How to Select a Stepper Motor

What speed do you need? What do you mean by "light weight"? A stepper motor giving this kind of torque (14Nm) without gearing would be massive (~ 5kg at a guess) - if you can find one!

I am forever collecting stepper motors from hardware, plotters, printers, floppies , stiffies & hard drives for possible projects when I retire.

Have a look at the specks (get the name and number on it) you might find that enough speed can be obtained from smaller and weaker ones.

There are also steppers with the controller on board.

To find a supplier in your country you should google for "stepper" locally.

I need to select a stepper motor too! For the "clock based dripper tracker" project.

I have being using a digital clock but the insides and gears are very weak. I need something that can step every 15 minutes or so and it only needs to hold up something like 50 to 200 grammes weigh

(depends on how it is set up). Anything like that out there?

Brian

Without knowing more details, it sounds like what you need is a ratchet driven by an electromagnetic solenoid, rather than a stepper. A stepper motor would be wasting a lot of energy holding position between steps.

Agreed. Another possibility would be a DC gear motor, doing one fraction of a rev (of the output shaft) fairly quickly, once every fifteen minutes or so (as required). A cam/microswitch or slotted disk/optoswitch could be used for shaft position sensing. Depending on torque & gear ratio etc., a brake may be needed.

This may be more expensive than a ratchet/solenoid setup, but it would have the advantage of being easily reversed to reposition the system ready for the next day.

Right! With appropriate gearing etc., it could be one complete rev. of the DC motor, or with a notched Worm Wheel, it could be several or many turns of the DC motor.

Still, if cost is a concern, the solenoid/ratchet is pretty hard to beat.

As John said, speed makes a BIG difference.

Also, the word 'hand' is open to lots of interpretations: If it is like the 'hands' on a clock, then add a counterbalance. The same is true if it is a waving 'hand' in an advertising mechanism. If it is a 'hand' on a robotic machine, then which of the many motions is the motor to control?

An equally important decision is the choice of a driver/controller for the motor, unless you choose to use a motor with built-in driver (more expensive, but guaranteed correct driver for the motor).

If the stepper motor is geared down a lot, it can be much smaller.

Stepper motor drivers (good ones!) can be set to remove a percentage of power when the motor is not being driven.This saves on power usage.

If a worm drive is used, which does not allow "backwards" movements (load moving the motor), the motor power can be completely removed between movements.

Stepper motors come in a variety of angles per step, the greater the angle, the cheaper the motor generally. 15°, 7.5°, 1.8° and 0.9° steps are fairly normal, also I have not listed all options. Current is also variable with motors, anything from a few milliamps to 10 amps or more......

Also, the driver circuit (at least the good ones) can be set to microstep, this means that the step can be further subdivided, often down to 1/16 or even smaller. Proper usage of micro-stepping can make movement smoother (particularly starting, acceleration/deceleration and stopping) and more exact.

There is a lot of useful infos on the web, just look around.

Ahh yes the stepping motor. The bane and present focus of my current job. Well here are the critical points one should know to successfully choose a stepping motor, driver AND power supply.

1. How much torque do you require at what speed of rotation? The numbers you state do imply an enourmous motor. I suspect that either the numbers you cruched are off or that with the use of counterweights or helper springs you won't actually require this much torque. Remember this motor torque specification is the added torque to the system to produce motion, include friction concerns and rotaional inertia considerations in your calculations. Keep in mind also that like all electric motors, torque nearly-linearly decreases with speed. But then again once you are at the desired speed you don't require much additional torue to maintain it.
2. How much torque do you require to maintain a position? If you don't intend on holding your arm at a fixed position then you shouldn't be using a stepping motor.
3. How fine of an angular increment do you require and how accurate should it be?
4. Do you require encoder feedback to determine the precise position obtained? If you do how fine of a resolution does the encoder have to be?
5. How do you intend to control the motor? A few examples; serial interface, step and direction bits, quadrature bits, Ethernet.
6. How do you intend to power the driver electronics? (Power supply selection)
7. Last but not least, what physical size is available for the motor, coupler and possible gearing?

Now after determining at least the first item and half of the remaining concerns can an attempt at choosing a motor be made.

I hope this helps.

