Mill knee driving with a stepper

One of the things about steppers..torque falls off with "rpm"

So if torque ok at 20 rpm then torque should be ok at 5 rpm

If torque ok knee down at 5 rpm then torque will be the same at -5 rpm ccw vs cw

This leads to a "mechanical" type problem

Ok if a counter weight is not possible for test of knee move then maybe a extention spring..fixed to the quill say and clamped to the table. ..now back off the knee...to tension the spring..now tryslow up on knee..if now works you know that some help or bias would answer the question

Hi Trevor,
I had a thought overnight about some other tests you could do, You could remove the stepper motor from the mill and attach an arm and spring balance to it. Also have a dial gauge touching the arm about 63.7 mm from the axis. (So 1mm movement corresponds with one step.) With the driver powered up but with no step pulses being sent to it pull on the spring balance with a force that corresponds with a torque of about 6 Nm (I think you said that you measured the static torque required to raise the knee was about 5Nm) Note the change in reading on the dial gauge. If the deflection is a significant fraction of a step then there must be too low a holding current being applied. If this does not show a problem then you could see the result of sending single steps to see if it moved and then went back to its original position.

Les.

Hi,

I'm pretty sure that the Z axis screw on mine is 5mm pitch. You say that you have no acceleration ramp, I found that I had to fiddle around with acceleration in mach 3 to get things working nicely.

Regards.

David

Hi Jason,
This is my calculation for how long the knee would take to fall the distance of one step.

Acceleration due to gravity = 32 ft per sec^2

S= u + 1/2at^2 (Assume u = 0)

For 8 TPI 1 step = 1/(8 x 200) = 1/1600 = 0.000625"

0.000625" = 0.000625/12 feet = 5.21 X 10^-5 (0.0000521 feet)

So 0.0000521 = 1/2 x 32 x t^2

So t^2 = 0.0000521 X 2 /32 = 0.0000521/16 = 0.00000326

So t = Sqrt 0.00000326 = 0.0018 seconds = 1.8 mS

Les.

The knee will be very highly damped by the slideway friction, and indeed stiction effects if it has to reverse, so I think this is a very idealised calculation.

If the stepper is slipping it is not happy and being operated near its limit. I think you will need to increase the torque on the screw with gearing between the stepper and the Feed screw. There are all sorts of nasty striction for want of a better word for the varying friction you get from a metal on metal slide way.

The drive I posted earlier in this thread used two stages of 3 to one reduction there was no problem at all with missed steps.

Regards
John

Trevor,

You didn't say exactly which driver you were using just that it has a minimum microstep of 2. I diod ask if you were using the 2M542 type which are commonly available on eBay - these have a function that reduces the idle current when stepping stops (presumably after a short timeout). This means that their holding torque characteristics at very low speeds could be quite anomalous. As far as I can tell (don't have my data sheet to hand right now) it is "SW4" that selects this function - if you are using this type of driver then this could be worth checking and maybe just toggling sw4 to see if it makes a difference?

John.

Had not forgotten I was tied up yesterday. The X&Y Axis are running DQ542's on a 35V supply. The knee was originally running a DQ860 but I am now trying a more advanced version DWD872 both on 55V. Changing controller used seems to have no effect on the problem.

Although the X&Y are quite happy to have SW4 set for reduced current mode I am not running this on the knee stepper. I decided I would wory about energy efficiency once I had the problem sorted out. As you noted all these controllers use SW4 for this function. However, they do not all operate the same. Most cut by 50% but others cut by 60% or 40%. The idle function cut in time also varies (if quoted at all!) it is stated as a value between 0.1 ->0.5 sec. Which always seems very fast to me.

For the DQ860 it is qoted as 0.2 sec which could have an effect at the very lowest speed I run. Although I usually run with SW4 reduced current I did try some tests with it enable but it did not seem to change anything.

Later today I plan to do some measurements with my DSO. i will let you all know how it gets on.

Trevor

I done a check with my DSO.

Voltage measurements across the windings was never going to show very much. All you see is a variable markspace square wave providing a PWM drive.

