THC for Mach3 - full external Z control vs Mach Z control

I'm on the design path again, this time to build a plasma torch height controller for Mach3.

I'm going to start with letting Mach do the Z movements so my THC will simply monitor the torch volts and send the up/down signals to Machs input pins.

I've heard a certain manufacturer of a THC say Machs built in THC was too slow (???? no more details). His model fully controls the Z motor, leaving Mach to just do the XY moves during the cut.

Does anyone have any actual experience with one verses the other. What shortcomings would a THC have when it used Mach to do the Z moves. Has anyone came across this way being too slow as just mentioned.

I know Machs THC does not have acceleration / deceleration on the Z moves, and this can limit the speed at which a stepper can operate. Try to make a stepper go a certain speed instantaniously and it stalls / loses steps. Would the closed loop of a servo motor help in this regard because it can catch up to the steps commanded.

Cheers,

Keith.

The MACH3 THC logic running with the parallel port is not "too Slow" for cutting thicker plate (14ga and thicker) that is not badly warped. The THC coupled with the Z control from MACH is a closed loop. As in any loop a delay interjects problems. So while not large via the parallel port it is there. Then because most THC moves are short and fast there is little time for acceleration and deceleration so it creates a "Bang-band" type of loop control (its either full speed in a direction or off) Those two factors result in a lot of instability in the loop that precludes moving the Z very fast. About 20 to 30% for the normal Z velocity is about all you can expect.(maybe less with a course Z leadscrew) That is the case if your input signals are fast and turn off quickly, or the instability gets worse. Inputs from relays are much slower than something like an Opto Isolator In a true servo control loop you have what is called PID that provides proportional control to prevent the over shoot and stabilize the loop and allow much faster response

The option of having the THC outside MACH presents added problems because a lot of the required THC functions (like the Touch-off , move to pierce height , move to cut height, and lift at the end, along with ARC OK sense, have to be moved to the THC itself. Basically its a 2 axis machine with a standalone Torch height control and lifter. In short you lose the toolpath aware feature of having MACH involved and any other THC related moves.

There are over a dozen MACH based THC options from cheap to expensive . Before you ask several of the external pulse card providers for MACH have added "THC inputs" that directly send pulses to the Z . It solves none of the inherent problems and in some cases makes them worse

TOMcaudle
www.Candcnc.com

To add to your post. You can get slightly better performance from a servo on Z with the conventional MACH THC because your velocity is higher so even though the % of velocity under THC won't go up much the overall speed can. You can get on the top end about 60IPM of Z speeds with a carefully tuned servo on Z.

Thanks very much Tom,

I'll stick with making a basic THC that sends the up/down signals to Mach for now. May try something a bit more ambitious later on and get into external PID control. With a servo motor for Z, a Lava parallel port card, and high speed optocouplers getting the signal to Mach, it should be about the best I can get with such a system.

Cheers,

Keith.