Not another Arduino or Pi question!! (Relax - I'm not that annoying, honest)

Question

Before you all shout at me, I've done some digging, read similar questions on here and have actually made up my mind. This is really just to see if I made the best choice.

I've been digging around looking for a way to recreate a vehicle dashboard using Switec X27.168 steppers (nice low 5V and 20mA/coil with 315degree swing).

I initially looked down the route of the Laika Explorer but soon realised it was only capable of driving 1 stepper per board! Next I was looking at stepper driver boards for the Pi - I already have an old Model B lying around.

The more I looked and the more I read, the more I became convinced that the Microcontroller basis of the Arduino (Uno in particular) was the best option.

I also discovered that the steppers I'm using can be driven directly by the Uno - no HAT required (only 20mA/coil - source current is 40mA max according to specs); essentially if I protect the outputs with diodes then I'm golden (plus the Uno can drive 3 of them)!

The final nail in the coffin of trying to do it with a Pi was I found a nice resource of driver libraries for the motor I'm using - all for Arduino - and that does kinda help A LOT as I can learn what the code is doing and manipulate it instead of trying to figure out how to code it from scratch.

So, in summary, with the 6 available analogue inputs (I can use F/V convertors to turn the Revs / Speed into corresponding voltage levels - or maybe there is a way to use the frequency directly into Uno - I'll have to dig some more there), pre-made libraries and a proven application for exactly this, I'm hoping the decision I came to is the best one?

Any thoughts all?

Anyone?

Regards

shabaz · Accepted Answer

Hi Andrew,

Using the Arduino for this sounds like it should work!

You could use I2C for the comms between Pi and Arduino (with pull-ups to 3.3V, not 5V), some information on this is here:

https://blog.adafruit.com/2014/02/21/connecting-an-arduino-to-a-raspberry-pi-using-i2c-raspberry_pi-piday-raspberrypi/

jc2048 · Answer

According to the datasheet, the high output for the Atmel part on the Arduino will sit down to about 4.5V if you draw 20mA from it. So if your circuit is single-ended (coil down to ground), you'll be operating on 4.5V, not the 5V that you would think. [To be picky about it, the lower voltage will then reduce the current, so you'll probably end up with it balancing out at something like 4.6V and 18mA.] Might not matter, but it may have at least some effect on the rate at which you can step. (You're already at the low end of the voltage range of 5V-9V for the motor.)

If you use the motor in a bridge (which you might have to, depending on how the coils are arranged), with an IO pin at each end of each coil, the other end that's being taken low to GND will rise up to about 0.5V sinking 20mA, so then you'll only have 4V across the coil. [Again, a little more, probably 4.2V and 16mA.] Doubt you will be able to swing it around as fast as was originally intended [without missing steps], but try it and see. As long as you protect the IO pins from going outside the rails, you're not going to do too much harm with a 260 ohm coil and the Ardino is a nice easy thing to experiment with (says me, never having used one).

jc2048 · Answer

I was looking at this (which I found by searching). Is that the same one?

http://www.jukenswisstech.com/JSTFiles/downloads/2011/06/X27_Flyer_v1.3.pdf

If it is the one, I was wrong about the voltage range [you'd think by my age I could read a datasheet properly, wouldn't you] - it's 5V typical, 9V max.

It looks like a nice thing to play with.

jc2048 · Answer

That 'motor shield' you link to isn't a driver. It's the motor and eight diodes (for protecting the four IO pins) on a board ready to connect. So if it works for them connected to an Arduino, it'll work for you.

This page has a link to a much better datasheet. It's got curves of the torque versus the voltage, so you can see what you lose when you move down to 4V rather than 5V.

http://guy.carpenter.id.au/gaugette/resources/