With a motor you are interested in torque which is related to current not voltage so I decided to try my home made current probe ( commercail current probes cost more than many DSO's!). Unfortunately faced with the pulsed magnetic fields the current ends up buried in noise and I was unable to make any useful measurements. Maybe one day I will build a better current probe, but not today.

There is a limit to what the DSO would show. As has already been mentioned steppers run in an open loop mode. Consequently the only effect seen in the driver is due to adjustment of the PWM to get the required current through the winding. A function of winding inductance and resistance only (as long as we don't get magnetic saturation occuring).

My hope was that I would be able to see how the current varied after a step had been applied if Jason theory is correct I would expect the current to drop slightly. Unfortunately it was not possible to do this measurement.

Time to review:

The original objective (easy to forget this) was:

1) To find out what was causing the problem with slow speed running.

2) Come up with a modification that will solve the problems.

Although I don't have the evidence to prove it Jasons theory is the most promising answer to (1). I would very much like to get a definitive answer but that is beiginning to look like a major research project and consequently not justified.

My decision on (2) is that direct drive was a bad idea. I had already come to that conclusion before this started. As a result I will change to a pulley drive system with a big reduction ratio and use a high speed NEMA23 stepper, this still gives an increase in torque at the handwheel shaft. If this does not solve the problem I will then fit a balance weight.

Trevor

Hi Trevor G

Last year I made a couple of Stepper motor drives for an artist friend of my Sons, He needed a to move large objects One was over 150 KG.... slowly. they were not guarded in any way so I made the drives failsafe by using steppers that would stall rather than say a worm reduction box driven by an AC motor, far too much torque when geared down (Yes I could have used a pre-set slipping clutch) but that would have added to the cost and or a VFD with torque control also required for speed control. Speed required was about 1.5 rpm. At very low speeds an AC motor can overheat and they often hum loudly when driven by a VFD

The art object itself was supported on standard self aligning cartridge ball bearings...Fairly cheap and easy to obtain.

In the end I used a Nema 34 step motor like this one **LINK** .

It has a half inch shaft supported in larger ball bearings making it strong enough to resist the lateral force imposed on it by a timing pulley directly mounted on it. (Unlike the around 5mm shafts on smaller step motors) Again I used a connected intermediate pair of two three to one reductions as used in the router I posted earlier on this thread.

This is a pretty powerful drive with the motor used I found that the T 2.5 belts used were actually the weak point, space was at a premium and I had used the minimum allowed driver pulleys for that size belt as specified in the Gates catalogue. T5 with larger timing pulleys would have been better If I had the room.

At full motor torque the belt would skip a tooth on the driver pulley if the art object was stopped with my hand, OK the safety was inherent no chance of mangled visitors.

Luckily I had used a Stepper controller similar to this one.. **LINK** It was a simple matter to program the torque to a figure below the belt slipping point. The speed was set using a small pulse generator board from the same supplier (I know easy to make but it was there! Job done). I also sourced a switch mode power supply from them.

OK the solution I used was not cheap compared by the bare board approach, I was working with the public and safety and fire was an issue I wanted UL tested components.

My supplier Ocean Controls is in Melbourne Australia, (I have no connection with them) Their service was very good. Similar components can be sourced from many suppliers around the world.

Oh and the drives worked perfectly and almost silently. If you want silence make sure the power supply does not have a noisy little cooling fan some do some don't I found out the hard way. Without the fan with the drive in a box it could not be heard if anyone was talking and even in a quiet setting there was only a slight murmur.

Regards
John

Edited By John McNamara on 09/09/2014 14:12:18

Good question Jason. As I stated earlier SW4 is in the disabled position so it should not be. On the other hand the manufaturers descriptions of how their products operate is so vague and wooly it is difficult to be 100% certain. It is one of the questions I had hoped to answer when I did my test with the DSO. I am getting some new equipment for anopther project which may help in the future.

Thanks to everybody who has contributed. I am sure I am not the only one to learn more about steppers. So I am not going anywhere as I suspect there may be an ongoing discussion of this and other aspects of steppers. The popularity of this thread indicates the interest in the subject.

I will of course post an update on the final set up.

Trevor