Wireless Powering of a Stepping Motor

In simpler forms?

Are you saying having a 'passive' induction stepper motor?

Similar to a RFID?

Let's say that we have the system shown in the second image I posted, but without the possibility of accessing inside the area limited by the black lines. We must stay outside it, at a distance of 10 centimeters.

Thank you for your answer, can I ask you a good link where I can learn something more about the RFID? A first research on google leads me to strange results and I would not like to misunderstand you :)

Thank you very much, have a nice evening!

The link doesn't work.

How much power is required to run the motor?

Will the motor ever be "parked" in one position long enough to charge a small on-board battery?

I manage to open the link, anyway it is this one: http://www.stepper-motors.net/low_cost_stepper_motor_nema_8.htm

Let's say we choose the first motor of the list, which requires 4.8 V and 0.2 A/Phase.

Thank you :)

You might look at Wireless Charging Powers Forward for Mobile Devices | PCWorld ...

This is not very powerful, but may give you some ideas.

I take it this is attached to a screen? Will the path/track be flat or curved?

We have two plates. The bigger one is limited by the black line and hosts the second one, which has a curved profile described by the green line in the second picture I posted. In the new image below here you can have a better idea of the 3D shape of these plates:

The green part of the rack and pinion mechanism, which is the part that moves, translates along a lateral direction left-right and is fixed to this smaller plate. The result is that this plate "slides" laterally inside the bigger one by following their common curvature.

Unfortunately I had started reading about these principles of energy transfer applied to battery charging processes, but I hadn't found any useful answer :/

Note that if you're doing the control and power wirelessly you'll need 2 coils for the motor and two transmitter coils.

I would like to better understand this point. Since I don't know anything about electronics, I'll just ask you... why this? :)

I mean, which components are necessary for the wireless control of the motor? Where do they have to be positioned with respect to it? How much space to they occupy? If we have two internal coils, how many external ones will we need, is one enough or again do we have to couple them?

Thank you very much!

Wireless power transmission to a NEMA size stepping motor!?!? Nope, not gonna happen. Have you considered at all that what you're asking for is an air core transformer to be fabricated that has nearby an iron core motor. Even if you could obtain a free space B field large enough that one could convert enough transmitted power to a coil that will power the driver electronics, this same B field will saturate the core of the motor.

Now wireless RF control of a battery powered motor is very straight forward and common practice.

I was thinking more along the lines of a CD drive head positioning motor. Agree any of the NEMA frame sizes is a non-starter.

I was thinking more on the lines of nano motors.

Actually, as I wrote in my first message, I have no background in this domain and I just wanted to see if my initial ideas were practically feasible, at least some sides of it. Your answers are very useful in this sense because they let me save time without getting mad on useless researchs :)

I would like to learn more about "wireless RF control of battery-powered motors", as redfred wrote. Do you have any existing application in mind which I could read about on the web? It would be great! In any case, I'll start looking for it right now :)

Thank you!

RFID, technolgy the limitations would be the distance between the emitter and reciever.

Normal disclaimers... don't read the below in a negative tone, but rather one of casual differing opinion:

I'm sorry... but I disagree with your view of these forums. Although we do not do other's homework... this is a place for the educated... or rather knowledgable to assist those less knowledgable.

I come here more to learn than I do to educate. And I think Andre has done quite a bit of up front work but is stuck in a certain area of his project and has come here seeking enlightenment regarding that particular area in which his knowledge is not as strong.

In fact, the whole reason I am here is because the title of the thread hinted at a topic I know little about so I clicked on it simply to read it and maybe learn somthing.

Plus, after receiving some input from Phoenix911 on RFID, he invested additional research looking into RFID elsewhere, rather than simply asking for clarification right off the bat. Only after his search resulted in inadequate information did he come back and ask for clarification.

Rather than discourage these types of questions... we should encourage those that do the upfront work but get stuck in an area outside of their expertise to come and ask questions. I'd rather have them come here than go someplace else on the web... this forum is one of the few that can be counted on the give sound advice, based in science and fact.

"Its a BBS for engineers, and clearly you have little engineering or electrical physics education."

