Fullscreen HTML5Med Res (??MB)HTML4

Stepper Motor Controller By Ross Tester With so many stepper motors in use (and disposed of) these days, one of our most-asked questions is “how do I use one?” Here’s how . . . S tepper motors are everywhere. For example, every computer contains several (in the floppy and hard disk drives – one popular hobbyist source). They’re used because it is easy to achieve very precise positional control – far better than you can achieve with a “normal” motor (if you can control it at all). Unlike a conventional motor, where you simply connect an appropriate voltage and “away she spins”, stepper motors require considerably more effort to get them to work. So what is a stepper motor? First of all, think of a conventional motor. It has two main components – a stator, which sets up the magnetic field, and a rotor, which by magnetic attraction or repulsion turns toward or away from the magnetic field. But there’s also a commutator (actually part of the rotor) which keeps switching power from one coil to the next, moving the magnetic field as well, so the rotor has to keep moving, or rotating. Yes, that’s a pretty simplistic explanation – but will suffice for our purposes. Stepper motors are similar in many respects – they have stators and they have rotors – but they don’t have commutators. The magnetic fields which cause attraction/repulsion, and therefore turning, are set up externally by the motor controller. 76  Silicon Chip A stepper motor operates a little like a chaser: one stator coil is energised, repelling the rotor. Then that coil is de-energised and the next one energised, again repelling the rotor. Keep this up and the rotor turns continuously. The rotor may be either a permanent magnet, a variable reluctance or a combination of both. By controlling which field coils are energised and when, the rotation and stopping position of the rotor can be extremely closely controlled. You will hear stepper motors referred to as 0.9° degrees, 1.8°, 3.6° and so on. This refers to the rotation of one “step” in the motor – a 0.9° motor will have 400 individual steps to make one full rotation of 360°. As you can see, 400 steps in one rotation is a lot of steps, especially as each one can be individually accessed. And many stepper motors operate through a gearbox, multiplying that yet again. The speed of rotation is obviously directly related to how fast you can switch current between the coils. At low speeds, there is no problem – but as the switching frequency increases, we can start to get into difficulties. Loss of power At low speeds, most stepper motors are generally quite powerful devices – ie, lots of torque – especially when driven by an appropriate supply. However, most hobbyists tend to drive a stepper motor from a fixed voltage supply. This is fine at low speeds (low frequency) but as the speed increases, the torque drops off, often dramatically, due to the impedance of the coils. There are three main methods used The stepper motor controller is shown here with a typical 6-wire disposals stepper motor. www.siliconchip.com.au Fig.1: three ICs, four Mosfets and a handful of other components make up the controller. It can be use in “stand alone” or computer-controlled modes. to overcome this reduction in torque: 1 – The use of a higher voltage switched-mode power supply that increases the duty cycle at higher motor speeds. 2 – The use of a higher voltage power supply along with a power (ballast) resistor in series with the motor. The resistor limits the current to that of the nominal motor current. 3 – The use of a constant current source that maintains constant current to the motor at all speeds. As the speed increases, the voltage also increases. There are disadvantages in the first two methods. Switched-mode (some times called "chopper") power supplies need to be carefully tailored to suit the specific motor being used and its torque curve. So it’s hard to make a “universal” supply. The ballast-resistor method results in less current being applied at higher speeds and is therefore less efficient. However, the use of a constant current source ensures the motor current remains constant throughout its speed range. Although this method is perhaps wasteful compared to the "chopper" due to the heat generated in the driver it requires little or no setup. In practice this type of circuit is already proven and is in common use in industrial drivers. How it works This circuit can be used in either free-standing or computer-controlled K&W HEATSINK EXTRUSION. SEE OUR WEBSITE FOR THE COMPLETE OFF THE SHELF RANGE. www.siliconchip.com.au May 2002  77 Parts List - Stepper Motor Controller 1 PC board, 72 x 42mm, coded K179 (Oatley Electronics) 2 3-way PC-mount terminal blocks 1 2-way PC-mount terminal block 1 4-way PC-mount header pin set (or 4 PC stakes) Semiconductors 1 4093 IC(IC1) 1 4030 (IC2) 1 4013 (IC3) 1 7805 5V regulator (REG1) 4 IRFZ44N Mosfets (Q1-Q4) Capacitors 1 100µF 35VW 1 100µF 10VW 1 1µF 20VW 2 0.1µF polyester Resistors (0.25W, 5%) 1 10kΩ 1 1MΩ PC-mounting preset pot modes. In the free-standing mode, an internal square-wave oscillator based on IC1b supplies timing pulses to the “OSC” output. The frequency of these pulses (and therefore stepper motor speed) is controlled by preset pot, VR1. (A standard 1M linear pot could be substituted to allow external speed control at any time). Either the oscillator pulses or control pulses from a computer are fed into the “STEP” input which in turn are buffered and inverted by IC1d, a 4093 Schmitt trigger. This helps prevent false triggering. Similarly, IC1c buffers and inverts the “DIR” (direction) input which once again can be either manually set or taken from a PC. Taking the DIR input to +5V causes the stepper to turn in one direction; taking it to GND will reverse the rotation. IC2c and IC2d (4030 exclusive OR gates) invert the outputs available at the Q and Q-bar outputs of each of the flip-flops, IC3a and IC3b. The incoming step pulses clock the flip-flops thus toggling the Q and Q-bar