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1.1.4 Problems and quibbles

There is much free play in the shoulder

The shoulder bearing, with the screws giving excess free-play circled

There is some free play in the overall robot construction, but by far the most noticeable is in the fixing of the upper arm to the shoulder. This is caused by the holes through which the servo horn screws pass are bigger than the screws themselves, and the cohesion between the bracket and the (smooth nylon) servo horn is insufficient to hold the arm in place.

Because the control of the motors is local closed loop, it is the position of the motor shaft, hence the horn, that is controlled; there is no direct control over the position of the arm itself. If this kind of feedback were available, it would be relatively easy to compensate (to a limit) for this looseness. Perhaps we can find a way to take the potentiometer out of the motor housing and use it to act directly on the arm? But that would cause real problems when an attempt is made to hold the arm at a position (near equilibrium) where it will never stay.

The base and shoulder are underpowered, show lots of ratcheting

Without a doubt the two motors which are asked to do most work are the base rotate motor (should have been mounted to the edge!), and the shoulder which must lift the entire weight of the arm. When moved at moderate slow speeds, both these motors ratchet, the arm tends to have a natural oscillation frequency corresponding to the ratchet, and the entire motion is hugely unsatisfactory, occuring in jerks and jumps as it does.

The base is not so stable

Top: think of the forces on the pivot at the centre of the turntable when the robot is in this position; Bottom: the turntable, bearings just about visible.

If the turntable on the base, on top of which the arm is mounted, is screwed down too tightly, it won’t turn. It must be screwed down loose. But the turntable has to withstand a large effort to lever it from the base, both statically and dynamically. Although the base is well engineered for what it is, it is not strong enough. Ideally want all-aluminium construction with metal bearings including down-bearers; the motor would be better placed at the edge, and this would require a motor which can turn all the way around at controllable speed.

As well considered and manufactured as the base is, it is ultimately under-engineered. If nothing else, it needs something to stop the back of the turntable from pulling away from the base.

The wiring is not all long enough

Ouch! This wire is not going to last long at this rate… see the position we are trying to put the robot in?

Some number of wiring extenders are provided with the kit, but one less than enough. It is not possible to permute the wiring so that the robot can attain all possible poses. We can rectify this with a soldering iron.

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