I picked the above out simply to make a spicific point, and inquire into your thought process... my background is in Law Enforcement, Environmental Compliance, and Mechanical Design. If I'm working on a design that has an electrical component, I will come here and seek guidance and ask electrical-based questions. But having little electrical physics education, (read: none)... where would you suggest I go, if not here? And I'm not trying to be a smart-@$$ but I'm serious... Based on your statement, if only upper-level electrical questions should be asked here by those with a solid foundation of electrical engineering education... then where should us Mechanical guys go to ask the more rudimentary ones?

Sure, you mentioned books... and I, just like OP more than likely, have learned the basics of electricity from books. But when you get into questions regarding the application of principles... books become a little less-efficient and it is quite beneficial to have a place such as CR4 to come and ask those with applied experience.

In closing though, I do want to give you props for providing some guidance at the close of your post. Too many here simply dismiss the OP. And even though we disagree in our opinions of the intent of the forum... I am glad you didn't take that road.

Respectfully, JavaHead

Your speculation is not quite correct and are making some assumptions, but I understand where your coming from.

I am currently working on a project using RFID because it sounded like the OP wanted to power passively. The title listed here, like yourself, again, peak my interests on a new application. But I can see where you coming from.

If anyone has ever been in a brainstorming session where anything goes, this was it.

Was it a foolish question is in hindsight? maybe.

Was it a question that sparked interests? definitely.

Was anything gained by the question? again, most definitely.

In a brainstorming session, this fits.

I don't mind people such as daveca in the know putting it in prospective, just as long as they are in the know.

but I believe there was some side banter going on that leaves possibilities open for at lease speculation.

To daveca: I am sorry that you didn't appreciate my message, actually the situation is exactly as JavaHead (and apparently the other users too) interpreted, so I invite you to read again what he wrote. As I declared in my first message, unfortunately I am uneducated in this subject and I appreciate your critics and the indications that you gave in the second part of your post. They orient me in the right direction, letting me save the time that otherwise I would spend on useless researchs.

I am 24 and I've just started a PhD project which is strongly multidisciplinary. Inevitably, thus, I find myself facing with subjects that I never studied. Since Electronics, as well as other domains, is HUGE, I thought that posting the problem on a forum could be a good way to find some useful keywords and focus on the basic principles where to start from. Luckily, forums are virtual places where if you don't like something you can just skip it.

Don't take it personally,

daveca did list your short comings.

There's no problem, of course! Indeed I prefer to realise that the way I chose is not good at the beginning instead of getting lost while trying to make feasible something that is unfeasible :) so I appreciate all of your answers.

the 'Start' is always the hardest

Frequency, isolating substance plus gap distance (and other geometry factors) sets limitations to how much power can be transmitted efficiently with no wires. Now duty cycle is a limiting factor for your specific app, but if detent torque of your stepper is enough to keep it still when you don't want it to move, AND your duty cycle is very low, it could be done, you need of course a means to accumulate energy (batt or capacitor) and another, also wireless way, to signal to an integrated control cirquit when and how to move. S.M.

Thanks for your answer. I had thought that a capacitor could be employed in a circuit connected to the motor, I will try to explain this idea even if it may sound really strange (I apologize for the "kid-style" explanation I am going to give, but I lack all the technical terms and notions):

- from outside I start "sending electricity" by induction. The capacitor starts charging and the motor doesn't move yet;

- after some time (I guess calculable by knowing the circuit structure) the capacitor will be fully charged and it will "release" its energy to the motor, which will turn of one step in the desired sense. After this, if I keep the external induction system on, the capacitor will restart charging... Otherwise, the motor will stay turned off and won't move any more.

in your opinion, could this idea be realised or is it impossible?

I do write with bizarre vocabulary and a bit too "concentrated" to follow sometimes, but I already answered affirmatively in your question on my last post, on what conditions and roughly how. Now for specifics you can use a microcontroller that sleeps until battery or capacitor is charged, and then gives the next pulse sequence etc. Remember though what I said about stepper detent torque and duty cycle. S.M.

Ok, that's interesting... thank you, at least I'll have some good parameters to look for in the tables that list the features of the motors :) it's a better starting point than before!

Stepper motors need a two phase driving signal. I don't see how you can transformer couple that.

Here's another idea. Instead of a stepper motor on the inside, how about a shaft with a permanent magnet attached. Rotate this with an external magnetic field from another magnet on the outside which you can turn with a stepper motor if you wish.

