best TORQUE-WEIGHT RATIO

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The torque/copper ratio is not so much affected by number of poles for a normal motor design. With fewer poles you will need more iron, however, since the field lines become longer. On the other hand, at a fixed and high speed, there is more iron loss for multipole motors, since there are more field reversals per turn. So efficiency normally drops with nr of poles for high speed motors at fixed weight.

But for maximum torque/weight at low speed, use as many poles as possible.

Stepper motors are made differently, but they are still extreme cases of low speed and high torque.

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-- Sven Wilhelmsson

thanks, I'm realy thinking of a stepper pancake type motor, hopefully to get the benefit of both, the point about iron path length is a good one, and the speed is extremly low as there is no gear reduction.

I cant quite get my head around what makes a stepper motor more weight/torque efficient I know the construction of them is quite different as I have taken many apart.

reluctance type motor is easier to see why increasing poles increases torque.

maybe a similar reasoning can apply to the types of configuration of PM steppers ?

Colin =^.^=

The key point is the 'air' between the cogs. It does about the same job as copper & current, however much better. To have one tesla over 1 mm airgap would require about one kA, and it is hard to get such a high current through copper in this very small volume. So a 'normal' motor with 100 poles is not practical, unless the diameter is very big.

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Sven Wilhelmsson    http://home.swipnet.se/swi

Given the limitations of cooling the copper and of the maximum available B fields in permanent magnets or electromagnets, the torque of an ungeared motor is pretty well proportional to its volume. The reason for this is that the total force available is proportional to the area of the armature (maximum current density times maximum B field times area), and the torque is equal to force times perpendicular distance from the shaft. You can get factors of 2 by going to rare-earth magnets or water cooling or intermittent service, but otherwise this simple rule is surprisingly accurate.

The best way to get lots of torque from a small motor is to run it at high speed and gear it down, unless you really can't manage the noise and slight backlash, in which case you sort of have to take what you can get.

Cheers,

Phil Hobbs

yes I would go along with this, however different types of motor have very different torque/weight ive looked at quite a few, and some have as much as 4NM/kg wich would mean it supports its own weight at the end of a ~ 400mm arm.

im looking for quite accurate positioning, so any backlash would cuase problems, especialy in a positional feedback loop, I have had the idea of having an aditional geared motor to take most of the weight but with some springiness to allow the direct drive motor to control the fine movement.

the direct drive motor would only need to overcome the inertia the geared motor would strive to keep the direct drive motor at zero load, so the direct drive motor could have quite a low duty cycle.

2 small motors and simple gearing compared to one probably huge and unwieldy motor.

Colin =^.^=

Are you talking about a mounting scheme for something off-the shelf, or are you talking about inventing a new motor?

Thanks, Rich

an off the shelf motor would be nice, im not sure id actually be inventing a new motor, that would be like inventing a new type of wheel lol, im just not convinced the ones ive seen available so far are as light as they could be for a certain torque.

might be a while before i go through some text books and remind myself of the maths.

im still sure the more poles the better, as there is less rotational distance between the poles being aligned and not, if the energy required to sepereate the poles is the same this means more torque, although the degree of seperation would also change a bit with the number of poles.

the monting scheme is nothing to fuss over tbh.

Colin =^.^=

yeah thats sounding exactly like a pancake motor, also looked at pcb motor wich again are pancackish.

basicaly it needs large air gap area with large effective diameter, and minimal iron path length, maximum steps etc ... multiple discs would give good surface area for the volume.

by supermagnets I assume you mean neodymium ?

Colin =^.^=

A coarse-fine strategy like that would probably work fine, but it'll be more complicated to get working. Maybe try a preload spring to make sure you're always at one end of the backlash travel?

Cheers,

Phil Hobbs

yes, basicaly im interested in trying to come up with new ideas, theres quite a lot of antibacklash options :-

very precise machining lowers backlash considerably, springs can take up the slack, but this increases the torque needed, a strong spring would be needed to always keep at one end of the backlash, the weight of the arm would tend to do this to some extent, vibraton might still defeat this.

a counterweight can reduce the holding torque needed but increases the inertia, a spring (like on an angle poise lamp) would have to take into account the orientation.

other ideas come from an old disc drive head actuator wich had flexible thin steel strips as belts wich were fixed to each pulley so they cldnt slip, limit is about 1 turn of the motor.

Colin =^.^=

Taut band drive can go further than that if you do it right--stiffer than belt drive, less backlash than gears.

Cheers,

Phil Hobbs

yes im sure it could, the most that is needed is almost one turn on the larger pulley, the band can wind itself down the motor shaft so it doesnt overlap wich would cuase ratio problems, a guide on the larger pully might be a help.

ive got some shim steel wich is thin and flexible, its only 12" long, but might be long enough given that 2 lengths can be used.

this probably seems the most atrractive I wonder what the pitfalls are to avoid ...

another advantage is the motor can be closer to the sationary end of the arm.

Colin =^.^=

well I dont think ive got any fresh ideas to come up with and I dont think direct drive is going to be so advantagous, even without doing the maths,( although i may look into it just to satisfy myself i can stil do it lol), except for rotation that isnt fighting against gravity.

so now im looking at existing robot arm actuators. there doesnt seem to be much details about the construction of commercially available ones, maybe they like to keep it secret although you would think someone might want to boast about how their design is better.

plenty of hobby details about though, plenty of varations on gear reductions, steel belt fixed to puley at the top of my list atm.

Colin =^.^=