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View Full Version : The right voltage for my Stepper Motors in order not to lose steps



jhonansaro
09-12-2017, 05:44 PM
Hi there
I have put together a 3 axis router CNC machine. It is a 4 meter By 2 Meter long machine. The machine has 4 Leadshine stepper Motors (Model 86HS45 4.5 N.M --- 637 OZ-In --- NEMA 34), The motion system for Y and X axis is Helical rack and Pinion (spring loaded with 3:1 Pulleys). The Drivers are Leadshine M880A with Electrical Specification as follow:
Min current of 1.8 A --- and Max 7.8(5.6 RMS) A
Supply voltage --- Min 24 VDC --- Max 72 VDC --- and Typical 68 VDC
The Machine is working fine in Short Runs. It can actually cuts 16 mm MDF in one Pass without any Difficulty. But In Long runs (Over one Hour) For example when is Cutting a Relief Like a Lion Heads and so on, IT Loses some steps and when it moves to its zero Position it is off target by few mm mostly in X Direction. I am wondering what would be the probable cause or Causes?
I have some questions on its Voltage supply.

1. My power Supply is a Hittachi 32 A --- 57 VDC. I think I am OK with Current, But as far as the Voltage goes as you can see My Driver Typical Voltage requirements is 68 VDC, and my PS has a Max of 57 VDC, Could this be the cause of the problem?

2. Can I add a 10 or 15 VDC --- 10 A in Series with my Original PS? And If I do so, Wouldn’t this sort of burns my Smaller PS?

3. I am also considering of Upgrading my motors to a Higher NEMA 34, Like 86HS85 with holding Torque of 8.5 N.M --- (1203 Oz-In ), On this issue what do you think of Stepper Motors with Brakes? Are there any better for my machine? And How about the Closed loop Steppers? And If I use them do I need to change my BOB?

Regards
Jhonansaro

Doddy
09-12-2017, 08:07 PM
If you don't mind an external link, http://www.machinedesign.com/motorsdrives/misconceptions-about-stepper-motors-explained (read the whole page) offers a good insight.

m_c
09-12-2017, 10:23 PM
If it's always off in the same direction, you need to check your step/direction signal timing.

If you have the timing wrong, then every time the axis changes direction, it can move one step in the wrong direction, which will accumulate over long jobs.

Neale
10-12-2017, 12:01 AM
I would agree with the last comment, but I would also check that the step signal polarity is correct as well. You don't say what your motion controller/BOB is, but particularly if you are using differential signalling (which is a good idea if your hardware supports it) it is possible to get this wrong. However, you do need to find out if your missed steps are coming from hardware or electrical problems. Generally, hardware/mechanical problems are worse with increasing load while load doesn't make much difference to electrical problems. So, try air-cutting with a short piece of gcode that just moves one axis backwards and forwards say, 50 or 100 times and check that the axis returns to the same position at the end. I used a dial gauge to check - the gcode moved the axis off the gauge, did the back and return moves away from the dial gauge, then moved back to the original position to check the gauge reading. Keep the speed and acceleration down to avoid mechanical issues. If this does not show any problems, then either your speed and/or acceleration are causing missed steps by overloading the motors, or there is some other mechanical issue like a slipping coupler or something like that.

Good luck - this kind of thing can be difficult to diagnose but check systematically and carefully, trying to check just one thing at a time. I had exactly this kind of problem with my Z axis. I didn't see any problems with "normal" cutting where Z only changed direction a few times, but when I started doing intricate 3D work the zero position gradually drifted. In my case, it was the step signal polarity, but the symptoms could have been something like a slipping coupler. It was only careful checking that found the problem.

jhonansaro
10-12-2017, 07:50 PM
Hi Neale and m_C

Thanks a lot for your valuable comments.
m_c : as matter of fact it is almost off always in same direction. Could you please explain How can I check
for my step/direction signal timing?

also Neale could you tell me how can I check for my correct step signal polarity?
I will also try that air cutting that you mentioned

