[RCaffin]

Hi Daniel

I understand the Asian Rotary problem, having been there myself. In the end, my thoughts were along the lines of silk purses and sows ears.

Their basic design is good for a certain amount, but once you go beyond that it can be extremely difficult to make the design any better. You could take out all the bearings and replace them with far better (more expensive) ones, but then you have to ask whether the bearing seats are good enough. Or will they all need remachining as well? I dare say the rotaries will do what they advertise; they just don't go any further than that.

I guess you could say that I cheated, in that I decided to use someone else's very high quality machining systems to get the basic precision I wanted. True, very true, but I can't make a spindle showing ABEC grade 7 performance by using ABEC grade 1 bearings, can I?

Cheers
Roger

[louieatienza]

Hi Louie

What size drive? Just curious.

Yes, design coming.

Just a thought: I suspect the 200 W motor will be at least 10x more power than will ever be needed. With 100:1 gearing, the torque from the motor will be multiplied by ~100x, and that ought to be enough to serve as a planer!

Cheers
Roger

It's CSG-20,93mm OD, 20mm bore through. I have a 200w servo, but yes probably overkill. Would be handy to use as an engine hoist! My only concern was their catalog stating that the backdriving torque ahould not be used to provide holding torque, though admittedly it would take quite a bit of torque to backdrive. The other concern was the end seal on the input side does provide some resistance which I haven't measured yet. I cannot backdrive it by hand however.

The nice thing about these units is that the cross roller bearing on the output and deep groove on the input are all preinstalled. For me the challenge will be to find a timing pulley of sufficient size that I can machine the 6 bolt holes concentric. My plan is to make an adapter plate for an old Atlas 4 jaw chuck I have, and have the motor mounted chuck side to allow mounting flat to the table.

Edit... It's SHG-20

[daniellyall]

correct I may in the end just chuck the bearings and bearing block out and start again probley be the east`s way your thread is helping to make the decision.

I found a gearbox in the shed the other day what I need to check to see if its ok if it is better than whats there now I may just use that so I may end up running with your way to do it as I wont to add a 5th axis to the machine using the rotary table as it is it will just kill it. so more info please on your plan or keep up the good info

[RCaffin]

Hi Louie (and Daniel)

> backdriving torque ahould not be used to provide holding torque,
Oh, probably quite right, IF you are holding a robot arm in position way up in the air with 500 lb of steel in it.
But in a rotary table , at 100:1 reduction ... not a problem imho.

All the other things you mention are covered in the rest of the chapters. ALL of them.

My rotary table is complete and tested, and all the chapters are written as well. I am posting them at 2 day intervals. This was chapter 7: the series goes to 13 chapters.
If you want to discuss details, PM me your email address.

Cheers
Roger

[daniellyall]

ok will do after you finish putting up all the chapters may not need to ask any question after that.

if you wont you can remove my comments anything I have put in so it makes this thread a bit cleaner

[RCaffin]

Hi Daniel

Oh, feel free to comment! No problems.

Cheers
Roger

[RCaffin]

Harmonic Design Mounting and Performance

Recall the coloured drawing of the HD unit from the previous chapter. The largest diameter part was in blue, and is the obvious part to bolt down to the frame. It not only supports the magic spline, it also supports the Crossed Roller Bearing (CRB). In fact, the 16 M4 cap head bolts used to hold it onto the frame are also required to hold the CRB together. Yeah - this thing is engineered. The green bit is equally the obvious bit to be the rotating output, and the 16 cap head M4 bolts there are also required for holding the bearings and the Harmonic Spline together. (So I went and bought a bag of non-Chinese M4 cap head bolts and matching nuts.)

Which is all very well, but this configuration means the division ratio is actually N+1:1, not N:1, so my unit has a slightly awkward 51:1 reduction, not the basic 50:1 shown in the unit designation. That is due to the way the spline inside works, and there is no getting around the N+1 bit. That means that a unit quoted as being 100:1 will, if used this way, have an actual reduction of 101:1, and so on.

I did look at mounting the unit off the green part (to get the 50:1 ratio), but putting a chuck onto the blue part was mechanically more difficult. While thinking about this I discovered that the 51:1 ratio was actually quite manageable, so I left it like that. More on that later.