As many have already pointed out its a matter of correct gearing to increase torque to the desired level. Meccano sets may have worm and wheel gears and spur gears to practice with and steppers are readily available from printer/scanner recycled parts so practice makes perfect. Maths is one thing does it work is another. I have found that small steppers in printers are not very powerful but can be used with counterbalancing etc and the right driver circuit.

thank you everyone for contributing

we need the stepper motor for hand (to be precise finger) movement. (to be working when placed on the hand) we are not sure about the speed we want. but just a little movement is all we require. so any help will be really appreciated. our project is a sort of robotic arm, using a stepper motor,microcontroller, worm gear and a few other parts. thanks again

Are you sure about your force & radius requirements? It must be one hell of a robotic hand to be able to lift ~28kg with one finger!

That helps a bit, but I'd hardly call it a 'hand' unless there are at least three movable 'fingers', in which case size becomes critical.

You might consider having the stepper turn a fine-threaded shaft (either directly or through gearing). These threads would push and pull a nut, with a link to the finger.

A sketch of your concept would help a lot. The more detailed info you give, the more appropriate will be the resulting answers.

As John says, review your numbers. Is that 279N the force applied perpendicular to the fingertip, or at some other point in the linkage?

we are not trying to build a robotic arm. we are trying to move the hand of a stroke patient. mainly the fingers only. to facilitate their recovery. the maximum grip force we found from research books is 279N. basically we need low speed n high torque n light weight. anyone willing to help let us know n we will send u a drawing of concept. thanks again. this site n all you guys have been very helpful

this is a sketch of our concept

Now that I know what you intend to do I don't think that you want a stepper motor. I think a reversable DC servo motor will do your job. (I don't think that you'll need to close the servo loop though.)

Your diagram though places the rotational torque from the motor in an odd direction. My suggestion is to make a glove that fits over your patient's hand, actually a gauntlet that extends at least halfway up the forearm. Add sleeved stings to the outside of the gauntlet that mimick the tendons of the hand. One sting running from finger tip to gauntlet sleeve running along the palm of the hand. Add an opposing string running from finger tip across the finger nail, knuckle and up to the gauntlet sleeve. Pulling on one string or the other will open or close the digit to be exercised.

Building this kind of a rig will also permit measurement of the forces required instead of attempting to calculate these forces. Once the forces are known you can clockwise wrap one string to the motor shaft and counterclockwise wrap the other. You may find that the string length deltas are different so you may have to add a collar for one sting to make up this difference.

OK, now we're getting somewhere! as I said "The more detailed info you give, the more appropriate will be the resulting answers."

Certainly a commendable project, but I agree with Redfred that this is not an appropriate application for steppers. An ordinary DC motor with appropriate gearing/linkages will do the job better with significantly simpler controls.

I assume the 'liquid sensors' are some form of pressure transducers. Their outputs can be used to control the motors without any need to keep track of motor or linkage position.

You should spend considerable time in designing the mechanism such that it is physically impossible for the motor to move the fingers beyond their natural limits, and of course there must be limit switches so the motor does not continue using energy pushing against the physical limits..

I forgot to mention that some stepper motors are built with gearboxes integral to them, this gives high power and small size.....

Having now seen what you are trying to achieve, I am of the opinion that you need a motor with a feedback loop so that your software will know exactly where each finger is at all times.

Stepper motors generally operate without any feedback circuit, but if correctly dimensioned, this need not be a problem with most things, but here not I feel!

The speeds needed and the powers needed should easily be covered by what is basically a radio control servo. The circuits are digital and there are even chips around to simplify the adaption of almost any DC motor that can be reversed. So mainly permanent magnet motors. I do believe that there are even brushless versions nowadays that will improve reliability, reduce wear and tear and also reduce the possibility of sparks generating false information to your interface electronics.

Interfacing is a simple 3 wire interface generally.

You can get a better idea by looking at some of these links. In the last one is a simple circuit to build that you could use as a basis for a computer controlled version:-

http://www.epanorama.net/documents/motor/rcservos.html

http://en.wikipedia.org/wiki/Servomechanism

http://www.seattlerobotics.org/guide/servos.html

http://www.everything-rc-cars.com/07-rc-servos.html

thanks for the information. so we have now decided to use a dc servomotor. but we are not sure about the ratings needed to be specified while ordering it because we don't want to make a mistake which will lead to further expenditure. what rating would you guys recommend using for this project?? (as in voltage power torque current etc) thanks again. u guys have been really helpful!!

It depends on the linkages, gearing, etc., and on the speed desired. How fast do you want the fingertip to move?