05-03-2008, 02:19 PM #1
- Join Date
- Feb 2007
- California, USA
Stepping Motors In Simulated Instruments
Steam-gauge style instruments used in hobby flight simulators are generally based on air-core movements, RC servos, or stepping motors. This tutorial is an overview of stepping motors.
There are several varieties of stepping motors. In this tutorial I’ll be referring to the permanent magnet variety. These motors provide easily controlled, low to moderate RPM rotary motion. They are widely used in printers, scanners, early generation PC hard drives, floppy drives, VCRs… The list goes on; these ubiquitous little beasties are cheap and versatile! You can use them to make all manner of simulated aircraft instruments.
There are two types of permanent magnet stepping motors. The gray round motors pictured below are “can-stack” or “tin-can” motors, so called because they are made of stamped sheet metal cans which are stacked and spot welded together. The cans contain bobbins for the field windings and a multi-pole ceramic magnet rotor. It’s an easily built motor that doesn’t cost much. Can-stack motors generally provide 10 to 48 full steps per revolution, depending on model.
The black square motor is referred to a “hybrid” stepping motor because it combines a permanent magnet rotor with the construction and field windings of a synchronous AC induction motor. It costs more to build but has greater positioning accuracy. Hybrid stepping motors typically provide 100 to 200 full steps per revolution. A few models offer 400 or even 800 full steps.
The next picture illustrates the basics of stepping. A two pole magnetic rotor is progressively stepped through 90 degree steps by alternately reversing the magnetic polarity of opposing field poles. The field poles are magnetized by (invisible) windings. Switching the direction of the currents through those windings reverses the magnetic polarity of the field poles.
The hybrid and can-stack stepping motors are more complex inside, but they are stepped in exactly the same manner. They key difference is that the resulting step size is much smaller
The fact that a stepping motor only steps when the field current is switched leads to some interesting and useful results. If we don’t switch the current, the rotor stays locked in its current position. If we switch the currents faster, the rotor spins faster. If we change the order in which the currents are switched, the direction of rotation changes. If we count the number of times the current switches, we know how far the rotor has turned.
By using slightly more complex switching waveforms than the square waves pictured above, we can half-step and even micro-step the rotor leading to smaller step sizes. More detail on that (and other stepping motor topics) can be found in “Control of Stepping Motors” by Douglas Jones (http://www.cs.uiowa.edu/~jones/step/).
An inexpensive micro controller like a Microchip PIC16F648A is an excellent motor controller chip. It provides all the logic needed to put a stepping motor through its paces. It can’t directly power the motor, but it can control an H-bridge power driver like an L293D which can. The PIC16F648A has serial communication capability built in. With the addition of a data transceiver chip you now have a three chip solution for interfacing and controlling a stepping motor from your PC.
If you were to mount a compass rose disk directly to the shaft of a 400 step/rev hybrid stepping motor, you’d have an easily assembled direction gyroscope or vertical card compass. The motor bearings are strong enough that no additional support for the compass rose is needed. If you half-stepped the motor, the compass rose would move in 0.45 degree increments.
There’s nothing that says you should use stepping motors only to make instruments that require high torque. You could simply put a pointer on the shaft. This seems like massive overkill; a 3 gram pointer certainly doesn’t need the available torque, but there’s a lot to be said for ease of construction. One pointer, one motor, three chips, some fiddly stuff, and let’s go fly
Stepping motors are particularly useful in more complex projects. The torque and tight positioning control of these motors are very welcome when gearing losses and friction from concentric shafts need to be overcome.
Stepping motors can be salvaged from defunct computer and entertainment products. They are often sold by surplus electronics vendors like All Electronics (www.allelectronics.com), The Electronics Goldmine (www.goldmine-elec.com), BG Micro (www.bgmicro.com), MPJA Online (www.mpja.com), and Alltronics (www.alltronics.com). They are also sold by electronic parts suppliers like Jameco Electronics (www.jameco.com), and motor suppliers like Anaheim Automation (www.anaheimautomation.com).
There is a geared stepping motor being used in automotive instrument clusters may be of interest. The Switec M-S Motor (http://www.switec.ch/products/switec/switec.html) is an indicator driver that has 1/3 degree resolution. It’s primarily an OEM component with limited retail availability. However, there are a few after-market instrument repair businesses. I’ve notice a couple such companies marketing repair parts on Ebay. Search on “stepper motor” or look for the company names “Speedometer Sales and Service”, and “Digital Speedometer Repair Service”. Caveat: I don’t know these people and am not recommending them, merely pointing out that the items are of interest.
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