Comment by Animats | Hacker News Reader

Comment by Animats | Hacker News Reader

Where did they get a shaft encoder with over a million counts per rev? 130K is considered "ultra precision" in encoders.[1]

The article misses the point. The purpose of the high resolution sensing is not positional control. It's for force feedback. Here's the spec page for the robot, with more info.[2] (But too much hype. Seeing "In the loop, On the loop, Out of loop, Big Data Roadmap" on a data sheet makes me think "phony".)

Also, it's not $2,999. It's $4,888 for the base kit, and $9,800 complete and assembled. You don't have a usable robotics system at that point - you need another computer to control it. The FPGA is just motor control. Still, that's better pricing than Universal Robotics, which starts around $35K for a mechanically comparable system.

There's no reason these things should cost more than many cars. They're not that complicated. It's mostly volumes being low. That problem took down Rethink Robotics. I have a $400 3-axis uArm on my desk. This is a low end robot with RC servo grade motors. I built a six-axis force feedback end effector out of a Space Navigator for it. Controlled by ROS. The functionality is there; the precision and useful life are not.

[1] https://www.mouser.com/new/broadcom/avagoAEAT9000/ [2] http://hdrobotic.com/tech-hub/

The encoder is my very favorite part of the robot. I've written a page about how it works in detail: https://github.com/HaddingtonDynamics/Dexter/wiki/Encoders

In summary, it's a hybrid of digital and analog. To the standard quadrature encoder slit counter, we add sensing of the light levels as each slit covers or obscures the phototector. So we get e.g. 200 slits times 8096 light levels inside each slit equals over a million positions. It's so much data that we need an FPGA to process it fast enough. You can breath on the arm and it can sense it.

There is a constant battle between writing for sales/investment and writing for technical disclosure. Sometimes we get some chocolate in the peanut butter. It's certainly not phony, and I'm happy to answer any questions that anyone has about the real technical abilities.

The $2,999 price was for the Dexter 1 which was available at the time the article was written. The current generation is the Dexter HD which is more capable, much easier to build, and (as you noticed) more expensive.

Seems like a device of this capability will be an easy sell to anyone who is going to use it to make money. It only seems expensive to a hobbyist.

So, actually, servos with >1 million counts per rev are not that uncommon. They are now standard.

Example: the mr-j4's on my CNC machine have 22 bit encoders (4 million counts per rev) and are 4+ years old at this point.

These are fairly standard equipment, and not that expensive as servos go (300 a motor, 300 a servo drive, in bulk).

It will happily position repeatedly at to any of those counts, and it's repeatable to better than 5 microns (i am only using equipment good to about 5 microns because that's what i need. I have no urge to go set up the laser interferometer to measure this better, but i have no doubt it is repeatable to better)

Yasakawa's sigma7 series are 24 bit encoders (so 16 million count per rev).

Basically current generation servos are easily in the millions per rev count.

(and since the robot are in turn made from these servos, ...) To be fair to haddington: this is a pretty wild leap from the last generation of servos in most cases.

For comparison, the melservo-j3 was only an 18 bit encoder (262,114 count per rev), and the yasakawa sigma 5's were 20 bit (1 million count per rev)

So each got better by a factor of 16 in the newest generation

and FWIW:

The near state of the art that i'm aware of is the renishaw resolute:

https://www.renishaw.com/en/resolute-absolute-encoder-system...

Being rotary encoders, they are measuring accuracy/etc in arcseconds. For reference, 1 arcsecond is ~0.0003 degrees (1/3600th of a degree). So the +-5 arcsecond on the largest rings is 0.0015 degrees, and the +-0.5 arcsecond on the smaller is .000015 degrees.

Both are fairly ridiculously small angles.

To give a sense of scale of this error. On a circle with a radius the size of the earth, 1 arcsecond would only make an arc 30 meters long.

On a circle with a 1 meter radius, it's about 5 microns of arc length per arcsecond.

So yeah, you can do pretty well now in encoders :)

> Where did they get a shaft encoder with over a million counts per rev? they use about 200 tick encoders, but instead of measuring on or off they measure light intensity. Because each encoder slot has a bit of variation they get absolute positioning.[0] This is what their FPGAs are for.

As for the reason this robot is so expensive, the inconel harmonic drive gear boxes are probably a good portion of the cost. Harmonic drives aren't made in big production runs. The assembly instructions are somewhat horrifying and involve quite a bit of manual labor.[1] There's a lot of cutting carbon fiber rod and gluing it into the 3d printed bits. The horrifying bit is that you have to glue stuff to the motors and it appears that some assemblies aren't exactly made to come apart.

[0]https://github.com/HaddingtonDynamics/Dexter/wiki/Encoders [1]http://hdrobotic.com/dexter-v1-assembly-manual/

> Where did they get a shaft encoder with over a million counts per rev?

They didn't. They use a wheel with fewer counts per rev, read an analog value, and interpolate. It's like micro-stepping a stepper motor.

https://hackaday.com/2018/11/15/an-in-depth-look-at-dexter-t...

Oh, like a daisy-wheel printer.[1] The daisy wheel is mostly driven as a stepper motor, but fine positioning uses an analog sensor.

[1] http://laughtonelectronics.com/oldsite/comm_mfg/commercial_d...

The difference between hobby/research robot and similar industrial robot is the cost of maintenance and uptime.

If you run cheap robot continuously, it wears down quickly.

The FPGA enables higher sampling rates, which allows for the finer resolution when paired with a lookup table which contains systematic error.

RLS make magnetic encoders going up to 600k CPR (ring with dozens of magnet poles + multiple sensing elements): https://www.rls.si/en/products/rotary-magnetic-encoders

That would still need very accurate homing switches though.

Yasakawa has some very neat batteryless absolute encoders (so never need rehoming) in the latest sigma 7.

http://www.e-mechatronics.com/download/datas/catalog/cheps80...

They use the power generated by the motor spinning to store the position.

> Where did they get a shaft encoder with over a million counts per rev?

The article says they put the encoder directly on the motor, before gearing it down. So the effective resolution after gearing is magnified. But the encoder moves much faster accordingly, hence the need for higher speed / fpga decoding.

No, the article says "Dexter’s optical encoders are placed directly on each joint, and have resolutions that aren’t possible with any other control system." The usual approach is an encoder on the motor, before the gear train. They probably have one too, so they can see what the motor is doing vs. the joint. Remember, none of this is rigid under load.

There's just one encoder per joint. The encoder disk is 3D printed and the cruft from the print is used to refine the encoder ticks. Instead of fighting noise they save a map of all the systematic noise and then can use a lookup table to localize.

*I went to Vegas and built an arm with the founders.

Like an optical flow applied for radial motion?

Like optical flow in that features are used, not like optical flow in that there's one standard by which all measurements can be compared to.

Does this introduce risk of future noise (fluff, chad, whatever: it's industrial applications == dirt == hostile environment) being misread, and require recalibration?

Not saying wontworkbecause. I think this calibration trick is very good and I am imagining it's applications to other things eg more precise disk head placement from noise maps of the interblock separation reads

Ya recalibration is definitely a necessary aspect of this approach if the optical reading changes. For example if IR light levels change.

Which is guaranteed with aging.

Encoders at the joint only. Motors are steppers, so we have some idea where they are but that is open loop.

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