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Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

motorsformakers.com

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Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#61

Post by Steltek »

doctorwho42 wrote: post #44
FatActor wrote: post #38

One problem I always run into with rotary motion: gearing. Finding gears of the right diameter, thickness, and thread pitch is always such a stumbling block for me. Most sites that sell gears to fit their motors have a small selection, and sites that sell gears are very hard to navigate (only to find out they don't sell in quantities <1,000 units). I end up having them laser cut out of HDPE online. Wish there was an…

It really comes down to the fact that gear shaping/cutting machines are quite specialized machines. Therefore demand a higher cost to buy (machines cost) and setup (tooling cost). For example a single gear shaper cutter is on average $700-$1000, and that's for a pretty standard and brand new cutter. So without taking into account actual time to set up the machine, program it, and feed it material. You are already hav…

If you watch enough tool teardowns (AvE, etc), most gears are sintered metal or plastic. I'm sure large industrial applications use machined gears but it looks like consumer-prosumer space goes for much cheaper fare.

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#62

Post by GeorgeTirebiter »

adrian_b wrote: post #55
oakwhiz wrote: post #36

Earlier quoted context omitted.

This is fascinating, I hope someone knows the answer. Is it possible that they magnetized the teeth of the rotor as a Halbach array?

Any permanent magnet demagnetizes partially whenever it is taken out of a closed magnetic circuit. If the magnet is made from a material with very high coercivity, the demagnetization may be negligible, but it is always recommended to store permanent magnets only with a piece of soft iron in contact with their N and S poles. To reach the maximum remanence possible for a given material, a permanent magnet must always…

What about, say, Neodymium magnets? https://www.kjmagnetics.com/neomaginfo.asp

What I'm asking is: are Neodymium magnets NOT used in these applications?

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#63

Post by mvidal01 »

frankus wrote: post #18

VESC (vesc-project.org) is worth checking out if you need (low) triple digit watts and a PMSM/BLDC motor is suitable. You can interface via UART and CAN and a bunch of other analog/digital options as well

The url is https://vesc-project.com/ I think. vesc-project.org didnt work.

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#64

Post by gwynplaine »

I feel like there's a gap in the hobbiest motor market.

On the one hand there are stepper motors which can withstand lots of radial load thanks to steel shafts and bearings. They also provide some relative position control and no absolute position feedback (at hobbiest prices).

On the other there are servos, which typically have weak components. Plastic gears, shafts and bushings which cannot withstand radial load. But they do allow control of absolute position.

For many robotics projects, a hobbiest has to choose between servos which are physically weak, or steppers which can tear the project apart if they don't home correctly. And of course the hobbiest has to roll their own safety/endstop system every time.

I'd love something physically like a Nema 14/17 stepper but with a servo interface. However I can't find them anywhere at hobbiest prices. Why are there thousands of good value steppers and thousands of models of servos, but nothing in between, I wonder?

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#65

Post by sacrosancty »

dragontamer wrote: post #30
CubsFan1060 wrote: post #11

For someone wanting to get into "making",is there a good set of starter resources like this? Software I'm comfortable with, but I'm not really familiar with all of the physical components or how to put them together, when you might use each, etc...

My issue with motors is that all of their problems seem mechanical in nature to me. I don't think anyone will really have a problem wiring up an H-bridge or whatever to run a motor. I'm personally wondering how to use the physical properties of a motor to like, move Magic The Gathering cards around, or other real world tasks. It very quickly becomes a mechanical levers / pullies / motion kinda problem, rather than el…

A lot of that stuff is internal industry knowledge that people learn by working for a company that already does it. It probably started out with people experimenting. You don't learn that in a mechanical engineering degree.

Though printers use rubber rollers on the top sheet in the pile so the friction between the rollers and sheet is greater than the friction between sheets and therefore only the top sheet is fed by the rollers.

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#66

Post by GeorgeTirebiter »

abakker wrote: post #52
Animats wrote: post #51

Earlier quoted context omitted.

It depends on the kind of load and the job. Harmonic drives tend to shear teeth if they get a shock load. It's good to have a back-driveable gear train, so that if the arm hits something or is overloaded, the forces push back to the motor, and the controller can see them and stop or deal with the problem. For something like a pick and place machine, you don't need that, but if you're trying to put screws into holes a…

Very good point about harmonic drives and backdriveability. I had forgotten that problem.

When we build actuators in the Leg Lab, every actuator had some electrical feedback element. For example, leg length actuator had a resistive strip that was as long as the leg's travel. A fixed 'wiper' allowed us to therefore have a potentiometer, giving a leg-length-proportional voltage on the wiper when top and bottom were driven by a constant current source. This was used in a 500Hz (sometimes 1 kHz) servo loop for position control.