I swear that this is the other approach I am working on. Something "passive" which could passively stay inside the system and be activated from the outside only when necessary. Everybody has played at least once with magnets and has moved metallic objects through a separating surface, like a sheet of paper or some plastics object... theoretically if I have a super powerful external magnet, the interaction with the one connected to the rack and pinion mechanism should be possible... but again, it's difficult to dimension and have an idea about the order of magnitude of this magnetic interaction force.

I've played a lot with NIB magnets and they're pretty strong. As others have said, you can generate the external field electrically if that's more convenient. And if you need more torque, possibly another gear in the internal mechanism might help.

I think I'll look for more info about these magnets: I need to understand if they are biocompatible and how to dimension the system in order to have a precise control on their interaction (so that to precisely control the movement of the small magnet inside the system). Is it possible, in your opinion, to have a precision of fraction of millimeter?

Thank you!

I think this comes back to my point about how you keep the external (driving) part in a fixed position relative to the internal (driven) part.

If the external part is worn (on a belt, jacket or whatever) it could potentially move around all over the place.

The only way I can think of keeping it fixed (relative to the skeleton, anyway) would be to screw it to a big chunk of bone .

Yes, I see what you mean... What if the driving part was worn in a "solid" way, like something wrapped around the knee and able to stay fixed in the proper position during the actuation process?

Better, I guess, but I can't imagine it being accurate to less than about 1mm at the best. (Just clamp your hands around your knee and feel how easy it is to move them a few mm either way).

Would the "wearer" be immobilized while the device was in use, or is it continuously operating?

Yes, the patient could be instructed to stay still while the necessary operations would be performed. The idea of wrapping something around the knee (like this) was to have a kind of "rigidly coupled" device.

As i understand your ideas, it won't ever work.

Many others have already said the same.

Think more in the direction of laying a power cable or using solar energy to charge a battery.

I do not understand why the need for "no contact" power, you have not explained that well enough for me.....sorry.

The need for "no contact" power is motivated by the fact that it is a biomedical application. The piece I quickly drew will be put inside the human body so we cannot have any cable permanently connected to the motor.

The project itself is quite difficult but challenging, so I am looking for feasible solutions. Putting a battery, as you and someone else said, is a good way to face with the problem but here it comes out a problem of biocompatibility... I need to think about that, thank you for your answer :)

something way out there!

Have you ever thought about I believe its called memory wire as an application here

nope, but I'll add it to the list of things to look for, right after finishing answering all of the messages here :)

I let this go earlier because I needed to mentally compose everything I wanted to say here and you do say all the right things here.

Placing anything inside a human body requires many disciplines and expertise that you will not get on a public, free, non-certified forum. Having said that, I applaud that you are looking for feasibility of your ideas here. I am very concerned of your apparent level of comprehension of the critical parameters for basic motion control, let alone bio-mechanics. You've yet to clearly articulate how much mechanical energy, power, momentum and other aspects of the load you expect to interact with this system. There's also efficiency concerns in translating one energy form into another. While many mechanical systems are prototyped without knowing all of these parameters, they will effect the performance of the design. The only parameter you have indirectly provided, an apparent 100mm of travel, is not a dimension I would anticipate with any bio-mechanics done inside a human. Possibly a limb prosthetic might have this far of a travel but that will be a dangerous guess on my part.

I understand that you're in the brainstorming phase of your project and are looking for both feasibility insight and to not reveal too much on a public forum the details of your project. You need a trusted electrical engineer/mentor on your project team to work on the real details of your project. We cannot be a substitute for one. We can translate what your electrical engineer member tells you.

I completely agree with you and in a certain way I am sorry, because I recognize that my messages are "foggy" . Unfortunately, for many reasons, "a trusted electrical engineer/mentor" is exactly who I lack here: in this moment I am in a laboratory where nobody studies mechanical and electrical engineering. I am supposed to contribute to this project by exploring these subjects but actually, from the academical point of view, I am alone :/

It's also difficult for me to articulate the system parameters and needs in a proper way, but I am doing my best in order to make my first steps into this domain.

