Buildlog.Net Blog

Step Pulse Extender – PSoC Style

The TB6600 stepper drivers I have in the X-Controller have a “Torque” feature.  You set the motor current with a reference voltage and the torque feature allows you to easily switch between that current and 1/3 third of it. This is typically used for an idle current reduction feature at the system level.

Why Idle Reduction?

Stepper motors have a lot of hold torque, but that torque quickly falls off with speed.   Therefore you typically size a stepper motor and set the current for your maximum cut or rapid speed.  This means your motors will have excess torque when idle and will tend to run hottest at idle.  You basically the the current as high as possible until the motors get too hot.  If you could reduce the current at idle, you would reduce the temperature and could set the current higher than normal when spinning.

This is great, but the machine will never be in idle during a long job.  At least one of the motors should always be running. If you could figure out when each individual motor was idle, you handle each motor independently.  That is not easy in firmware, but there are tricks to do it in hardware.  You could tie the feature to the step pulse.  Whenever the step pulse is active, the full torque could be active.  That has two problems. The step pulse is extremely short, in the range of a few microseconds.  The other is you might want the current high for a a short bit after the motor goes idle just to make sure the machine is stable in the new position.

The trick is to use the step pulse, but extend it to the desired duration.  It should stay on through all the step pulses and extend the last pulse.

Discrete Hardware Solution

The X-Controller uses a discrete logic chip to do this. It uses a retriggerable monostable vibrator (74HC123D).  The R/C circuit on the right of the schematic snippet sets the duration. It works great, but this adds a lot of parts and things are locked down and not easily adjustable. If you needed to override this function, you have to break out the soldering iron.

PSoC Solution

With PSoC, when you hear “discrete logic” you should know there is probably a good way to do it on the chip. In this case I designed a custom component using verilog.

The verilog code is quite simple.  The best part is none of this is done on the CPU, so there is no impact on the motion control performance. What the video to see the details.