Attachment 272012

Obviously, the thing to do here is to carve a shaped hole in a thick plate of metal to hold the HD and be a 'front panel' for the rotary table. Yes, the outside rim is on the other side of this plate. With no less than 16 cap head bolts holding the HD into the front panel, the basic design looked fairly robust. A thick front panel will be needed of course to handle the profile of the mounting.

In this context it is worth checking the details on the engineering drawing I showed before. Certain dimensions have a fairly tight h7 tolerance quoted: these are the probably best faces to use for the mounting. So machining the big hole and drilling the small bolt holes to that accuracy was an obvious requirement.

Attachment 272014

So I will just mention here that all the machining was done on my CNC, which easily holds 10 microns. That meant everything was machined to 10 - 20 microns accuracy, and all the parts just went together without any adjustment. Bolt holes were perfect: holes for M6 were drilled 6.0 mm and so on. I could not do that with manual marking and machining - no way!

Having bought the HD unit and decided on using it, I wanted some preliminary idea of how good the HD might really be. In particular, what sort of backlash does it have and what sort of load compliance does it have. Backlash is reasonably well known, so I won't explain that, but maybe I should explain what I mean by 'load compliance'. It means rotational stiffness: how many degrees will the output twist when torque is applied to the output?

The easiest way to measure small rotations is to measure small movements at the end of an arm bolted to the output stage - while placing the load somewhere else! You can't use the same arm for creating torque by loading the arm and for measuring the rotation: all you will do is measure the bending of the arm. I tried using a conventional dial indicator for this at the end of an ~200 mm arm, but the DI was only sensitive to 0.01 mm (smallest unit on dial). "Only", he says. So I made up an electro-optical sensor able to go down to well under 1 micron. You can skip the description if you wish.

Attachment 272016

I had some old IR LED emitters and photo diode detectors designed for fibre optic cables. These have fibre-optic-type glass fibres at the output and input, and the fibres are about 100 microns in diameter. The fibre diameter means you go from unblocked to blocked in 100 microns. That's moderately sensitive. I mounted one of each in a coaxial arrangment and put a blocking vane through the gap between them, as shown here.

Some electronic circuitry was needed to drive the emitter and to amplify the sensor output: this is not shown in the photo (because the photo was taken while I was building the sensor). The output was read with a DVM.

Technical explanation: when you shine X amount of light on a photo diode, it gives out a current Y. Within broad limits, any changes in the amount X will cause similar changes in Y.

Do I really expect to get sub-micron accuracy out of a lash-up like this? In short, yes. But note the restrictions on use: there are no forces on the sensor and it is not meant to be a commercial unit, able to withstand rough treatment. It works for me, but that is because I built it and I understand its limitations.

Attachment 272018

Then I mounted the HD on a steel plate, added some arms and the sensor, and was ready to measure. Well, actually, this photo was taken only part way through the construction as well, as the gold leads on the emitter don't yet go anywhere. Note that the upper arm going to the black sensor block is purely for sensing: any loads go on the lower arm. Note also that all metrology stuff is referenced to the flat vertical plate to which the HD is bolted. They are out of the bendable regions. Trying to put a magnetic DI stand on the horizontal base part was a complete failure, due to flexing of the steel around the bend.

Before going any further I had to calibrate the sensor: convert microns of movement at the flag to milliVolts out of the sensor. Better note here that this calibration will apply only to this exact arrangement. If I change the arms at all, I have to recalibrate. But this set up gave me an idea of how well the sensor arrangement worked, and it also gave me some idea of how well the HD worked. That's all I wanted at this stage.

Attachment 272020

To do the calibration I mounted a third arm, at the back on the input, and drove it with a long-throw (Mitutoyo) micrometer. Now I could rotate the input axle in steps of 100 microns at the input arm radius (or smaller), go through the known 51:1 reduction ratio, and measure the change in output voltage from the sensor. By knowing the length of the input arm I could convert the microns of movement at the micrometer to degrees of rotation at the input to the HD. In addition, by knowing the 51:1 reduction ratio, I could also get the output rotation in degrees from the input rotation. Yes, that assumes that the reduction ratio is as claimed, but that seemed reasonable.

(Interpolation: I am a retired PhD physicist, and I have spent most of my career developing measurement technologies. They all worked, so this is 'fun games' to me, and I had all the necessary stuff in stock.)



The first thing to do was to look at the linearity of the sensor. Since the light beam is round, I did not expect a simple straight line between input movement and output signal: rather I expected an S-shaped curve. And the graph here shows just that: the vertical axis is mV while the horizontal axis is microns of movement at the end of the output sensing arm. Microns of movement at the output were calculated from microns of movement at the input micrometer, the ratio of the lever arm lengths, and the HD reduction ratio.

