mesa + battery

StepperOnline also sells an inline box for their 23-bit encoders which can make them into battery-backed absolute encoders.

www.omc-stepperonline.com/1-5m-encoder-c...e-encoder-ceast1m5-2

I'm not familiar with the details of how they implemented this, but it seems like something trying to solve a similar problem.

PCW wrote:

The DYN adapter has a fixed resolution and does seem to be single turn.
(its absolute nature is mainly for commutation which it seems to acquire via Hall inputs)

 

Yes it's "fixed" but input resolution is specified at "time of ordering" which sounds to me like it's updated via firmware. Multiturn still seems like a possibility to me via firmware, if they offer it.

Absolute encoders are typically not just incremental encoders with processing, as this would not
have the correct position count at startup.
 

OK, I don't use servos with absolute encoders (well techincally I guess I do lol) so I have to ask....On a servo system that uses a single turn absolute encoder, what is the position read out from the drive on power-on? You have to be able to tell the angle of the motor shaft otherwise what's the point right?

A single turn absolute encoder will read the correct angular shaft
position at power up. Magnetic encoders are often absolute as they
have a single 2 pole magnet on the shaft, influencing an array of hal sensors.
Its actually sort of like a 1PPR encoder with a very high interpolation
ratio. Optical absolute encoders typically have multiple tracks  with 
Gray coded data on the tracks. to get absolute operation.

The Dyn adapter  gets it "sort of" absolute operation by having
very low resolution absolute inputs (UVW or Hall)
 

PCW wrote:

A single turn absolute encoder will read the correct angular shaft
position at power up.

 

From the DYN4 manual regarding the "Absolute Encoder" equipped Servo

The motor power on position is the default absolute zero position, or it is the position set by using set absolute
zero function (0x00).

That sounds like the behavior of a purely incremental encoder. Now if you stick an ASIC inside the encoder housing that does the actual counting, maybe you can get away with calling that servo encoder "absolute". If you stick a battery on it, power on no longer affects it's so it's semantically the same thing as a physically reading absolute encoder except for the fact that when it woke up the absolute position was not physically predefined.

The only way to reset the drive position on a DYN4 is to hard reset the CPU. I'm guessing that hard reset drops the encoder power and allows it to reset as well. That sort of backs up my theory about it being a mostly incremental encoder with an ASIC processor I think. I'm not really smart enough to get into all that commutation business but I don't see how their standard motor encoder resembles an absolute encoder in the traditional sense.

I make those points because I'm assuming the converter is based on the same ASIC and basically does the same thing.

I just got a response from the guy at DMM, he just says no, the ENCV module can't do anything with a battery.

He also says that the motors have "traditional" absolute encoders. I can verify from working with quite a few of these drives and motors that there is exactly zero visibillity of absolute shaft angle from the drive power on, I just went through the DMM software as well. The only thing you will ever see from that drive after power on is a 0

What am I missing? How is that the behavior of a drive connected to a motor with an absolute encoder? What's the point?

I think the point for them is that the commutation angle
is correct at power on.

They could forward the (1 turn) absolute position but that
would require a fieldbus interface of some kind  or some
absolute encoder format output (some drives do this with SSI)

As a curiosity, some older Fanuc absolute encoders used a normal
quadrature interface and sent the absolute position as quadrature 
counts when reset.

PCW wrote:

I think the point for them is that the commutation angle
is correct at power on.

They could forward the (1 turn) absolute position but that
would require a fieldbus interface of some kind  or some
absolute encoder format output (some drives do this with SSI)

As a curiosity, some older Fanuc absolute encoders used a normal
quadrature interface and sent the absolute position as quadrature 
counts when reset.
 

Can't commutation just be derived from hall sensors? It is an RS485 encoder "SN75176 or equivalent" 5v, gnd, S+ and S-. They're protocol isn't published in the manual, it says "contact" for that info but they didn't bother to give it to me.

The converter module which I still speculate uses the same ASIC says "Brushless AC motor types should be used and commutation can be generated from incremental, or hall sensor signals." which sounds exactly like what I was saying about the standard motor encoder, which the motor probably has a 3 hall arrangement like a brushless DC motor.

The multi-turn encoder does explain the protocol layout with some function code, CRC, 16bits of position, and 16bits of turn. I suspect the 2bytes of turn is omitted by the encoder or ignored by the drive if either is single turn. AFAIC it doesn't matter whether it's ASIC or magic that makes it an absolute encoder, If you turn it on and it can't tell you the shaft angle it doesn't seem like an absolute encoder to me. An M16 with a 2 position selector wouldn't do anything an AR15 wouldn't do.

Halls are fine for fan motors and the like, Good servo drives typically use a higher
resolution commutation angle source to avoid torque ripple. Of course if you have Halls and
an incremental encoder, you can use the Halls for initial movement and then calibrate
the incremental count on the first Hall edge.