Regards
jhonansaro

Neale
10-12-2017, 08:35 PM
In the M880A manual, it says that the minimum step pulse length is 1.5us and on direction change, the direction signal should change at least 5us before the next step pulse. There are two pieces of information that are very important here that you have not given us. First of all, what motion control software are you using? Typically, this is going to be Mach3 or LinuxCNC but you might be using something else. Secondly, how does the motion control software connect to the stepper drivers? This might be parallel port to breakout board (BOB), or USB or Ethernet to a motion control board. The signal timings are generated by Mach3 or LCNC if you are using a parallel port, and by the motion control board if you are using one. Without knowing what these are, we cannot say much more at this stage. Typically, if there is a pulse timing issue, it is because the motion controller does not wait long enough after changing the direction signal before it starts sending step pulses. However, the M880A is pretty fast according to the manual, so I would be surprised if the default settings for whatever you are using are not OK. However, as I say, without knowing what you are using and if it has been changed from the default, this is all just guesswork.

If the step pulse polarity is wrong, then generally you are going to see a drift of position in one direction only. It is not easy to test this directly - you can measure the "drift" but that doesn't tell you where it is coming from. One way to check is to change the step pulse polarity (which is fairly easy to do in the "Ports and Pins"->"Motor Outputs" page in Mach3, and the equivalent in LCNC but I can't now remember exactly how you get there) and see if the problem goes away. Alternatively, what I found on my own machine when I had this problem was that I lost the equivalent of one step pulse every time there was a direction change from + to -. On my machine, one step pulse is about 0.003mm. By using a dial gauge to measure the position of the axis, I could run gcode that just went backwards and forwards, and I found that the position error after X direction changes + to - was X times 0.003mm. In other words, I could actually see that the error increased by one step per direction change, whatever number of direction changes were made. Because a single step error is very difficult to measure, you do need to do, say, 100 direction changes to get a measurable error. I must say that I did check as many possible mechanical areas as I could, particularly where something could slip a tiny amount - things like motor shaft couplings, for example - before I tracked this down to a wiring fault (I had a twist in my cable, in effect, but I could cancel this out via the ports and pins setting as above).

jhonansaro
11-12-2017, 02:33 AM
Hi Neale

I am sorry not to mention those. My controller is Mach3, and I am connected to my BOB via a Parallel Port.

Regards
jhonansaro

jhonansaro
11-12-2017, 04:25 PM
Hi neale and m_c

On the issue of Step Pulse and Dir Pulse, Do I have to set these 1.5 us and 5 us in my Motor Tuning Setup config.?
If it is so, presently their value is set at 0 for both of them.

Please confirm.

Also Neale, Could you tell me how can I set the value for step signal polarity?

Regards
Jhonansaro

Neale
11-12-2017, 05:54 PM
On the motor tuning configuration page for each axis there is a box for pulse timings. I'm not sure if you can set 1.5 for the step pulse length but 2 would be fine (1.5 is a minimum). There is an internal value for pulse timing that Mach3 uses anyway and the value you give here is added on. I would be surprised if it makes much difference to your problem but it is such an easy thing to do, it's worth checking. Set 5 for the direction pulse length.

For step signal polarity, go to config->ports and pins->motor output. There is a box for step signal polarity which has either a tick or a cross. Click to change it for the axis that is giving problems.

jhonansaro
11-12-2017, 07:24 PM
Hi Neale

thanks a lot for your reply.
do you have any recommendation for the general Logic configuration page settings?
also on sherline mode, and Kernel speed do you have any recommendation?

Regards
jhonansaro

Neale
11-12-2017, 07:48 PM
I'm sorry but I can't really help with those things. I actually use an external motion control board (a CSMIO-IP/M) which means that some of the parameters that are used with a parallel port do not apply and even if I gave you a copy of my settings, they might be wrong for your machine. Same for Sherline - you are more likely to find that information from a US site where Sherline is much more popular. Kernel speed - 25KHz is probably a good starting point but again, my system does not use that value anyway.

I don't want to sound unhelpful but this is reaching the point where I just do not know the answers. I have never used Mach3 in parallel port mode. Good luck!