zaterdag 16 november 2013

Stepper motor with slider

I bought five of these on AliExpress. It's a stepper motor attached to a mechanism that makes a lineair movement very easy. I usually try to combine some plastic toy gears, or use a lever and a servo to create a linear actuator, but this seemed like a very neat idea. Too bad that usually these things don't come with a manual or a bit of technical data to get me started. However, this picture gave me enough info to know that I could definitely use it in a small project that will require four of these. I want to make a small platform that moves up and down, and these sliders could just do that.

Here's how it looks in action. This is not a live picture, but a small 360° animation I made while it was moving. But it's actually very close to the working speed of the motor. You can make it go faster, but then it loses a lot of its strength.

The tests I did were actually meant to see how much weight I would be able to move about when I use four of these. Although speed is of no importance for my project, I was also curious how fast it could move with no load attached to it.

In that case, it's a good idea to start the bipolar stepper motor at a lower speed and move it into the slew range (the shaded area in this graph). The highest speed was 18 cm/s, a point in the shaded area. I started at 10 cm/s (in start/stop range) and accelerated in a few steps.

For testing the strength of the mechanism (torque of the motor), I tried a very low speed of 2cm/s, a point far to the left in the start/stop area. The maximum load was 128g before it started skipping steps. This is pretty good for a small motor like this. My four motors would be able to lift 0.5 kg, which should be sufficient for my plans.

This is the result of the test:

The range of the slider is 5 cm. It takes 464 steps of the motor to complete one slide. This gives an accuracy of 0.11 mm per step. The windings have a resistance of 10 Ohm, the test was done with a 6V supply and at its maximum load, it drew about 450mA current from the power source, which would mean about 225mA per winding.

I used an Atmega8 microcontroller (compiled in Arduino environement) like this one and a L293D dual H-bridge to drive the motor. The scheme was exactly the same as in the previous link.

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