Don't pay a lot of attention to the actual numbers, as they depend on several factors which could and did change later; just note the nice S-shape of the curve. Obviously this is not a general-purpose linear sensor, but the middle region is quite usable, even if the outer edges are less so. The shape is what matters, and it confirmed my expectation of an S-shaped curve.

Around here I had to repeat the whole first set of measurements. I was finding a steady drift in the results: a drift I did not believe. (Long experience helps.) After some ferreting around I realised that I was driving the IR LED emitter rather hard, and that it was warming up. That meant its output was slowly changing. Oops. So I reduced the drive current and the sensor became satisfactorily stable. If I left the system just sitting there for 5 minutes, the DVM reading stayed about the same (give or take 0.1 mV). The moral here is to never believe the results from the first few experiments: always repeat them a few more times until the results are stable - ie until you have the bugs out!



Having first driven the HD in one direction to look at the shape of the sensor response (to see what was happening), I was then able to drive the HD back and forth a few times to look at the hysteresis in the drive. That is, the micrometer went up and down a few times, in very small steps. The bent bits at the extremes of the loops are probably the non-linearity of the sensor, but the fact that the loop does not retrace exactly on itself in the middle tells me that there is some hysteresis in the spline drive. The path in the forward direction is not exactly the same as the path in the reverse direction.

The data starts at the top open-ended curve and goes in a 'loop' anti-clockwise. The 'loop' actually represents data from about 5 cycles. After the first lead-in, the loops overlay each other, which was really pleasing. It meant the data was fairly reproducible.

Well, it would be pretty amazing if there was no hysteresis at all. In fact, this preliminary graph suggests there is about 0.0062 degrees (0.37 arc-minutes or 22 arc-seconds) of hysteresis, under no load. To be sure, the result might be out by 10 - 20% but it is not incompatible with what I was able to get from the Harmonic Drive data sheets. I dare say some commercial RT units can do much better than this - but they also cost a whole lot more.

Actually, getting this sort of information from the HD data sheets is difficult: they are far more concerned about operation under high torque loads and lifetime under overloads. I guess that makes some sense: this is a gearbox, after all. I was not expecting to load the HD spline quite that hard anyhow.

If we are going to fuss about the details, I had better add that it is possible that the hysteresis might be slightly different if I rotated the HD output by 90 degrees. I don't have any guarrantee that the HD spline is absolutely uniform all the way around. Well, true, but I am not quite that fanatical.

You might say 'so what?' to all these measurements. Well there are good reasons for knowing about this. If the rest of the hardware (ie the motor) could only turn the HD in 10 arc-minute steps, I would be wasting some of the performance available. On the other hand, if I tried to get a resolution of 0.05 arc-minutes from the system, I would wasting a lot of my effort and getting no-where. The thing to do is to find what is a reasonable expectation from the HD, and to design everything else accordingly. Equally, with this sort of good performance, the rest of the housing had better be rigid enough to match. Tin plate will not do.

In the next chapter I will look at the performance under load. How much will the HD deflect when machining? (The answer to that question really depends on how hard you bash the cutter into the workpiece! If using a small cutter - say 5 mm, I suspect the cutter would break before the HD suffered.)

[louieatienza]

Now you lost me Roger... If the circular spline is fixed, and you use the wave generator as the input, then the output of the flex spline is indeed the stated ratio. If you mount the flex spline fixed (which doesn't look to be the case in your pics) then the circular spline is the output and the ratio is is r+1/1.

[rowbare]

I guess you could say that I cheated, in that I decided to use someone else's very high quality machining systems to get the basic precision I wanted. True, very true, but I can't make a spindle showing ABEC grade 7 performance by using ABEC grade 1 bearings, can I?

I can't either but clearly someone must have been able to or else we would never have gotten where we are. Granted it may have take an iteration or two... :-)

bob

[RCaffin]

Now you lost me Roger... If the circular spline is fixed, and you use the wave generator as the input, then the output of the flex spline is indeed the stated ratio. If you mount the flex spline fixed (which doesn't look to be the case in your pics) then the circular spline is the output and the ratio is is r+1/1.

