Fullscreen HTML5Med Res (??MB)HTML4

An introduction to: Ever wondered how stepper motors work? Here's a practical run down on these useful devices. By STEVE PAYOR Most semiconductor manufacturers produce a range of power ICs for driving stepper motors. The data sheets for these ICs cover the drive requirements for stepper motors quite well but the purpose of this article is to provide some practical experience with the stepper motor itself. By practical we mean just that if you want to learn all there is to know about stepper motors, you will need to wire up 4 pushbuttons, 4 diodes and a 1.5V dry cell. With this simple test circuit you can demonstrate half-step and full-step drive modes, and regenerative braking. DIRECTION OF MAGNETIC FIELD FROM PHASE 1 1 Fig.1 shows a "conceptual" model of a typical 4-phase stepper motor. (They are not actually built this way but the operation is easier to visualise). The rotor can be thought of as a permanent magnet which aligns itself with the direction of the applied magnetic field. By energising one winding at a time, the rotor can be made to move to any one of four positions, 90° apart (Fig.la). By energising two adjacent phases simultaneously, the rotor can be positioned in another four orientations, mid-way between at 45° as shown in Fig.lb. The latter driving scheme is more commonly used since the torque is RES~~[tNT~ MAGNETIC FIELD FROM PHASE 1 MAGNETIC FIELD FROM PHASE 2 ~ N ~~] rooofooo7 $2 V+ (A) ,t,4 $~ .,.. Y+ $4 (B) Fig.1: simplified representation of a 4-phase stepper motor. When phase cf>l is energised, the rotor aligns itself as shown in Fig.la. When cf>l and c/>2 are energised, the rotor moves to the position shown in Fig.lb. 14 SILICON CHIP greater when two windings are energised. By alternating between the two driving schemes, it is possible to obtain twice as many steps per revolution - this is known as the "halfstep" mode. We'll have more to say about this later. A typical stepper motor has a step angle of 1.8 ° so if you can imagine a rotor with 50 poles, and the coils of the stator duplicated 50 times around the circumference of the rotor, you have a pretty fair idea of how it works. By the way, the term "4-phase" stepper motor is really a misnomer. It is actually a 2-phase motor to a power engineer. Yes, it does have four coils, but energising <J,3 or <J,4 is no different to energising <J,1 or <J,2 with the current reversed. Having four windings just simplifies the drive circuitry, since only four SPST switches to ground are needed - these are usually just NPN power transistors. However, only half of the windings can be active at any one time, so large, high power stepper motors usually dispense with the centretapped windings and use a single heavy-duty winding for <J,1 and <J,3, and ditto for <J,2 and <J,4. The drive circuitry must now be capable of reversing the current through the windings so two "H-switches", each containing four power transistors, are required. (See the Railpower train controller circuit, SILICON CHIP, April 1988, for a typical example of an H-switch, using PNP and NPN Darlington devices). Demonstration circuit Anyhow, enough of the theory. The best way to learn is by doing, so if you have a stepper motor lying around somewhere, dust it off and wire up the simple demonstration circuit of Fig.2. A 1.5V "D" cell will provide enough drive to demonstrate the torque capabilities of the motor. Four pushbuttons enable you to drive the motor manually through "full step" or "half-step" sequences. Diodes D1-D4 prevent sparking at the switch contacts, and actually return the stored inductive energy back to the battery. For example, assume that the cpl switch is closed and a current of 300mA is flowing from the + 1. 5 V battery and through the et> 1 winding to ground. The instant the switch is opened, a current of - 300mA flows through the c/>3 winding via D3. The direction of the current is back towards the battery. The current rapidly drops to zero and the stored energy is returned to the power supply. During this period, the voltage across the open-circuit c/>2 switch is twice the supply voltage (neglecting diode drops). This is something to keep in mind when selecting transistors for unipolar drive circuits. The tables beneath Fig.2 show Below: this simple unit can be used to demonstrate all the characteristics of a stepper motor. It uses the circuit shown in Fig.2 , wired up on a piece of perforated board. The stepper motor is a surplus commercial unit. the required button presses to move the stepper motor in a clockwise direction, in either full-step or halfstep modes. Reversing is easy just walk your fingers backwards across the buttons. At this stage it is worth fitting some sort of lever securely to the motor shaft so that you can check out the torque characteristics. Notice that the holding torque is much greater than the " working torque" (the torque developed when moving on to the next phase). Most stepper motors will develop an impressive torque with only 1.5V applied to the windings. The maximum rated continuous DC voltage is usually only 5V. Why then do some drive circuits use 50V supplies? The answer has to do with speed. Try this simple experiment: leave the pushbuttons open circuit, and turn the motor shaft briskly using the attached lever. At around 60 RPM you will feel a distinct drag and the ammeter will show around half an amp being fed back into the battery. The speed at which the current just starts to flow backwards is the speed it would run at when driven as a motor from the 1.5V supply. In this respect, the mot or and drive combination AUG UST 1989 15 An introduction to stepper motors - ctd OPTIONAL AMMETER 0.5A-0-0.5A +1.5V 0 CELL + 1.5V - D3 ,j,1 <1>2 ,j,3 J,4 ,j,1 q,2 <1>3 <1>4 ON - - - ONON-- , ON - - ON - - - ON ON - - - - - ON ON ON ON - ON ON ON ON - - ON - ON - ON ON ON - ON FULL STEPS TWO WINDINGS ENERGISED FULL STEPS ONE WINDING ENERGISED ON ON - - ON - HALF STEP SEQUENCE Fig.2: this simple demonstration circuit will allow you to take almost any stepper motor for a "test drive". In addition to the stepper motor, it uses just four pushbutton switches, four diodes, a meter and a 1.5V battery. The accompanying tables show the step sequences. - - - - - - - + V MOTOR R- +V LOGIC STEP INPUT DRIVE LOGIC DIRECTION INPUT -;- .,. Fig.3: a typical stepper motor drive circuit. The NPN transistors and their associated clamp diodes are usually incorporated in a "power-pack" IC, along with the necessary logic to generate a 4-phase drive sequence from a series of input "step" pulses. 16 SILICON CHI P behave exactly the same as any normal DC motor. To go faster, you need a higher voltage. Of course, you will also need some means of preventing the motor from burning out when it is standing still. The stalled current is usually limited by two resistors (R) as shown in Fig.3. Only two resistors are needed for 4 phases, because cpl and cp3 are never both on at the same time; neither are cp2 and cp4. Fig.3 is the most commonly used drive circuit for low to medium power motors. The VcE rating of the NPN transistors should be greater than twice the motor supply voltage. High-power motors require a different approach. Dropping resistors would waste too much power, so the driver transistors are used as switching current regulators instead. Shaft torque There are numerous applications for stepper motors. In models, for example, a gearbox is often unnecessary because of the high shaft torque. And don't forget their braking ability. Try this experiment. Remove the 1.5V cell from its holder and depress all four pushbuttons. Now try to turn the motor shaft. The damping effect with all windings shorted is amazing. The braking torque saturates at only a few RPM. A constant current load circuit would enable the stepper motor to be used as an adjustable brake on reel-to-reel tape hubs. Put the 1.5V cell back in its holder and try turning the motor really fast. You will notice that it feels like a slipping clutch. This is because the generated AC current is limited by the winding inductance as the frequency increases. This constant current generating characteristic is very useful for bicycle lighting systems - just choose the total bulb wattage to suit the saturation current of the " alternator" , and the bulb brightness remains constant above a certain threshold speed. With the larger sized motors this may be as low as 60 RPM - suitable for direct drive from the wheel hub. ~