I will soon have to purchase some stepper motors for the robot I’m building; I’m not an engineer, so the torque ratings mean precisely nothing to me; I have the choice between a motor with Holding torque of 1600 gf.cm and Detent torque of 150 gf.cm and another of 3000 and 200 respectively.
While it’s true that more is probably better, it’s also true that more is heavier and much more expensive; I don’t want to fork out more than I need to.
What kind of useful work (in layman’s terms if possible) would I get out of the above motors? - maybe it would help if somebody could explain the way in which torque is measured (the ‘old fashioned’ way, as opposed to just using a torque measuring device or something.
In general torque is a measure of the “twisting power”. As you know it’s a lot easier to rotate something with a longer lever, than by grabbing hold of the shaft.
The torque is defined as the force applied multiplied by the (perpendicular) distance to the rotational center. Thus the SI unit is Nm (Newton meter). I have no experience with “gf.cm”, but according to this site it’s equivalent to 9.8*10[sup]-5[/sup]Nm.
If I remember correctly the torque you can get with a normal screwdriver is a handfull of Nms.
That would mean that none of your motors is stronger than a human hand with good grip on a screwdriver.
From what I’ve seen on Robot Wars and other fighting robot shows, I would go for the higher torque over higher speed.
Lower torque is usually compensated for by high gear ratios, but this is at the expense of good control at low speed.
I’m not saying this very well. Let me try again.
Think of a sports car vs. a diesel pickup. The motor in the sports car has low torque (it does poorly at pulling a load). In order for it to have good acceleration, it is designed with a large bore and a short stroke. Because the stroke is short, it reaches higher revs quickly. The diesel has a small bore and a long stoke, can provide power at low revs, and can pull a heavy load from a standing start because it doesn’t depend on revs for power.
If you translate this to your robot, you will be able to outpower faster ones at low speed, and be able to out-maneuver faster ones at most speeds. They will reach a high speed too quickly to properly control, and in a pushing contest, will be spinning their wheels while yours digs in.
This is still relevant, I think, if yours is not a fighting robot. Maneuverability is probably still its most important attribute.
Quite useful, thank you; so a motor with a detent Torque of, say 200gf.cm is equivalent to 0.00941192 Nms, or (assuming the by ‘handful’ you mean 5 or so) 0.001882384 ‘screwdrivers’; about 1/500th of the force that I can exert with a screwdriver (that does seem rather low; could someone check my maths there?).
Ah, I’ve just looked at a PCB drill in the catalogue that sells the Stepper motors and the drill has a stall torque of 800gf.cm - I think that what makes the stepper motors look poor is that the amount of torque that a bloke with a screwdriver can exert is actually quite considerable (in the scale of the kind of work I’m doing).
It’s difficult to compare the torque of a motor with a screwdriver, as a hand with a screwdriver can only produce decent torque at little or no speed.
The power is the torque multiplied with the rotational speed (ideally in radians per second, to produce Watts). For a powerdrill the speed is easilly a hundred times faster than for a lonely hand with screrwdriver.
However, as you said stepping motors, I gather that they will be used at low speed, when the torque becomes the limiting factor. That’s what makes the screwdriver analogy work. If I’m correct when I remember that a hand with screwdriver can exert half a dozen of Nms (and I can’t remember where I got that from[sup][/sup]), then it means that a bloke with a screwdriver can without any problem stop the motor from starting.
[sup][/sup] here’s an adjustable torque-limiting screwdriver, that goes all the way to 36 in.lbs, which in my book would be 4Nms - and I see no use to have a screwdriver go that far if it’s more than a normal human would exert.
Torque is primarily a function of displacement. The amount of air-
fuel mixture you can put into the cylinders and ignite causes the
resulting explosions to push against the cylinders, and this in turn
rotates the cranckshaft. Part throttle reduces torque, and turbo
or blower boost raises it.
Horsepower equals torque times RPMs divided by5252.
Displacement gives torque and therefore acceleration.
Horsepower takes revs and gives top speed.