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Moving Parts – Part 1: Stepper motors

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After the new Atmel ATMega was installed yesterday, I’m now ready to experiment with some stepper motors. First out, the Mercury Motor SM-42BYG011-25, a medium sized motor with quite a bit of torque and also half decent speed. Second, the smaller but much faster Mercury Motor ST-PM35-15-11C. Both driven by the small EasyDriver card, which really lives up to its name. After soldering on a few pins, it was literally plug & play. The eight-line example to get started could not be simpler.

There are of course plenty of experiments and tuning which can be done with this motors and the driver alone, so initially I had a look at the RPM speed. Here I was slightly puzzled to find that both performed very well, even though comments on the product page suggested the speeds I saw would be hard to reach. So if my calculations are off, please do add a comment below.

When using a 12 V power supply, and the dead simple example of write, delay, write delay (or rather delayMicroseconds()), I found that I could go as low as 90 microseconds with the bigger motor, or 180 µs for a complete pulse without it stalling. The example continues: “the Easy Driver will default to 1/8th microstep mode. That means that [for each pulse] the stepper motor will move 1/8th of a full step. So if your motor is 1.8 degrees per step, there will be 200 full steps per revolution, or 1600 microsteps per revolution”. At 180 µs per microstep, there is time for 5555 steps in a second, or 3.47 revolutions per seconds. That means 208 RPM. In summary:

1000000 µs / ( 2 * 80 µs ) / ( 360.0º / 1.8º * 8 ) * 60 = 208 RPM

Again, this seems surprisingly fast, and it’d be interesting to measure it using an external device. When I go down to 80 µs it starts to skip, and even lower it will stall.

For the smaller motor, I simply swapped the wires on the driver card (after the power supply had been turned off), and ran the same code. Here I could go as far down as 35 µs delay, or 70 µs per microstep. The motor has a step size of 7.5º, which means an RPM at 2230. It is indeed spinning very fast.

1000000 µs / ( 2 * 35 µs ) / ( 360.0º / 7.5º * 8 ) * 60 = 2230 RPM

Funny anecdote at the end: The smaller engine came pre-wired to a 4-pin plug, while the bigger simply had loose ends. When putting that together, I found myself comparing the two data sheets to get the correct wire order. Only problem being that the colour notation on the smaller was in Chinese. Luckily, the PDF had kept the text intact, including the Chinese characters. Copy/paste and Google Translate to the rescue, and I found which was which.

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Scavenging: Printer

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More scavenging of old parts, this time from an Epson ink printer. Trust me, if you get your hands on a used printer, it’s best to just throw it away. It’s the dirtiest computer junk there is, with all of the inside covered in ink. The motors in there are probably not worth more than 10 Euros. However, if you’re reading this, it might already be too late.

Stepper EM-438

The stepper motor for the paper looks solid. However, there is no documentation what so ever on it. I just became yet another guy asking for a data sheet. I did find one video from somebody who have gotten it to run, though.

DC RS-455PA-15200

The second motor was a simple DC motor. The data sheet was easy to find. It’s from Mabuchi Motor, has a 12 – 42 V operating range, and draws 330 mA at maximum efficiency.

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Scavenging: Scanner

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HP ScanJet 4500C

Why throw away things when you can keep them? Or at least get some useful parts!? :-) I recently found an old HP ScanJet 4500C which was about to be thrown away. After I had opened it and unscrewed every single screw, there was just some plastic left to get rid of. And some nice parts left for me.

The scanner included a document feeder as seen on the image above. Great, even more parts! In the end, I found two stepper motors, a two digit LCD display (with what seems to be a strange data interface), a few small IR blocking sensors, the scanning sensor, a very narrow focusing wide angle lens, lots of buttons, lots of plastic gears, and a unknown amount of screws. More details follow.

Mitsumi Stepping Motor M42SP-5

The stepping motor from the document feeder (right mote in the picture above) has the following text on its sticker:

Stepping Motor -  Mitsumi
M42SP-5        -  7.5° Step
3140-1093      -  10Ohm
T1C2522        -  Made in Taiwan

As far as I can tell from the various data sheets, it is a 24 V, 2 phase, unipolar driving stepper motor. There’s a bit of discussion about whether it has 4 or 5 wires or is bi or unipolar, however it it is difficult to tell when the controller board is mounted on top. Consensus seems to be to remove that though. The torque seems quite high, so I hope I can fit it into one of my projects.

Neocene Stepper Motor 2T4242

The second motor (to the left in the picture above) was from the scanner, moving the scanning head back and forth. As can be seen from the image above, it already has two gears neatly attached. I haven’t counted, however I’m guessing the gear ratio is more than 1:150 to the top gear. It has four wires coming out, and has the following on the sticker:

Neocene 2T424217
4 Ohm

The motor code number is explained on the Neocene web site. In this case, 2T indicates it is a PM-type stepper motor; 42 is the outer diameter; 4 is the step angle code; 2 is the hight code; and 17 is a serial number. It means it is 8.3 mm high, and has a step angle of 3.75. From the data sheet, it  is a 12 V motor with 4 Ohm resistance. The excitation is bipolar 1-2.

Penguin LCD

From the control panel of the scanner, I got a small two digit LCD. It has 9 legs, and by applying voltage to different pairs of legs, I can turn on what seems to be predefined segments of the two digits. I guess I’ll have to create the map of legs to segments, and see what I can draw. Maybe there is a more clever way to drive it. However, I don’t have any specific enough search terms to go on.

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Motors and robotics

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Looking at the 12 V stepper motor at SparkFun, I came across a few other fun links. Including instructions for stepper motor wiring, a DIY surveillance camera, and an example project for the Arudino, using an EasyDriver Stepper Motor Driver. Also interesting, were the hardware suppliers: here’s some clamping shaft couplers from ServoCity, and lots more from McMaster-Carr.

Finally, I was eyeing up on of the robotics kits at SparkFun: “The POP-BOT is an Arduino compatible, mobile robotic platform. It comes complete with wheels, motors, sensors, software, documentation, etc. The POP-168’s pin-out is similar to the Arduino Stamp“.

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