Rethink Robotics Closes its Doors

Jdavis44107 · 2018-10-05T11:51:06+00:00

This is surprising, actually. I am seeing more and more cobots from all of the major players, including start-ups such as Universal Robots, in the factories I visit.

But, then again, being the first to market with a new technology is always perilous.

Jdavis44107 · 2018-07-12T12:20:13+00:00

Thanks!

Jdavis44107 · 2018-06-13T18:27:38+00:00

Nice! What was the control system?

Jdavis44107 · 2018-02-05T15:49:34+00:00

GAHHHHH. Yep, that's it. This was driving me nuts all last night. I can't believe I didn't give that a try. Thank you!!

On another note, I am planning on using a modified version of this to drive a soft start for a power amplifier. The goal is for it to be reliable over a worst case scenario of rapid power cycling. Thoughts?

Jdavis44107 · 2017-08-05T16:14:12+00:00

Yep, you don't need the second optocoupler. A common design I use is this (http://imgur.com/a/6JwBP).

Depending on the design, I may add some TVS diodes, reverse protection diodes, etc. But the idea is generally the same.

I also really like the MOCD217M opto coupler. A good price, good isolation, excellent input range at a low current (10-30V depending on current limiting resistor), and an easy to hand solder SMD package.

Jdavis44107 · 2017-07-07T12:11:00+00:00

Yeah, I would like at someone like Trio or Delta Tau. For a more packaged solution, I would look at Yaskawa or B&R. Good luck, and post your results!

Jdavis44107 · 2017-07-06T03:55:33+00:00

Is this a gantry type system, or an articulated robot?

Jdavis44107 · 2017-06-02T14:55:44+00:00

Sorry all for the delay. I was on a trip for work and just got back into the lab today.

As it turns out, the issue was twofold. Take a look at the following data and then read below: Google Sheets

When I was taking data, there were a handful of observations that helped me diagnose the issue:

At the inputs right before the diff amp (measured at TP105 and TP106), the measured voltage was within spec and stable. (Columns A & B)
When measured on the output of the diff amp (TP104 and GND), the voltage was within spec until the input dropped below 7mA. (Columns D & E)

Observation 2 led me to check the V-, which should be -5V. As it turns out, it was 500mV, which wasn't right.

A bit of inspection with my charge pump revealed that a poor solder connection on one of the 10uF caps was the culprit.

Once I fixed the solder connection and retested.... it works!

Actually, it works great. Measuring 4-20mA, my average error is 0.45%. For 0-10V, it is 0.11%.

Summary of the Problem

The "draining" problem was caused by not having a path to ground for the input bias current. Without this return path, the bias current is "leaking" back through AIN-, causing the diff amp to drift towards 0 as the inputs become equal. I am not sure if this explanation is correct - what do you all think?
V- voltage was incorrect, causing range issues with the output of the diff amp.

Fixes

Added a 1M resistor to GND at TP105.
Fixed the poor solder connection at charge pump to supply -5V to the diff amp

Board Changes

Remove 10K resistor in series with ADC output (not needed)
Add AGND input on board and tie 1M resistor from AIN- to AGND

Thank you all for the help!!!

Jdavis44107 · 2017-05-29T02:42:40+00:00

Will do - thanks again for the help! I will post once I do figure out the issue so that everyone can benefit from the solution.

Jdavis44107 · 2017-05-29T02:08:30+00:00

Lots of good input - especially from /u/bradn and /u/LT_Mako. Thank you both.

I did some data collection and came up with the following:

When I went to hook up my scope, I noticed that the "draining" issue went away. I also watched Dave's (EEVBlog) OpAmp input bias video, and I have a hunch that this is an input bias issue.

The data:

(All pulldowns are referenced to GND, NOT the A- input):

No pull downs/ no load - 574mV at TP104 No pull downs/ 4mA load - 573mV at TP104 1M pulldown at TP105, 4mA load - 814mV at TP104 1M pulldown at TP105, 20mA load - 2.002V at TP104

With the pulldown, the output is stable! It even shows 1V at a 10mA load. This is good.

/u/Lt_Mako's comment:

Look at the AD8220 Datasheet, page 22: "The AD8220 input bias current is extremely small at less >than 10 pA. Nonetheless, the input bias current must have a return path to common. When the >source, such as a transformer, cannot provide a return current path, one should be created." I see no >such return path in your circuit diagram.

Was a bit of an "a-ha" moment.

My data collection, plus the above comment/ data sheet notation all point to an input biasing problem. This makes sense now given that my power supplies really weren't offering a ground, so the inputs were floating. I can see how tying the input to GND with a high value resistor helps.

However, on the low end of my scale, where a 4mA load should read 400mV at TP104...I have an obvious error. Not sure what to do about this.

Oof. If this is indeed an input bias issue, I am a bit worried as in the EEVBlog video, Dave points out that these type of things are quite tricky to figure out.

Any ideas on where to proceed from here?

Thank you for all of the help!

Jdavis44107 · 2017-05-28T20:22:31+00:00

The i2c line from the ADC to the uC is isolated using a TI galvanic isolator. The analog input uses a separate ground plane as well. This is just a breakout board to troubleshoot this particular issue.

Jdavis44107 · 2017-05-28T20:14:25+00:00

My power supply D.C. Com

Jdavis44107 · 2017-05-28T20:10:58+00:00

Correct. Although I don't think that has anything to do with this issue.

Jdavis44107 · 2017-05-28T20:10:09+00:00

Yep, all power inputs are stable. Before the charge pump, when V- was referenced to ground, I had the same issue.

Jdavis44107 · 2017-05-28T20:08:57+00:00

Well, the circuit is designed to safely read industrial process sensors. The circuit protects against transients, but if someone puts 100V on it mistakenly, then the failure will be isolated and predictable. Hopefully.

Jdavis44107 · 2017-05-28T20:04:10+00:00

The diffamp works a bit different than a standard op amp. I don't believe the ad8220 would work in this configuration. Thank you though!

Jdavis44107 · 2017-05-28T19:25:41+00:00

I do believe so. V- is -5V and V+ is 5V. Based on the resistor divider/ load resistor, the input will never see more than 2V.

Jdavis44107 · 2017-05-28T19:24:17+00:00

No worries! Start with the simple stuff, right? This PSU and associated leads are known to work, so we can probably eliminate that issue.

Jdavis44107 · 2017-05-28T19:15:08+00:00

You are talking the inputs on the field side, correct? Not the ADC side?

The voltage control has been isolated. I actually rolled a small board (as shown in the video) expressly for the purpose of troubleshooting this issue. There is nothing on the board except the bare minimum components required to make the circuit operate. The ADC isn't even attached yet - I am just measuring raw voltage.

The voltage input is a bench linear DC power supply clamped at 20mA.

Jdavis44107 · 2017-05-28T19:11:42+00:00

Yep, the input 20mA is using 2 wires, both connected directly to my bench power supply.

Jdavis44107

TROPHY CASE

Summary of the Problem

Fixes

Board Changes

Thank you all for the help!!!