Re: Combining shaft+linear encoders

> > I am currently designing my own servo control chip (step/dir and encoder
> > inputs, +-10V or PWM output). I consider adding additional input pins to
> > connect a second (linear) encoder. The idea is to mix the two position

Jon Elson wrote:

> Why don't you just use a DC tachometer? That is a lot better, as it is
> not a discrete, but a linear device. That means, it gives valid output down
> to zero speed. The shaft encoder only gives a valid output when it is moved
> to a new count position, and no output when between these positions.

Hi Jon

I Know, but...

1) tachometers cost money. Most motors come with either encoder or tacho
but not both. So you can't upgrade an existing system with only (shaft)
encoders to linear encoders + tachos.

2) There are servo drives that can cope without tacho like the geckos. I
know they have some disadvantages compared to your aproach but they
can't be beaten in simplicity.

3) I want to build the whole circuit for a brushless drive (except the
power transistors + bridge drivers) in a single chip. I cannot afford a
mixed signal ASIC process, so I must implement the whole control loop
digitally.

> So, you get no velocity info for a while, then suddenly a velocity change
> is noticed. My DC tachs allow me to have absolutely smooth motion down
> below .01 IPM, where stick-slip friction develops. You can stop the position
> loop update through the computer, and you STILL can't move the machine
> manually due to the tachs.

Wow, impressive. How do you detect a broken encoder cable?

> Since the tach is continuous-time, there are no
> nasty stability problems. An encoder has a variable update rate depending
> on velocity, therefore it has to give a rough output at low speeds. There are
> ways to compensate for this, but they are not so easy.

Correct. I'll have to plan for some spare gates and spare time to figure
this out.
But life would be boring I there was nothing left for research... :-)

> > damping is calculated from the faster input from the encoder mounted
> > directly to the motor shaft.
> Ugh! I think there could be all sorts of "sensor fusion" problems with this
> approach. If you derive velocity (and its derivative d-vel/dt) from the shaft encoder,
> that might not be too bad.

> If you are using position from both linear and shaft
> encoders, the leadscrew errors + backlash could drive you nuts!

Yes, of course. Position info of the both sensors must not be mixed.
Shaft encoder should be used solely for velocity and linear encoder for
position and integral.

> With the DC tach, all you need is a gain pot (and maybe a loop compensation
> pot, too) for balancing velocity with the position loop from the CNC control.
> You'd need some method of making very small incremental changes to this
> gain if you use an encoder for velocity. otherwise, the loop will be forced to
> hunt (faster - slower - no, faster ....) in velocity, as it can never get an exact
> match between commanded and actual velocity due to finite precision of the
> numbers. Put too much digital smoothing on it, and you end up with a sluggish
> system.

Thats the same reason why I don't want to use a tacho for a digital
control loop. An affordable A/D converter does not have the required
accuracy (resolution + linearity). I think, you either have to do it
fully digital or the "classic" way.

But many thanks for your knowledgeable advice. Now I know the critical
issues and can tune my design accordingly.

Btw. do you think that velocity feed forward improves control response
remarkably?

Regards

Bene

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