motor acceleration

EaziCNC, EaziCNCLite and other CNC related software

motor acceleration

Postby ptjw7 » Sat Nov 07, 2009 1:42 pm

I have tried to do some engraving with my mill nad have noticed that it appears to be losing steps on the z-axis.
I have been on another forum and people are saying it could be due to too high an acceleration setting on the steppers.
A search on the internet has so far given scant information so just what is the correct settings for acceleration and just what does it affect.

Anybody know.

Peter
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Re: motor acceleration

Postby alexmack » Sat Nov 14, 2009 3:12 pm

Well... yes, as it happens, I do :) .

First off I should point out that the acceleration/deceleration (we refer to it usually as ramping) is just one of the possible causes of lost steps. It could be that the motor is fundamentally too weak, it could be that you are trying to move it too fast or it could even be a mechanical problem such as a lack of lubrication on the slide-way or on the screw.

Stepper motors are at their strongest when they are stationary, they maintain a usable amount of torque through an operating range that is usually less than 450 RPM. It does depend on the motor but most motors start to lose torque quite drastically after about 300 RPM or so and will often be very weak at around 600 RPM. These are general guide lines - the load and the motor interact and effect these and motors can be quite different - a double-stack or triple-stack motor will maintain torque through a higher RPM than a single-stack motor. Stepper motors can also have resonant speeds where the motion of the rotor (they are called stepper motors because they move in a succession of discrete steps) causes a 'bounce-back' effect that makes the motor very weak - setting the motor to half- or micro-stepping can eliminate or reduce these resonance problems. If you have either STEP1(R5) or later cards or R3s or R4s with upgraded firmware then micro-stepping is used even when full- or half-step modes are selected (the card spreads micro-steps between the real steps automatically) so there will not be any resonance problems.

Ramping allows the motor to reach the top speed progressively and then to drop back to rest progressively. Most stepper motors have a limit on how fast they can be accelerated (a typical motor should be able to handle an acceleration of 250 full-steps/sec) and the load will also play a part - even on small machines there is considerable inertia to overcome when moving the slide-ways and there is kinetic energy in the system that needs to be lost when the slide-ways come to a stop. Some manufacturers data-sheets will quote the acceleration for the motor as 'pull-in' rate and may give it as an angular displacement - as with most data from manufacturers it will tend to be the best (quickest) value that they can quote and it is better to use a lower value in the real world :) .

Anyway, testing to see if it is the speed or the ramps is pretty easy - if you have one of the control systems that supports the feed-rate override from the hand-wheel then just set the override to 50% or even 25% and see if that cures or reduces the problem. If you don't have the override option then just set the max-feed and rapid rate lower. If it has an effect then it is either that the top feed is set too high for the motor and load or that the ramps are set wrong.

The ramps should be calculated so that there are enough ramp stages to allow a spread of step speeds between the pull-in rate (the starting speed) and the top speed (the number of steps/sec that the rapid positioning requires) without any of the stage jumps being more than the pull-in rate. The calculator in the ramps settings for EaziCNC should help you work it out. The steps/level setting will set how many steps are done at each ramp level before the speed is increased or decreased - this slows the acceleration or deceleration but is useful for larger machines where the limit is the weight of the machine rather than the acceleration capabilities of the motor.

As it is the Z axis that is giving you problems then it might be that the load is much higher than on the X&Y axes - a useful trick is to set the Z steps/rev higher for the Z motor driver than for the X&Y drivers. This will allow you to keep the ramp and feed settings that suit the X&Y axes but will slow down the Z motor to allow it to have a flatter acceleration curve and more torque.

I hope this helps. If you want more specific help then you'll probably need to email me your settings.

Alex
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Re: motor acceleration

Postby ptjw7 » Wed Nov 18, 2009 3:37 pm

Thanks for the description Alex. I think I have a mixture of things going on with the z-axis. I thought I was following what others had done with the Sieg X1 mill and no one I could read had mentioned lost steps on the Z-axis. Seeing as I have used ball screws throughout I thought I would be a step ahead obviously a rash assumption. I have a more powerful stepper to try yet before I think of gearing, theres also the use of counter weight system I could try. I am also thinking of fitting an encoder to the z-axis but so far found them a bit on the pricey side, I presume if an encoder is used then lost steps would be a thing of the past!

Peter
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Re: motor acceleration

Postby alexmack » Thu Nov 19, 2009 4:07 pm

An encoder would certainly fix the problem - the HEDS-9100 (HP/Agilent) don't seem badly priced - around £25 for an encoder head and slotted disk? Make sure you get one that is a multiple of the steps/rev - so for a typical motor get either 100, 200, 400 or 800 slots/rev (200 is usually best as the encoder reads 4 positions per slot [hence quadrature] so a 200 slot is actually 800 counts/rev) - don't get a 250, 360, 500 or 1000 slot - you won't be able to get steps and counts to match!

Z axes can be problem because gravity has to be taken into account unless the head is counter-weighted. The method I suggested of setting the Z stepper resolution higher (so that it moves slower) is similar to gearing but of course you don't have to change the machine mechanics about. If you are only losing a few steps rather than having the machine completely lose position then you are obviously not far off.

Alex.
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