Retro fitting a Closed Loop Drive Board to a Type 34 stepper motor

[GreensandsParticipant @greensands]

I am currently running a vintage 3 axis Open Loop CNC system fitted with Type 34 stepper motors activated from a parallel port Mach3 compatible controller but am suffering from the occasional lost of steps especially when cutting under an intermittent load. Although I would be the first to admit that under normal circumstances this does not present a problem there has been a recent case where the loss of steps has proved to be an issue and I have begun to think in terms of possibly substituting a closed loop system. Googling the problem I have discovered that it is possible to buy and retro fit a so called Closed Loop Drive Board to certain type of standard stepper motors and have wondered if anyone has actually tried doing this on a type 34 stepper. The boards appear to be readily available from China but a UK source would be preferable. All views and comments most welcomed

[GreensandsParticipant @greensands]

[John HaineParticipant @johnhaine32865]

I think you need encoders on the motors to close the loop?

[EmgeeParticipant @emgee]

Have you tried tuning the motors to get optimum torque without losing steps ?

You could try reducing the acceleration speed which will help reduce loss of steps in some instances.

You mention 3 axis so is this a milling machine we are discussing ?

Emgee

[John HaineParticipant @johnhaine32865]

If you mean this type of device:

**LINK**

…then they look intriguing and I hadn't come across them before. Essentially they are a board with a stepper driver plus a rotary encoder chip, of the type now available that can sense the rotational position of a small magnet on a shaft held just above the chip. The chip seems to be mounted on the centre of the track side of the PCB and the board is fixed to the back of the motor. The motor therefore needs to have a shaft extension at the back for the magnet to fit on. Joe Noci has written about these sensors on another thread. The boards completely substitute the standard drivers and in effect give you a closed loop stepper servo.

If your motors have a rear shaft extension then you could try one, but if not and you'd need to change the motors anyway it would be safer I think to invest in a servo or stepper-servo which would probably be about the same cost as one of these boards plus a new motor. All the "system integration" would be done for you.

As Emgee says though, if your problem is just occasional lost steps then maybe some mechanical attention to the machine and motor tuning would do the job.  What machine & control system is it?

Edited By John Haine on 28/04/2023 06:31:12

[Martin ConnellyParticipant @martinconnelly55370]

I would want a very well written data sheet for the board to read before I bought it. There are a number of different resolution encoder chips and I would want to make sure it had a fine enough resolution to match the motor and purpose it was to be used for and also that it was compatible with what you have already. If you have a 1.8° (200 PPR) stepper and a 1.4° (64 PPR, 256 edge, 8 bit) encoder chip will that cause problems?

If this is a new problem on an old system then what has changed?

Consider dumping the parallel port if you can. How do you know that this very old piece of technology is still putting out good enough signals to be sure it is not the source of the missed steps. Some of them were a bit close to the acceptable voltage limits even when new. It doesn't take much to interfere with the pulse signals from a parallel port. They were not designed for CNC signals after all. Also if you use these boards then there will be options for detecting lost steps or the magnet falling off the shaft. Do you have enough inputs on a parallel port system to respond to these error signals to stop Mach3 from carrying on as if nothing has gone wrong?

I would be wary of spending money on these boards before checking everything else in the system first. Start with the mechanical setup as suggested by John. Have you got an alternative computer to drive the parallel port for the purposes of checking if that improves the system? Can you decrease the acceleration and top speed of the axes in Mach 3 without causing problems. Could you upgrade to a well proven setup such as a Smooth Stepper? Could you use a gearbox on the stepper output to improve torque at the machine. What about the cost of matched closed loop motors and drivers compared to mixing and matching bits from different manufacturers in the hope they will cure the problem. Lots of things to consider before spending on these boards. I can just imagine having a problem and the board supplier saying speak to the manufacturer and the board manufacturer saying not our board that is the problem, speak to the motor manufacturer. What if one of them says it was not designed to be used the way you are using it? Bit of a minefield that you are going into as you may become the research and development department for other people and we all know they are an expensive part of any group. Have you found anyone else who has gone along this path?

You have mentioned cutting and 3 axes so CNC milling seems a likely use. Are you using climb milling or conventional milling. Conventional requires more torque from the axis motors but requires ball screws for low backlash. Something to consider is that if you use larger diameter tools, tougher material or higher material removal rates than the original machine design was meant for the higher the torque required from the motors. Have you put a larger vice or rotary table on the machine. These increase inertia and friction. Have you changed from sharp HSS or carbide to relatively blunt insert cutters? These increase the required torque from the stepper motors. Have you adjusted the gibs recently and set them too tight? Is there enough lubrication?

Martin C

[GreensandsParticipant @greensands]

The link referred to is the one I found but I don't think the boards in question would be the answer in my case as my current motors do not have the necessary shaft extension. Additional costs and aggravation would not make any change worthwhile and as said in the initial posting it is not really a big issue with the present (milling) setup but interesting never the less to know what is out there.

[Ronald MorrisonParticipant @ronaldmorrison29248]

Before you spend a lot of money changing the stepper motors, think of a cheap solution. Steppers lose steps because they don't have quite enough power to make the full step and then fall back to the previous step. If you are only losing a few steps occasionally, think how to increase the power slightly. I simply increased the power supply voltage a little to get rid of the missed steps.

[John HaineParticipant @johnhaine32865]

Also you may be over-ambitious with microstepping. The torque increment when you apply a microstep can be very small and not enough to move the screw. Trying to improve resolution by setting the microstepping too high can be self defeating.

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