OE-5.5 Robots & System Troubleshooting

What this is and why it exists

Degrees of freedom is the organising idea for the first half of this unit. It sets which poses an arm can reach, and therefore which jobs it can do.

Six is the number that matters: six degrees of freedom gets you arbitrary position and arbitrary orientation. Fewer, and there are poses you cannot reach at all, whatever the software does.

The second half is the most professionally useful part of this whole course, and the most likely to be skipped. A systematic troubleshooting procedure is worth more than any single fact in the syllabus.

The vocabulary

• Degrees of freedom — the number of independent ways a mechanism can move.
• Pose — a position together with an orientation.
• Workspace — the set of poses the arm can actually reach.
• End effector — the tool at the end of the arm.
• Symptom — what somebody noticed.
• Fault — what is actually wrong.
• Bisection — halving the system to locate a fault.
• Troubleshooting aid — a diagram, log or instrument that narrows the search.

The mental model

A robot in this context is a programmable mechanism placed inside a process. What it can do is set by its degrees of freedom. Three gets you a position somewhere in space. Six gets you a position *and* an orientation, which is what you need to present a tool to a surface at a chosen angle. Anything less means some approaches are geometrically impossible, and no amount of programming recovers them.

The workspace follows from that: the set of poses actually reachable, which is smaller than it looks because of joint limits and self-collision. Classifications and applications are then largely a matter of which workspace shape suits which job.

Sensing in robots brings the earlier units back. The arm needs to know where it is, and open-loop position control applies directly. A mechanism counting steps has the same silent failure here as anywhere.

Now the part worth staying awake for. Troubleshooting is a method, not a talent. The method:

Separate the symptom from the fault. Somebody reports what they noticed, which is rarely where the problem sits. Then check the dull things first: power, connections, whether it was ever configured. They are the commonest causes and the cheapest to eliminate. Then bisect: find a point in the middle of the chain, establish whether the signal is correct there, and you have halved the search. Repeat.

Change one thing at a time. This is the same rule as debugging code, and it fails the same way when ignored. Change three things, watch the symptom go, and you know nothing.

And use the aids. Diagrams, logs and instruments exist to narrow the search, and the person who reads the diagram first usually finishes before the person who starts probing.

Here is why this section is worth more than its length. Every system you ever work on will break, and this method transfers to all of them unchanged.

What you should now be able to explain or do

Say what degrees of freedom determine and why six is the significant number. Explain why a workspace is smaller than the reach suggests. Connect robot position sensing to the open-loop failure from the previous topic. Separate a symptom from a fault. Check common causes before rare ones, then bisect the chain. Change one thing at a time. Use the available aids before probing.

Check yourself