Concerning the travel, it is a few millimeters of translation while the 100 mm is the radius of the sphere of the inaccessible region all around the device. You got it right about the prosthetical application, except for the joint: it's for the knee :)

Thanks again for your patience, just the keywords and ideas that I raised up today by answering to your posts have made me look for many useful and interesting topics that I need to face with. :)

It seems that the wireless idea is not likely to be feasible. I do use air core inductors in my work that produce enough field to power your application but these magnetic fields are so powerful that you don't even need the motor, any metallic part moves (and heat) in its vicinity! It would induce relatively larges voltages and currents in any electronic devices placed nearby and probably fry it. Such a large fields might also be a health hazard.

You could consider using the support structure to carry the electrical power for the motor. Slip rings or other brushes arrangement could work. Once enough power is available to your motor driver, the control signal could come from a RF link operating in the uW of power or from another channel of slip brushes.

Good luck.

As I wrote in my answer to comment #19, the magnetic interaction is another approach I am taking into consideration. I am really interested in learning more about the action of these magnetic fields which you talk about:

- how much is their range of action, in general? (just to have an idea of the order of magnitude... millimeters or meters?)

- do they have standard dimensions or can they be dimensioned in order to be biocompatible? Or if you prefer: have you ever heard about these air core inductors employed for biomedical applications?

Thank you very much!

The strength of a magnetic field follows what is known as the inverse squared law in free space. In other words if your magnet is one millimeter away from a piece of iron and it generates a 1 Newton of force, at 10 mm away it generates a 1*10^-2 Newton of force.

Redfred is correct.

We can apply many newtons of force at a few centimeters with a powerful electromagnet, I am talking about flowing hundred of amperes into a coil the size of a suitcase.

This is not a precise operation and the movement is extremely difficult to control as the force increase rapidely as the magnetic object approaches the magnet.

There could be a way to use two or three coils to produce a rotating field and impress motion to your devices (or its internal coil / magnet) just like a loosely coupled motor (induction or synchronous). The range would be a few centimeters for the device you described. Basically, a rotatinf field generator placed below your device could induce a rotating force to the magnet or coil in your cone.

Magnetic fields disperse rapidely in space. The near field can be powerfull but has a very limited range. The success of your idea depends on the range you are trying to achieve.

We operate at 10 centimeters of distance from the center of the plate. Imagine we have the system inside a sphere of 10 centimeter radius; we cannot enter this sphere.

One month ago my idea involved a different architecture and a screw. The idea was to make this screw magnetic and (surprisingly) screw it without contact, from the outside, by using somehow a magnetic force. The inability to quantify the range of action and the practical feasibility of this external rotating magnetic field (with which guiding the movement) made me look for an alternative approach. But... maybe you say it could still be possible?

Yes, you could use two or three coils placed around a 10 cm sphere and apply a torque of many newton meter to a magnet attached to a screw.

You could probably use a variable frequency drive or a servo drive (low voltage high current) to excite the coils. The coils would need hundreds of turns of wire (diameter depends on the current) on a ~5-10 cm form.

Potential problems will be coils overheating and finding a drive that can operate a low voltage 10-50V and high current (10-50A). The exact figures will depend on your torque requirement, the speed, the exact distance ...

Depending on what you have access to, it might even be an easy experiment to do. But you seem very novice in this field and I advise you to get the help of a good electrician / engineer. You are getting close to dangerous voltage and current levels.

Make sure nothing bad will happen if your coils catch in fire. We use large air blowers to cool down our large air core inductors. You may need some small fans for your coils.

What kink of project are we talking about? Hobbyist, Inventor, Undergrad project, Master project, PHD? Do you have any budget for the electrical devices?

Wao! Yes, I am "extreme novice", if you let me use this definition :) But yet I am also creative and extremely motivated and I thought of this "simpler" solution... However, I understand that it is not so simple as it may sound, initially. Just two questions before, potentially, going on:

- would we be able to have a high level of precision for the control of the induced rotation?

- as I said, it is a biomedical application. Do these external coils risk to set up an unpleasant barbecue? It would not be funny.

It is a Phd project and I am quite free to find a feasible solution... this means that I can use or not a micromotor, a magnetic induction, and so on... About the need to ask for help to a good electrician/engineer... well, that's why I posted the problem in this forum ;)

-The level of precision can be to within a fraction of a turn. It is actually mostly dependant on the friction and backlash of the mechanical system. If you use a servo drive, the field orientation can be controlled continually through a revolution within fraction of degrees. But if the change in the force is not enough to overcome the friction, the device will not move. Eventually, you will change the field angle enough to generate the force to overcome the friction and are likely to "slide" past your intended target, spring back and so one...