The problem here is that the HD literature which I have is clearly wrong. Yeah, that sounds unbelieveable, but inspection of the brochure 'SHF and SSHG Component Sets Housed Units', page 6 will show what I mean.

That page shows 6+ configurations for a Harmonic Drive, with the different parts fixed and rotating to get ratios of R:-1, R+1:1, R+1:R and so on. But while the arrows on the diagrams are all variedt, the labels underneath are all exactly the same, which is clearly wrong. For instance, all of the labels have 'CS fixed'. Also, while the arrows show the direction of the output being either the same as for the input or the opposite, the labels all say 'opposite direction'. Yes, the labels are correct for the first diagram - but not for the rest! Someone in the graphics department has forgotten to fix the labels after doing a cut-and-paste of the diagrams. The arrangement I am using is the R+1:1 version. I guess that has led to a few misunderstandings. It fooled me too at first.

The real test is whether telling the RT to go around 360 actually works. Last time I checked, it seemed to do just that. But I will go and check again today. A later chapter will cover this in more detail anyhow.

EDIT later on:
The ratio is definitely R+1:1, or 51:1, in my RT. The tech detals of pulley ratios will appear in a later chapter, but for now I can say that telling the RT to rotate for 10 cycles (3,600 degrees0 brings it to the 'zero' with great precision. Very satisfying to watch actually. :-)

Cheers
Roger

[RCaffin]

I can't either but clearly someone must have been able to or else we would never have gotten where we are. Granted it may have take an iteration or two... :-)

More likely it took very careful selection of the best couple of bearings out of a full production run. The mfrs can do that; I can't.

Interesting factoid: one of the ways of QC testing a full ball race is to spin it at high speed and to listen to it with a sensitive microphone. The greater the noise, the poorer the quality, and v/v. So you pick out the really quiet ones and label them ABEC-1.

Another interesting factoid: go back 60 years and most ball races produced were ABEC-6 or ABEC-7. That's what they could make in those days. Today they can make ABEC-3 fairly easily.

Economics question: do you run separate production lines for ABEC-7, ABEC-6, ABEC-4, ABEC-3 and ABEC-1, or do you run just ONE production line the best you can and then select out the different grades?

Cheers
Roger

[louieatienza]

More likely it took very careful selection of the best couple of bearings out of a full production run. The mfrs can do that; I can't.

Interesting factoid: one of the ways of QC testing a full ball race is to spin it at high speed and to listen to it with a sensitive microphone. The greater the noise, the poorer the quality, and v/v. So you pick out the really quiet ones and label them ABEC-1.

Another interesting factoid: go back 60 years and most ball races produced were ABEC-6 or ABEC-7. That's what they could make in those days. Today they can make ABEC-3 fairly easily.

Economics question: do you run separate production lines for ABEC-7, ABEC-6, ABEC-4, ABEC-3 and ABEC-1, or do you run just ONE production line the best you can and then select out the different grades?

Cheers
Roger

Answer: it doesn't matter... That is exactly what Intel, nVidia, and other chip makers do... They make top of the line chips, and dumb them down for the lower grade models. The cost of fabbing chips are high, as the rate of chips that meet spec may not be. So it's very possible a slower chip can clock higher, though it would be hard to activate deactivated pipelines.

[louieatienza]

The problem here is that the HD literature which I have is clearly wrong. Yeah, that sounds unbelieveable, but inspection of the brochure 'SHF and SSHG Component Sets Housed Units', page 6 will show what I mean.

That page shows 6+ configurations for a Harmonic Drive, with the different parts fixed and rotating to get ratios of R:-1, R+1:1, R+1:R and so on. But while the arrows on the diagrams are all variedt, the labels underneath are all exactly the same, which is clearly wrong. For instance, all of the labels have 'CS fixed'. Also, while the arrows show the direction of the output being either the same as for the input or the opposite, the labels all say 'opposite direction'. Yes, the labels are correct for the first diagram - but not for the rest! Someone in the graphics department has forgotten to fix the labels after doing a cut-and-paste of the diagrams. The arrangement I am using is the R+1:1 version. I guess that has led to a few misunderstandings. It fooled me too at first.

The real test is whether telling the RT to go around 360 actually works. Last time I checked, it seemed to do just that. But I will go and check again today. A later chapter will cover this in more detail anyhow.

Cheers
Roger

Hmmmm.. My literature does not have such error... Will have to wait for your next post, but I'm all but positive the center ring mount is the flex spline, which is the normal output method. Being you have the outer ring fixed that's the only way I see you can output rotation...

