The high torsional rigidity of your the drive line is the cause of the missed steps and is one of the factors that needs to be taken into account with steppers.

Stepper motors (whether in single or micro step mode) move in a series of jumps. Regardless of the size of the jump (or step) the rotor tries to move instantaneously from A to B as the magnetic field changes. The magnetic field does change almost instantaneously and the rotor has to accelerate and decelerate to move from step to step. The laws of physics dictate that it cant actually do that.

If the rotor is rigidly linked to a significant mass it just cannot accelerate quick enough to get moving. Many home brew CNC systems have a toothed belt drive so the motor is only carrying the weight/mass of the pulley. Even though the belt is virtually backlash free it has enough 'give' to allow the stepper rotor to start moving at each step. Without some compliance in the drive between motor and load the easy way to ensure consistent performance is to use a bigger motor. (The sledgehammer to crack a nut technique)

Without knowing more about your setup I can only guess, but if your motor is the 200 steps/rev type does that give you enough resolution? If you incorporate a belt reduction not only do you get an increase in torque but very often you can make a much more compact arrangement as you are not stuck with the length of the motor protruding.

Ian P

Murray

My description is based on my knowledge of stepper motors learned from reading about them rather than using them so is purely from memory.

You are completely correct in stating that when energised the rotor will develop significant torque. The torque is greatest when the fixed and moving pole 'tips' are aligned (in each step position) though it reduces slightly in between step positions.

Some stepper drive circuits use clever circuitry to boost the beginning part of each pulse in order to help overcome the load of moving the rotor from rest, which it is (sort of) between every step.

Regardless of anything else the electric current flowing through the windings in order to move it to the next step, the electrical signal is only present for a finite time (for each step). If for some reason the rotor has not moved in response to that particular pulse its too late do do anything about it because its an open loop system. Syncronous motors, which steppers are a type of, only run or move at the speed dictated by the frequency of the signal and nothing else. They don't accelerate in the normal sense.

I did once watch a demonstration of a stepper motor that would not move even though it was not coupled to any load, it did however have a flywheel on its shaft. With the flywheel removed, the motor worked perfectly.

I think Trevor problem would best be solved by incorporating a reduction stage.

Ian P

I have no practical experience of driving steppers, but some genuine experts I have known claim that driving them is the darkest of electronic's dark arts.

If a single pulse is not enough to move the knee on its own, then you will always lose at least one step at the start of any continuous motion.

Such a large mass will take time to accelerate. Sophisticated drivers ramp up the pulse speed to allow for this, to avoid lost steps. Obviously if you were just using this as a 'power feed' rather than CNC then it would be unimportant.

My instinct would be to balance the load as much as possible – with such a big mill this won't compromise rigidity, and it will reduce leadscrew/nut wear as well.

Bear in mind that a simple weight on a pulley give a virtually constant load, few spring systems will.

Neil

Les

My explanation was gross simplification of how steppers are driven. There are umpteen methods and strategies used to get the desired performance for a particular application, mainly what I was trying to convey is the need to make an allowance for inertia,

I had mentioned that I had no experience of using stepper motor, by coincidence however, today I finished (well almost) putting together Steve Ward's indexer and I have it up an running driving a small rotary table.Most of the parts were from my scrapbox (I bought the PIC and PCB from J.S.) and the only part (I need?) to purchase is the 10MHz crystal. I have temporarily put in an 8MHz one. Am I right in assuming that this will have no effect on the actual number of steps and that the only thing that will be affected is the rotation speeds?

Ian p

Trevor,

Just a thought …

It might be worth using a worm drive instead of toothed belts.

… easy to get a big reduction ratio in a compact space.

MichaelG.

I'd class a counterweight as sound engineering; after all just about every lift and tower crane in the world has one. The bodge is mucking about with motors, belts and gearboxes, which don't address what is probably the real issue, but merely paper over it.

Never mind stepper motors; for a real black art look at RF, especially when you get into the GHz range.

Regards,

Andrew

Les: I'm not sure that the increase in mass is significant, as the accelerations are pretty low. Let's assume we want to move the knee (say 100kg) at 250mm/min and we want to get there in one second, from rest. A velocity of 250mm/min is equivalent to 0.004167m/s. From Newtonian mechanics we know that:

v = u +at

The initial velocity u is zero so we get a = v/t

So the required acceleration is 0.004167m/s/s, or about 0.0004g.

We also know that F=ma, so that gives F=0.4166N

Of course the above analysis ignores the effect of friction, of whatever sort, and gravity.

Andrew

PS: If I've made a mistake I blame the bottle of home brew red wine that I'm working my through, home grown grapes too.

The mass doesn't change with acceleration unless there is some form of warp in the space time continuum! And the addition of a counterweight will increase the total mass but reduce the static force the motor has to overcome to raise the table. That would be an improvement.

Andrew's sums are correct, as would befit a Cambridge Engineering PhD. Rather than trying to get your head around the physics, it's probably best just to get some stuff connected up and try it out!

It's a simple matter to accelerate a stepper motor. You start by sending it a low frequency pulse train (= slow speed) and steadily ramp up the frequency to a higher frequency (= high speed). You really aren't moving the table in sharp jerks – the inertia of the rotor and leadscrew ensures that the speed of rotation is reasonably smooth. The motor applies a force to the mass which results in an acceleration. Or not if there is not enough force to overcome the static and dynamic forces.

Murray

When I made the comment about the increase in effective mass I was thinking about its effect on one step of the stepper motor. Trevor, I do not agree that half the mass is moving up and half is moving down changes the force required to accelerate it at a given rate. Murray, The problem does not seem to be related to ramping up the speed as the problem does not occur at higher speeds. As the movement at higher speeds must start with a single step we need to know what is different in the way the stepper is driven between high speed (When it works) and low speed (When it does not work.) NOTE Trevor has confirmed that it does not miss a single step at high speed. I think it would be worth measuring the torque required to turn the knee leadscrew to see how this compares with the rating of the stepper. I also think trying to see what happens when the driver is given a single step pulse could be helpful. (Measure the coil current and note if it gets reduced from its initial value after a time. This will not be easy as the coils will be driven with a PWM signal. This would be need to be done with an oscilloscope to look at the duty cycle of the PWM drive to the coils.) Just out of interest what are your thoughts on my comment that the effective mass would be doubled no matter what the ratio of the pulley system was. ie If half the mass was used in the counterbalance then it would move twice the distance and hence move at twice the velocity.

Les.

A worm and pinion would work but the reduction ratio will probably be far too high. I suspect the best compromise will be in the region of 2:1 to 5:1 which is outside the worm and pinion practical range.

Ian P

Hi Trevor G

A number Commercial CNC Machines particularly the bigger ones have counterbalanced Z axis carriages often using a weight and cable or chain. The weight is usually buried in the column so you don't see it.

Fadal for instance **LINK**

I made CNC router using toothed belts for the X and Y drive, it works well.

Some images here a couple of lines down.

**LINK**

Gear train From the above link:
**LINK**

You may have to resize the design to accommodate the weight of the Z axis on your machine. Note the motor shaft only had to carry an axial load via the coupling which drove a shaft supported in inexpensive flanged ball bearings clamped to the side plates. This machine works very well there is very little backlash.

If you can read a DXF file can send a cad file.

Regards
John

Edited By John McNamara on 06/09/2014 09:35:51