(somewhat related): We also had a velocity sensor on the legs -- a simple coil of wire and a magnet (but COTS). This gave us a Velocity signal directly -- rather than differentiating the position measurements. Velocity estimates made by differencing position measurements tend to be noisy, and filtering esp FIR filters introduces phase delay -- things you do not want. Lesson was: If at all possible, get a sensor that senses the quantity you want to sense, so you do not have to integrate or differentiate to get your desired quantity. It's the same reason we had a REAL gyroscope, and not a rate gyro (as in most MEMs IMUs -- in IMUs they call them 'gyroscopes' but they are actually 'rate gyroscopes' -- they only directly measure "theta-dot").

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#67

Post by gwynplaine »

waynesonfire wrote: post #53

how does one protect the microprocessor from the back EMF? Do you need an optoisolator?

That answer would depend on the type of motor.

For a DC motor, they are rarely connected directly to the microcontroller. But you can put a diode across the motor terminals to minimise any back EMF.

A stepper would typically have a driver circuit between the microcontroller and the motor.

A small servo would usually just need one signal connection to the microcontroller, there's no back EMF via that path. You would of course also need common ground.

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#68

Post by jbay808 »

gwynplaine wrote: post #64

I feel like there's a gap in the hobbiest motor market. On the one hand there are stepper motors which can withstand lots of radial load thanks to steel shafts and bearings. They also provide some relative position control and no absolute position feedback (at hobbiest prices). On the other there are servos, which typically have weak components. Plastic gears, shafts and bushings which cannot withstand radial load. B…

It's worth distinguishing between two completely different types of "servos": RC hobby servos, and industrial-type servos which are robustly built like steppers but significantly more fast, accurate, efficient, and powerful. They are hard to find in Nema 14/17 sizes but not impossible.

RC servos typically have an analog or PWM signal interface. That would usually not be accurate enough for an industrial servo in positioning mode. Industrial servos, like steppers, have a digital interface which might be a serial format or STEP/DIR pulse train. Some drives will accept a -10V to +10V analog signal for velocity or torque control mode.

Anyway, here are some Nema 14/17-sized industrial servos with online pricing, which might be what you're looking for?

https://en.nanotec.com/products/2262-smart-servos-motors-wit...

https://catalog.orientalmotor.com/viewitems/l-categories-ser...

Or many companies offer stepper motors with built-in encoders and controllers to prevent missed steps:

https://catalog.orientalmotor.com/viewitems/az-series-absolu...

https://www.zaber.com/products/stepper-motors/X-NMS-E

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#69

Post by jbay808 »

GeorgeTirebiter wrote: post #62
adrian_b wrote: post #55

Earlier quoted context omitted.

Any permanent magnet demagnetizes partially whenever it is taken out of a closed magnetic circuit. If the magnet is made from a material with very high coercivity, the demagnetization may be negligible, but it is always recommended to store permanent magnets only with a piece of soft iron in contact with their N and S poles. To reach the maximum remanence possible for a given material, a permanent magnet must always…

What about, say, Neodymium magnets? https://www.kjmagnetics.com/neomaginfo.asp What I'm asking is: are Neodymium magnets NOT used in these applications?

They are used in these applications, typically very high grade, unless you're buying a legacy product line.

In, say, a hybrid stepper the magnet is usually a wide thin round disc, sandwiched between two steel rotor lamination stacks. With the link below you can examine the BH curve and load line for a Ø20 mm N52 disc magnet, 2mm thick, at 20 degrees C. (It's not exact because this assumes a magnet in free space and neglects the steel of the rotor - but it's an illustration).

https://www.kjmagnetics.com/bhcurves.asp

You'll notice that the load line (which assumes the magnet is in free air) is already landing within the "knee" where the intrinsic magnetization starts dropping rapidly. That's working too far along the hysteresis curve, where the poles are already starting to flip and demagnetize.

However, if that magnet were surrounded by the steel of the stator, the high permeability of the magnetic circuit would put the load line at a steeper angle, where the magnetic field through the magnet would be much higher. Small changes in permeability around that point would not damage the magnet, but allowing it to fall all the way down below the knee-point would. It would not be completely demagnetized by that, but it would lose some of its original strength.

Re: Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines (2015)

#70

Post by sbierwagen »

gwynplaine wrote: post #64

I feel like there's a gap in the hobbiest motor market. On the one hand there are stepper motors which can withstand lots of radial load thanks to steel shafts and bearings. They also provide some relative position control and no absolute position feedback (at hobbiest prices). On the other there are servos, which typically have weak components. Plastic gears, shafts and bushings which cannot withstand radial load. B…

>However I can't find them anywhere at hobbiest prices.

Might put this higher up in your comment. Dynamixel robotics servos exist, but at Dynamixel prices.

Weird stuff has weird prices. If anyone made a useful home robot that sold millions of units then robotics servos would get rapidly cheaper, but that hasn't happened yet.

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