-You can use magnetic material to guide most of the fields to your application but a certain percentage will escape. If you start with a few hundred VA of reactive power, a small percentage of it will be much more than your cell phone transmission power. The frequency will be different though. Some external shielding can be added (u-Metal as an example) but can you live with an enclosed setup?

-The people on CR4 are not providing engineering services. You cannot count on us to work on the small details or go troubleshoot your system. We can give general advises but you bare the whole responsibility for the applications and consequences. We will not be there to prevent you from sticking your fingers on the charged capacitors... You are on your own.

When you produce something like this, half the battle is to find the proper material and use it properly. This can take many years to learn. This is why I advise you to find people knowledgeable in electrical power. Your university must have some hand on teachers and lab technicians who would enjoy helping you.

-You can use magnetic material to guide most of the fields to your application but a certain percentage will escape. If you start with a few hundred VA of reactive power, a small percentage of it will be much more than your cell phone transmission power. The frequency will be different though. Some external shielding can be added (u-Metal as an example) but can you live with an enclosed setup?

I am sorry, but I don't understand what you mean here. Could you please say it in another way?

Someone here has already told me I need some electrician/technician... I definitely do! But the department here is about Biomedical Imaging, anyway I'll look for them in a really italian stubborn way ;)

The high power magnetic field needed to actuate a mechanical device will have some leak into the environment. It should be OK if you use it only temporary to perform a quick adjustment once in a while but might not be acceptable for continual operation.

I see that you may not have the resources available to test this system. I do here in Montreal. Power electronics is my field of expertise. I would be happy to give you a hand with your test setup if you can come here for a few weeks/month. I can supervise you on the electrical work, supply your university with most of the required electrical material for this research. You could arrange to have an exchange with a local university (We have two French (UQUAM, Polytechnique and two English McGill, Concordia) and probably find a way to cover your travel expenses with some grant. Both Polytechnique and McGill are very strong in biomedical engineering.

Regards,

Marco

You have two major issues to deal with to make this work.

First, you need enough power to drive your motor. Steppers are not low power devices. Note the coil voltage and current of the smallest NEMA 8 motor, 4.8V & 0.2A. That's nearly a watt. Now, in order to keep your motor in position, you're going to need to apply a good percentage of that 1 watt of power constantly (holding current). Otherwise you will lose track of the position of the motor as it will fall into its magnetic detent position.

Now that you have some idea of the minimum power needed you'll face the second hurdle, getting that power to the motor. Transformers are efficient power transfer mediums because they've been designed to encapsulate as much of the magnetic field as possible. Any portion of the field that does not pass directly from the primary to the secondary provides an avenue for power loss. This is why transformer coils are wound closely together and also why the core wraps around the coil as tightly as possible. When you open a transformer and separate the coils as you are proposing, you are now losing a significant portion of your input power. You are not transferring all of the power that you put into the primary over to the secondary (not very efficient). This is the major reason that we don't see a lot of inductively coupled devices on the market. See marcot's note that the fields he used for air core transformers are large. He needs that to get enough power from his secondary coils.

My suggestions:

-If you can get enough power to your motor, consider using a very small brushed DC motor with a gearhead. This may allow you to power down the motor without losing position.

-For better power transfer, if you can, I'd suggest using a flat-flex or printed circuit cable connection to the motor. This is style of cable that connects to a print head on an inkjet printer. Lift the lid on an inkjet and you'll see what I mean.

-Taking a slightly different track than Rixter, you may want to use your coil to position a magnet. Simply attach the magnet to your carriage and position a coil around it. Vary the voltage to set the position of the magnet relative to the coil. This is what's called a voice coil. The only issue I see with using it for your application is the travel distance. You haven't given us the total travel and I suspect it will be too much for a voice coil design.

Wao, thank you very much for this great explanation, it's very clear and I think I got it :)

About the magnetic approach without any motor, I've just watched some videos about voice coil. It seems to me that they always need to be wired, isn't it?

