Clinic

Recently, I had a problem with my BittleX robot and, after reporting it here, it was diagnosed as having a power circuit failure. I then requested a replacement from support, and they sent the replacement board. (Many thanks go to the Petoi team, from top-to-bottom, for their efforts towards getting me the new board.)

Replacement Biboard installation

The installation of the board was mostly uneventful with one, minor exception. The Ultrasonic sensor that sticks out at the front of the board is unadjusted and may interfere with the movement of the robot head. You should bend the sensors downward towards the shoulder in order to prevent interfering with the head. You can test the head clearance with all power off.

Mysterious Behavior

With the Biboard installed I ran my diagnostic software with terrible results; the bot moved in a chaotic manner, with many collisions between legs and body. I kept reducing the tests performed until I was able to isolate a single test that demonstrates some mysterious behavior previously unknown to me.

The first video (https://youtu.be/EmBHQHdSwBM) shows the mysterious behavior (n.b., the test is run twice for clarity).  The same test is performed for each servo, HEAD, RLEG, LLEG, in that order. The test places the bot into a pose that is suitable for performing the test without interference. This is done before and after the actual test to provide a visual cue the test has completed. When the test completes the test framework resets the pose to the "zero" pose.

The test itself is fairly simple. First the servo is set to the maximum angle (120 degrees for the head servo; 90 for all others). When that command completes the servo angle is read and compared to the expected angle. The smaller of the two is declared the maximum value. For the minimum angle the value is -120/-90 digress and the maximum result is used as the minimum angle for the servo.

The mysterious behavior is that the LLEG does not return to the test pose after the minimum angle test. Note that for all other servos the move is performed.

So what is the reported position of the LLEG (servo 15)?

The test pose command sent is verified by reading the servo positions. We can see this in the log. First, the -90 position is set

ftBittleX::ftfBittleX::on_send
TX command  : m15 -90
write count : 8
expected    : 8
actual      : 8
ftBittleX::ftfBittleX::on_response
Command completed: normal
description : INDEXED_SEQUENTIAL_ASC
cmd         : m
id          : 12
data        : 15 -90
    1747 ms : RX_latency
        0 ms : RX_elapsed
response    : 1 lines
m
  
response    : end

Then the position is checked

ftBittleX::ftfBittleX::on_send
TX command  : j15
write count : 4
expected    : 4
actual      : 4
ftBittleX::ftfBittleX::on_response
Command completed: normal
description : Joints
cmd         : j
id          : 8
data        : 15
        3 ms : RX_latency
        1 ms : RX_elapsed
response    : 3 lines
=
  
-80
  
j
  
response    : end
ANGLE MISMATCH: -90 != -80

The failure is the purpose of the test.

The test pose is then reset

ftBittleX::ftfBittleX::on_send
TX command  : i0 0 12 90 8 0 13 90 9 0 15 90 11 45 14 90 10 45
write count : 49
expected    : 49
actual      : 49
ftBittleX::ftfBittleX::on_response
Command completed: normal
description : INDEXED_SIMULTANEOUS_ASC
cmd         : i
id          : 6
data        : 0 0 12 90 8 0 13 90 9 0 15 90 11 45 14 90 10 45
    692 ms : RX_latency
        1 ms : RX_elapsed
response    : 1 lines
i
  
response    : end

The pose is verified

ftBittleX::ftfBittleX::on_verify
ftBittleX::ftfBittleX::on_send
TX command  : j
write count : 2
expected    : 2
actual      : 2
ftBittleX::ftfBittleX::on_response
Command completed: normal
description : JOINTS
cmd         : j
id          : 8
        5 ms : RX_latency
        9 ms : RX_elapsed
response    : 4 lines
=
  
0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	
  
0,	0,	-49,	-4,	0,	0,	0,	0,	0,	0,	45,	45,	90,	90,	90,	90,	
  
j
  
response    : end
ftBittleX::ftfBittleX::on_verify: joint list match

Disturbia.

The physical state does not match the reported state.

The result of this is that when the test framework tries to move to the "zero" pose, the LLEG is in a collision condition with the body and the movement "slides" the leg along the body until it suddenly moves properly. I have to conclude this is the reverse kinematic algorithm that is controlling this. The mystery is why it didn't happen when the test pose was performed at the test end?

But the mystery gets deeper. I puzzled over this behavior and observed it a number of times when I noticed that the LLEG servo was "twitching" (not "chattering") at the end of the test. I wondered if the servo was still trying to reach -90 and, as a result, it ignored or dropped subsequent commands. As a experiment, I lowered the target angle to one that should be achievable: -80 degrees. The second video (https://youtu.be/eLVuo9PSa1k) shows the result. Sure enough, the LLEG servo now returns to the test pose properly.

So the mystery is,

"What's happening to the servo?"

"Can the condition be detected?"

This behavior leads to undesirable results and potential disturbing collisions..