but sometimes other factors may be considered to be more important.

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Best wishes and take care to you both, Dave

@davee I don't want to waste any more time on this topic. @tasan is doing the right thing in trying to get Norse to implement his mode. I think he will be successful.

A couple of quick points so readers are not misled.

An EV motor is not a stepper.

When I looked up Isokinetic I was surprised that it was not a standard exercise machine feature.

Let me just add, that at one time I was an industrial electrician and worked on large overhead cranes. They used something we called 'plugging' to slow the speed of the motors since mechanical brakes were too small without going to extreme. I would call that generator mode, but that term may be confusing to some. Let's keep it simple and just call it resisting.

Tasan seems to have a good grasp of the situation, let's stand back and see how he does.

Hi Ron @zander,

   I agree that @tasan is likely to follow the commercial product, which is a perfectly reasonable decision. The original question was a relatively open inquiry as to other possibilities.

  As for motor types, it is true that electric vehicle motors are not 'stepper' motors, and whilst I haven't checked the details, I suspect the principles and drive employed by the two motor types are closely related, albeit they will be optimised for different principal characteristics.

Stepper motors typically have many poles, enabling relatively precise positioning of the rotor, but limiting the rotation speed and requiring relatively complex manufacturing.  Motors intended for higher speed rotation will have fewer poles.

I suggested a stepper motor as a convenient starting point to model the requirements and algorithms on a small, and relatively cheap, scale. A subsequent scaling up stage would include searching for the most appropriate device. It is plausible that a motor with fewer poles would also be satisfactory, particularly if some gearing mechanism was included.

Best wishes, Dave

@davee The Torque motor is probably more in line with what @tasan is wanting to do with his project, since it's dealing with tension and slow rotation.

I found this link:  https://www.orientalmotor.com/ac-motors-gear-motors/technology/torque-motors-winding-unwinding-applications.html

I thought it would be of interest to him.. But as usual, I'm kinda late for the party..lol

I hope you are good health, and doing well..

Kind regards,

LouisR

Thanks again for all the input. This has been a precious learning experience for sure.

I'll meet up with them and see where I go from there 🙂