There are actually 8 different output possibilities! Only thing I can think.of is you actually have an SHF, which doesn't have a full housing?

[RCaffin]

> So it's very possible a slower chip can clock higher,
Look up 'overclock'. hackers do that all the time.

Cheers
Roger

[RCaffin]

Hi Louie

See my chapter 6: Harmonic Drives. last drawing - my version of the HD in colour. The spline is drawn in red with a pink interior, and is firmly anchored to the blue section, which I anchor to the front panel. The green section is driven by the spline and goes around: that's what I use as the output.

> There are actually 8 different output possibilities!
Well, not really. You could argue for 6, but some of them do not make any engineering sense. Trying to anchor the input shaft to get a ratio of R+1:R or R:R+1 would normally lead to serious damage to the spline, especially with the higher values for R. Trying to drive the green output stage to get a gearing up from the input shaft would also destroy the spline. That's what comes of letting unqualified graphics people loose with engineering.

In reality there are just TWO options. You anchor the blue bit (also called the Flexspline I believe) or you anchor the green bit (also I think called the Circular Spline). The former gives R+1:1 while the latter gives R:1. I anchor the blue bit. The fine print (in my manual) does point out that the nominal ratio applies to the latter case. It is interesting, and probably quite reasonable, that their example drawings of robot arms etc show both versions in use.

I have an SHG-25-50-2uh drive.

Cheers
Roger

[louieatienza]

> So it's very possible a slower chip can clock higher,
Look up 'overclock'. hackers do that all the time.

Cheers
Roger

I know that, that's why I referenced that, being a "hacker" from the day. The best hack was the original AMD Athlon, which could be overclocked by jumping two contacts- with PENCIL LEAD! Yes, drawing a line... unbelievable. Messing qith clock speed and core voltage was a little trickier.

[louieatienza]

Hi Louie

See my chapter 6: Harmonic Drives. last drawing - my version of the HD in colour. The spline is drawn in red with a pink interior, and is firmly anchored to the blue section, which I anchor to the front panel. The green section is driven by the spline and goes around: that's what I use as the output.

> There are actually 8 different output possibilities!
Well, not really. You could argue for 6, but some of them do not make any engineering sense. Trying to anchor the input shaft to get a ratio of R+1:R or R:R+1 would normally lead to serious damage to the spline, especially with the higher values for R. Trying to drive the green output stage to get a gearing up from the input shaft would also destroy the spline. That's what comes of letting unqualified graphics people loose with engineering.

In reality there are just TWO options. You anchor the blue bit (also called the Flexspline I believe) or you anchor the green bit (also I think called the Circular Spline). The former gives R+1:1 while the latter gives R:1. I anchor the blue bit. The fine print (in my manual) does point out that the nominal ratio applies to the latter case. It is interesting, and probably quite reasonable, that their example drawings of robot arms etc show both versions in use.

I have an SHG-25-50-2uh drive.

Cheers
Roger

Looking at your drawing, the blue component looks to be the circular spline, and the green component the flex spline. The normal input turns the wave generator which is really ab ovoid ball bearing. The wave generator pushes the gears of the flex spline into the circular spline. The circular spine always has two more teeth than the flex spline, and always an even number,which keeps the flex spline centered. Thus a 50:1 drive has 100 teeth in the circular spline and 98 in the flex spline, so that 1 turn of the wave generator moves the flex spline two teeth (and this is true regardless the ratio.

Edit

Looking at the catalog again... Egad, the captions are wrong! No wonder my confusion! Makes sense now. I have the csd and shd catalog and the captions are correct. Damn catalog!

[RCaffin]

And for how many years have Harmonic Drive been circulating the PDF catalog I am using to potential customers without realising just how they are leading them astray? Would their Marketing Dept understand the problem I wonder?

Oh well, mine does work.

Cheers
Roger

[louieatienza]

And for how many years have Harmonic Drive been circulating the PDF catalog I am using to potential customers without realising just how they are leading them astray? Would their Marketing Dept understand the problem I wonder?

Oh well, mine does work.

Cheers
Roger

Maybe a great marketing ploy. How many engineers "discover" the flub and contact corporate? Way better than direct mail or cold calling...

[RCaffin]

> How many engineers "discover" the flub and contact corporate?
I could not even find out how to contact corporate from their web site. So I gave up.

Cheers
Roger