Concerning your proposal, if I got it right, the magnet would be fixed to the green part to be moved laterally. the total travel is a few millimeters, not too much! But a high precision level is required! Then, the coil would be positioned outside the whole structure and by giving it a current it will generate a magnetic field that will move the internal magnet, is this the way it works? If yes, it could be a great approach, but then we'd need to evaluate its biocompatibility and the range of action of this magnetic interaction.

I can see what may be a fundamental drawback to magnetic coupling for directly positioning something separated by some distance - and within the body - namely how to fix the relative positions of the driving and driven parts (magnets or the coil and magnet or whatever).

Uhm... what about tracking these components by MRI techniques?

You could track them (by various methods), but you'd need a pretty sophisticated control system to correct for relative displacements.

I'm not too happy about the idea of having magnets in the field of an MRI scanner!

Over how long a period is the device expected to operate?

I understand the drawbacks you're pointing out...

The device should be able to work from a minimum of 6 months to... many years :) no upper limits for its lifespan, the longer it lasts the better it is! But at least 6-8 months ! are you thinking about batteries lifespan?

"... are you thinking about batteries lifespan?" - partly that, and partly trying to pull as much information together as possible.

Not "having a go at you", but it seems we're getting drip-fed with information - the fact that it's to be operating inside the body, and that there's a 10cm 'no-go area' for instance. If these had been stated earlier, several of the suggestions made previously would have obviously been non-starters.

Don't know whether you have commercial or academic secrecy constraints, but giving as much of the full picture as possible will almost certainly yield more appropriate replies here.

You're right, I should have given these two necessary information right from the beginning. Nobody here in the lab told me to keep any secret, but as I understood it is clear that I cannot copy-paste the whole project description publicly on a forum... Actually I tried to focus on the necessary details and some of them simply got lost (even if very important, I recognize!) :)

Good morning all...

Don't give up yet... There have been many bright people on this planet that have been experimenting with that same project you have... Think more basic.. outside the box... I find that just making the logical approach to the challenge opens doors to alternative mean and ways. You wont get unless you try...

Your trying to transfer energy from one place to another with no physical connection... Radio Freq (Microwave) bad idea...... Light (to much work)

Magnetism... I know we have lots of people on this forum that can teach you all about rapid collasping fields and the power output of an attached transformer... add a some circuitry... maybe even that battery pack... RFID for command communication... I see the issue being the distance between the collasping field and the pickup.

Thank you for encouraging :)

Looking at your mechanics, I would instead consider first a cable pull trolley system where the motor and its power wiring was at a fixed location that pulled the trolley with a looped cable. The much more traditional approach will be a threaded rod with a traveling nut attached to the trolley/stage. If you absolutely must have your motor as part of the stage then there are two techniques you can use to get power to the motor, a guided flexible cable or a power rail with brush contacts to the rail.

Remember your control signals can be run with the power or they can be wireless.

The demands for wireless control of a stepper motor has already been solved with a wide variety of solutions in the telescope making community.

Scopes being used at high magnifications not only enlarge the image, they magnify vibrations as well.

Astronomers must focus those scopes, every time they change eye-pieces. The mere touching of the focus knob can set up vibrations that can last an eternity so some enterprising scope builders have used stepper motors attached to the focus knob to make fine adjustments possible without upsetting the optics. However, this solution still requires a cable to a remote box.

The steppers used on the old floppy disk drives have served this application well and some have found audio software that converts analog signals into digital signals that the motor can respond to. The result is a computer controlled stepper motor that thinks it's responding to music but really isn't.

Control of this device via a USB port is common. Since it can be driven through a USB port, Blue-tooth commands and other similar wireless adaptations are also employed.

You are asking the right questions and while you might get a solution here, my experience has been that you will get a wider variety of alternative solutions in a community that has long since solved the problem.

One enterprising ATM (amateur telescope maker) developed an even simpler solution for less than $10 at Walmart.

He purchased a pair of tiny robotic mice that chase each other around a flat surface, interacting with the walls and each other using high frequency transmitters built into each unit. He uses one of the robots to generate signals and secured the other to his focuser.

Wireless focusing!

Have fun

Laughing Jaguar

I am sure there are many possible solutions to this problem :) The great problem is that, for me, it's like diving in an another planet sea... All the technical terms that very kindly you're proposing to me sound, for the moment, as a foreign language to me... but I'm motivated to learn new things and I'll keep on studying. Thank you for your hints!