Selecting the motors

By | July 12, 2007

Typical size 17 frame stepper motors For general ease of construction and simplicity for the new mouse builder, stepper motors are hard to beat. They are also generally large and heavy. A bit of a search soon turns up a variety of possible motors for Primus.Ideally, we would have a small motor to allow the construction of a sleek, compact mouse. However, these motors, while available, tend to be harder to find, harder to find wheels for and significantly more expensive.It is no coincidence that many mice use the Sanyo-Denki 103H546 stepper motor. It is economical, easy to find and remarkably powerful.A very sketchy data sheet can be had from most of the online sellers of this motor. Farnell have a slightly better one at:http://www.farnell.com/datasheets/79514.pdfThe motor is in a size 17 frame which essentially means it looks like a metal box 44mm x 44mm x 32mm deep and has an output shaft 5mm diameter. In the UK, the easiest and (I think) cheapest supplier is Active Robots (www.active-robots.co.uk). They also have a wheel specially designed to fit this motor. the same motor and wheel combination is used on the AIRAT micromouse kit that they sell.The specification for the motor is

Step Angle Voltage Current Holding Torque Mass
1.8° 3.15V 1A 0.147Nm 0.2kg

Thus we get 200 steps per revolution in single step mode. The hugh current is what gives it its torque. However this, coupled with the low rated voltage means we are going to have to use a special driver chip todrive it. We could just connect it up to a simple driver running at 5V but we would not get terribly good performance. So long as a suitable driver chip is employed, we can use as high a voltage as we can get from the batteries we will have to carry.Two of these are going to contribute 400g to the robot mass all on their own. Fear not though, the motors have plenty of power for all that weight. In fact, at low speeds. the motors would be capable of accelerations of well over 6m/s/s. We will probably not get anywhere near that.The wheels have rims that are 9mm thick and have a diameter of about 50mm. Thus, the mouse can not be any less than (10+32+32+10) = 84mm wide. This is getting a bit big to be running the diagonals but it can be done. When you get to the diagonal running stage, you will probably be happy to find and pay for those smaller motors I was talking about earlier.The motors plus wheels will cost you £58.53 including delivery from Active Robots at the time this article was writte. Order part numbers:

[H546P] NEMA 17 2-phase stepper motor
[MRM-006] Aluminium Competition Robot Wheels

The same items are available from lots of suppliers around the world.

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