outputs, this turns the MOSFETs on or off in sequence. The IRFZ44 MOSFETs have a very low on resistance and can deliver 5 or 6A each without heat sinking. For higher current use, small clipon heatsinks could be used or the 78  Silicon Chip Fig.2: be careful when soldering in the ICs: the tracks are closely spaced! MOSFETs could be removed from the PC board and placed on a larger heatsink. The power supply is a conventional circuit using a 7805 3-terminal regulator, producing 5V output. A minimum of about 8 volts DC is required at the input and the maximum (limited by the 7805 rating) is 35V. Construction All components mount on a single PC board measuring 72 x 42mm, coded K179. This board is only available as part of a complete kit from Oatley Electronics. It is perhaps easier with this board to depart from normal practice and solder in the three ICs first. The reason for this is that there are tracks going between the IC pin pads and these will require very careful soldering and checking. Make sure you get the three ICs in their right places and the orientation is correct (all three face the same way). Next, solder in the resistors and capacitors and use some of the lead cut-offs for the three links. Solder in the header pins, the on-board terminal blocks and finally the regulator and the four Mosfets. Note that the Mosfets are NOT oriented all the same way – and it’s important to keep their drains (the metal tabs) separated from each other, especially if you fit heatsinks. (If you decide to mount the Mosfets on a larger heatsink for more power capability, as mentioned above, you will need to fit insulating washers and bushes to each Mosfet to ensure they are electrically isolated from each other). Before use, check and double check your component placement and soldering – especially the ICs as noted above. In use It’s outside the scope of this article to go into much detail. It’s sort-of like “if we need to explain then you shouldn’t be doing it!” However, a quick note on using surplus steppers: as you can see from the circuit diagram, the windings on most of the steppers you will come across are centre-tapped. This means you can usually identify the pairs with a multimeter, as well as working out which is the centre tap. Having got that far, connect up the circuit with the “OSC” and “STEP” pins shorted to each other, plus the “DIR” and “GND” pins to each other. Connect the centre-taps of your stepper motor windings to the V+ terminals and their pairs to the M1B and M1A, M2A and M2B terminals as appropriate. Set VR1 to half-way. Apply power and see if your stepper is continuously turning. If not (eg, if it is “hunting”), swap the M1B and M1A windings only (leave the M2A and M2B) and check again. Now it should be turning. Varying VR1 should vary the speed up and down. If it doesn’t work, check to see if IC1b is oscillating (an analog meter on a low voltage [<10V] setting connected between OSC and GND should show up and down deflection, especially with VR1 set to its maximum). If so, check the voltage between each of the motor terminals and ground with your meter set to a bit higher (say <50V) and see if the meter deflects. At the oscillator’s higher speed range, you probably won’t see any movement – the meter will read the average voltage. If this test proves OK, you probably have a dud stepper motor! Computer control There is quite a range of stepper motor controller freeware available on the ’net. Google “Stepper Motor Software” or words to that effect and see what you come up with. We have given a few sources in this article but www.siliconchip.com.au Software Here are just a few of the demoware or shareware downloads available from the net. Name: Download from: Runs under: Number of axes: Features: Imports: Name: Download from: Runs under: Number of axes: Features: Loads: Imports: Exports: DANCAD www.metalworking.com DOS up to 4 Extensive printable manual with printer port connection diaggrams etc. Able to be configured to suit most applications, (lathe and mill etc.) including angular and linear axis set up. Dancad is able to be set up with a tangential knife for sign cutting. HPGL KCAM www.kellyware.com/index.shtml WIN9X. 3 It gives a 2D or 3D view of the Item to be machined, manual jogging, controller and table setup to suit most machine tables including backlash compensation. Tool paths can be programmed in its Gcode editor, or imported as DXF, NC, and PLT files. Ideal for engraving signs and plates, drill printed circuit boards, mill parts, plasma cuts, PCB Isolation. Conversion from Gerber (RS274X) files or plot pictures. G&M code files, Excellon ASCII drill files, DXF files DXF, HPGL files PLT, Gerber files GB0 G&M code files Name: Download from: Runs under: Number of axes: Features: STEPSTER www.thegallos.com/stepster.htm DOS up to 6 Simple to use and set up, Able to be configured to suit most applications, (lathe and mill etc.), including angular and linear axis set up. Name: Download from: Runs under: Number of axes: Features: EMC www.isd.mel.nist.gov/projects/emc/ LINUX up to 6 Hard to set up. Able to be configured to suit most applications (lathe and mill etc.), including angular and linear axis set up. there are lots (and lots!) more. Most of the software available uses the same connections to your PC’s printer port: Pin Function 2 X axis step 3 X axis direction 4 Y axis step 5 Y axis direction 6 Z axis step 7 Z axis direction 8 C axis step 9 C axis direction 18-25 GND The other printer port pins vary according to the particular software – www.siliconchip.com.au they are often used for limit and home switches. Depending on the software used up to 6 motors (with 6 controller boards) can be controlled just by connecting the stepper drivers to the printer port of your computer. In other words, complete three-directional control is possible (we hope to have more on this in a future issue). Where do you get it? This project is available as a complete kit of parts from Oatley Electronics. Contact details can be found on SC page 15 of this issue. May 2002  79