Stepper Motor Driver

I'm currently using a Gecko G540 drive module to drive the stepper motors. This module has the ability to drive up to 4 stepper motors. They label the connections as XYZA -- we'll connect the C-axis stepper to the A output and leave the Y output unconnected.

Gecko provides a wealth of technical information about stepper motors in general and their products in particular. Take the time to download their information, read it, and understand it.

There are some nice things about this driver. It has the ability to provide 10 micro-steps for every full step of the motor, thus increasing the smoothness of cuts by 10X. It also has an adjustment to smooth the motion of the motor at slow speeds. This works particularly well because we tend to use the motors in this slow range where smoothness is important.

There are some things that I don't like about the G540:

• Their spec sheet says the inputs (for connection to the limit switches) are 5-volt (TTL) compatible. This is incorrect! The G540 tries to pull the inputs up to 12 volts from an internal supply. If your limit switches are not open circuit (or open collector able to withstand a higher voltage) then you will need some additional interface circuitry (see below).
• The G540 automatically goes into a power-saving mode where it reduces the motor current by 70% whenever the motor is idle more than 2 seconds. On the one hand, this is nice to keep your motors cool. However, there is a slight "twitch" in the motor when it is suddenly switched to a lower holding current. This can leave a line in your cut work. It can easily happen when you move the cutter into the work (Z-axis, for example) and then start cutting a rosette pattern. The Z-axis won't move for a while and the G540 "helps" you by reducing the current to the Z motor. You can't turn off this option.

The G540 is giving me good results, but when I get some extra cash I'll look for a different driver.

Emergency Stop Switch

Get yourself a big red ESTOP switch (I found them on eBay) and wire it to the driver. This shuts down power to the motors in a hurry when something goes wrong. Crashing into your work will not only damage the work, but can easily cause expensive damage to the XZ stage. My last "crash" caused about $200 worth of damage (I didn't hit the switch fast enough).

Limit Switches

Wire the 4 limit switches to the 4 inputs of the G540. If you have mechanical limit switches, optical slot sensors, or open collector sensors you can wire them safely to the G540 inputs (provided they can tolerate 12 volts on open circuit).

Traditional wisdom is to have the polarity of your sensors so that a broken cable will give you a limit error. Thus, you would want a "normally closed" limit switch that is "open" in the event of a limit situation. With an active sensor, a "normally low" output is desired and "open" in the limit event.

If you need power for your active sensors, look for a 5 volt "wall wart" on eBay. Caution -- check that it actually works and gives the correct voltage and polarity!

In my case, I had active sensors that were not "open collector" but were pulled to 5 volts when high. The only way I could make an interface to the G540 is with a circuit that presented a 12 volt tolerant open-collector signal to the G540. This was easily made with a single 74F06 hex inverter (open collector). I also needed a 5 volt supply (black wall wart) for the sensors and logic circuit. The parts were bought from DigiKey.