Quadbot 17 - Quadruped robot designed in Autodesk Fusion 360

**jwatte** · 05-13-2017

Very nice! (Despite the black-on-black color scheme ;-)

The servos initially juddered a lot with default AX-12 settings, particularly the ankle and knee.

You'll probably want to turn that back once you put real load on the robot. The load (and mass) of the full system will significantly change the dynamics parameters needed.
One way of somewhat emulating the real load in a test rig might be to add torsion springs on the joints. The benefit of that would be that you can then turn the spring direction around and make them counteract gravity when you build the full robot, to take some load off the servos.

I occasionally lose leg control and I'm not sure yet if it's serial failure or something else.

This is the point at which I usually recommend people to buy a Saleae Logic device :-)
You can tie it into a number of points in your chain (early serial output, other end of serial wire, DXL bus TTL line, various test pins, etc) and get a comprehensive look at where the data goes and what it looks like; where delays are; etc.

**m3atsauc3** · 05-13-2017

Thanks for the tips!

About the black colour, the intention is for most of the dynamixel parts to be blue. I have blue spray paint for metal (and primer), not sure how well it will work but will be worth a try.

As for the serial issue, I have improved some of the code, and also found a peculiar issue:
- start up the Arbotix (motors powered by Robotis power supply)
- start up the serial test program on laptop, works fine and motors move
- stop test program, wait 2 secs or more, restart program
- motors stop responding, Arduino doesn't seem to be responsive (the serial TX LED flashes, but the Arduino code doesn't process anything, which I tested by adding some dummy print() functions and observing the Arduino serial monitor window)
- (The issue doesn't happen if you kill and restart the test serial program quickly)

I narrowed it down to actually happening only when the laptop is working off battery. When plugged in, there is no issue regardless of length of time between restarts! Small but strange issue, but not sure how it can be power-related since the Arbotix is probably getting enough power through USB and is running in the first place.
It seems as if the Arbotix pauses execution if the Serial connection is interrupted for more than a few secs, yet this only happens when laptop is running off battery. Definitely could do with a logic analyser for this kind of thing!

**KurtEck** · 05-13-2017

Originally Posted by m3atsauc3

Definitely could do with a logic analyser for this kind of thing!

Yes I would not leave home without one!

Sounds strange. Wonder if the USB voltage is lower when you are on battery.

Sometimes in the past when I was trying to debug some issues like this, I played around using either a Teensy or an Arduino Pro mini and then wired it up to act like a servo (part of the HSR fun, with LED hands or FSR feet).

The program would listen to the AX buss like a servo and respond when necessary. But in addition I know at times I used it to debug AX buss, by having it print out the messages it was receiving...

**Brooks** · 05-13-2017

Originally Posted by KurtEck

Yes I would not leave home without one!

I took JWatte's advice and purchased one - it showed up last week. Just in time to debug NeoPixels driven by Teensy FTM and DMA. They'll have to bury it with me, as I'm never giving it up!

**jwatte** · 05-13-2017

I narrowed it down to actually happening only when the laptop is working off battery.

My guess is a ground loop, or a loose ground connection of some sort.

**tician** · 05-13-2017

Originally Posted by jwatte

My guess is a ground loop, or a loose ground connection of some sort.

Since it only seems to happen when the program is stopped (and the COM port released?) for a certain amount of time, it sounds more like a power-saving hardware/driver option than the physical setup. If it happens immediately after the USB dongle is plugged in (when the 12V SMPS ground shorts to the laptop USB port ground), then that would be a definite grounding hiccup likely causing the dongle to re-enumerate. Had that 'resets and reconnects as a different COM port' issue when attaching a 12V5A SMPS to a buss hub when the USB2Dynamixel was already connected, enumerated, and in-use by a program running on a PC.

**m3atsauc3** · 05-14-2017

I narrowed it down to the simplest possible case, and can replicate the problem easily, ignoring the test Python program entirely:
I uploaded this minimal program to the Arbotix:

Code:

#include <ax12.h>

void setup()
{
  Serial.begin(38400);
  SetPosition(254, 512);
}

void loop()
{
  delay(1000);
  Serial.println("alive");
}

Run, open serial monitor, messages appear each second. If laptop power is unplugged, heartbeat persists for another few secs, then stops!
The power LED on the Arbotix remains on. Note this only happens if the SetPosition() command is executed, and the AX-12's torque remains on.

This is really more of a curiosity than a problem, as it has no impact on anything I'm doing, but you know how it is, sometimes figuring out problems is half the fun!

As far as a logic analyser, does anyone have a good recommendation of one for beginner-intermediate usage?

**jwatte** · 05-14-2017

It does sound like some kind of driver timeout problem, except the bit about "it works forever if I don't turn on torque on the servo."

How does the servo get power?

does anyone have a good recommendation of one for beginner-intermediate usage

www.saleae.com

The cheapest one (Logic 4, $109) will probably be just barely enough. I've upgraded my way through their offerings (started at the bottom many years ago) and am now on the 16 Pro.
You can probably live with the second-cheapest (Logic 8, $219) for a very long time, especially these days when it also comes with analog input. (Used to be digital-only.)

**m3atsauc3** · 05-14-2017

So, if the SetPosition() command is commented out, the serial code continues working when laptop is on battery power.

I'm using the standard bioloid power supply to power the servos through the Arbotix (barrel jack). If SetPosition() is used, motors move and torque stays on (but serial stops after laptop power is unplugged).

Thanks for the recommendations, I'll check them out!