markh@csd4.csd.uwm.edu (Mark William Hopkins) (05/27/91)
I'm not very familiar with robotics in general, but keep seeing stepper motors mentioned in this context. How fundamental are stepper motors, especially in relation to the issue of precision in speed and positioning? Are they crucial to the whole process or producing robots, mobile machines, navigation controllers, and the like?
Frank2.Simon@arbi.informatik.uni-oldenburg.de (Frank 'Biber' Simon) (05/27/91)
markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: > I'm not very familiar with robotics in general, but keep seeing stepper >motors mentioned in this context. How fundamental are stepper motors, >especially in relation to the issue of precision in speed and positioning? There's a big difference between "normal" motors and stepper motors: The function of a "normal" motor is to turn around while getting current. For example to turn around one time it must get 1 second. To get a half round it must 1/2 second and so on. To get an exact round-partition you need a very exactly time-generator. But there's a next problem: The turn- around-time depend of the weight of the moved thing by motor. If the motor move empty a round takes for example 1 sec. If there is a big work to do it take a lot more for example 2 sec. Because of this a exact move isn't possible with different weights. A stepper motor don't depend of time: For example a 4-bit motor: it has 5 lines having these functions: the first is only ground; the following 4 are the direction-bit. If you put current to the first one the motor turns a little turn-partition; it's equal how long there is current it makes always the same turn-partition. To turn again you now must put current at the second bit. It now will make a second turn-partition. To turn again you must put current at the third on, to turn again you must put current at the fourth one and after that to turn again you must put current at the first bit again. The partition-number depends of the quality of the stepper motor. There are for example 360-part-motors, so you must put current 360 times at the right bits. So they work exactly and can be used in robotic. - Frank - -- FRONT242FRONT242FRONT242FRONT242FRONT242FRONT242FRONT242FRONT242FRONT242FRONT E-MAIL-ADDRESS: ~ I am ~ MAIL-ADRESS: Frank2.Simon@arbi. ~ because ~ Frank Simon informatik.uni- ~ somebody ~ Alexanderstr. 156 oldenburg.de ~ dreams me ~ 2900 Oldenburg
crs@lanl.gov (Charlie Sorsby) (06/06/91)
In article <biber.675348684@faramir>, Frank2.Simon@arbi.informatik.uni-oldenburg.de (Frank 'Biber' Simon) writes: > markh@csd4.csd.uwm.edu (Mark William Hopkins) writes: > >> I'm not very familiar with robotics in general, but keep seeing stepper >> motors mentioned in this context. How fundamental are stepper motors, >> especially in relation to the issue of precision in speed and positioning? > > There's a big difference between "normal" motors and stepper motors: > > The function of a "normal" motor is to turn around while getting current. > For example to turn around one time it must get 1 second. To get a half > round it must 1/2 second and so on. To get an exact round-partition you > need a very exactly time-generator. But there's a next problem: The turn- > around-time depend of the weight of the moved thing by motor. If the > motor move empty a round takes for example 1 sec. If there is a big work > to do it take a lot more for example 2 sec. Because of this a exact > move isn't possible with different weights. Well.... Not exactly. While it is true that a motor running at constant speed will turn a given fraction of a revolution in some specific period of time, that isn't the way they are generally used. When motors of this type are used for position control where exact position is critical, they are generally used in some sort of servo system together with position sensing. A position sensor such as a synchro or a shaft-position encoder or such is used to sense actual position. This information is fed back to the servo where it is compared with the desired position and the motor is driven accordingly. Drive is a function of the difference between the actual and the desired positions and, ideally, as this difference approaches zero, the motor comes to a stop. The problem with such systems is that they are relatively complex and, hence, costly. Most stepping-motor systems use no feedback but rather count pulses applied to the stepper and, therefore, count the number of rotational steps that *should* have been taken. Within their limits, such systems may permit precise positioning at lower cost than a full-fledged feedback control system or servo system. Best, Charlie Sorsby "I'm the NRA!" crs@lanl.gov