ABE Accessibility Mod: Continuous Drive Servo Configuration Windows

["What the hell happened to my feet???]

What’s the deal with drilling holes in your drive servos?

Continuous rotation servos are made by taking normal servos and replacing the feedback pot with a static pot and/or some extra resistors. In essence, when you tell a regular servo motor to move itself to a specific angle, here’s what happens:

1. It reads its current angle
2. It sets the direction of the motor in such a way that the difference between the current angle and the one that you’ve requested will be reduced
3. If the difference is large, the speed of the motor will be set high. If small, it will be set low

[pre-mod: notice the pot onboard and where it should line up to roughly once reinserted]

In a continuous rotation servo:

1. It reads its current angle the angle given by the static pot (usually inside the enclosure). The static pot is usually set such that it reports back about 90°, which is the zero-point of most standard servos.
2. As above
3. As above

[a completed accessibility mod - I see a pot dial!]

So, why are you drilling holes again?

Well, in the not-so-wonderfully-made servos, there is a tendency for the servos to (over time or perhaps with vibration/delivery) have the pot move or the impedence in some way change itself. That means that when you want a continuous rotation servo to STOP, it will often be just moving instead.

Now, this can be fixed by telling the servo to go to the 90° angle and adjusting the pot until the servo stops moving. Doing this is a finnicky operation as the four screws that hold a servo together usually release all the gears, the circuit board and the motor at once. Getting it back together is a pain in my arse.

SO! One simple hole later and we have a feature that some servo makers are charging extra for: Accessible tuning pot. Done!

Sneak peek: Upcoming project...

A pic from my upcoming project. A Cheyne-Stokes detector (experimental) of my own design. Project requirements: Mum to remind me how to use her sewing machine so I could go all stitch-bitch on this thing. :)

Bang! Narrowly-avoided Disaster

What the hell was that?

Not smart, I know. I was goofing around with an old transformer and I decided to hook it up to my WRPC to save on batteries whilst under development (replacing batteries gives me the shits). Anyhow, I wasn’t careful enough when connecting the leads (I forgot that 9V leads to 9V terminals invert) that I pumped a high-current 12V directly into the fuseless voltage regulator backwards (I seem to have a thing for backwards lately don’t I?).

The poor little bugger didn’t just burn out in the usual cloud of blue smoke. It went BANG!! and scared the crap out of me.

I was 70% confident that I’d only destroyed the regulator and it turned out I was right (thankfully). Swapping the regulator out with a new one (I keep these in handy supply) brought the WRPC right back to fully working order, however, it wasn’t without its twists and turns.

Track repair

Yep. Ahhh shit. Pulling out the dead regulator had some circuitwise side-effects. The traces attached to two of its legs didn’t like being disturbed and tore up away from the board.
Normally at this stage, the board is toast but if you read the previous post on how long it took me to make this damned board, I wasn’t about to give up on it.

Enter superglue

Superglue proved to be extremely useful in this case (sans the fumes when I re-soldered). It stuck the tracks back down and held them whilst soldering. I used one of the newly snipped pins of the replacement regulator as a jumper on the ground pin and it was as good as new.

Moral of the story

If you’re going to hook a transformer up to your ordinarily battery-powered project:

DO

• TEST the transformer with something that isn’t your current life’s work
• Use a multimeter and double check the polarity
• Use an inline fuse that will blow if you screw up

DON’T

• Be lazy
• Guess at the polarity
• Just hook it up and hope

I was fortunate that everything turned out okay and the WRPC was back up and running in under half an hour. It might not have been so chirpy though so I’m going to be a lot more careful next time.

ABE Signage

Randomly spotted this* whilst out and about.

It's just begging to be defaced. Not as clever as "rave outside the guard's compartment naked with a blue light", but meh. :)

*Those of you who know me well enough know that I rave about CamScanner+ for iPhone. This was a sign on a wall that I took a photograph of on an angle. App love!

First look at WRPC with ABE

This is a pre-pre-pre alpha look at the basics of communication between WRPC and ABE. Spot the bug at the end of the clip :)

Dual Battery Monitor Adapter

This is a design I sketched up tonight for a compact in-line dual battery monitor adapter for Arduino. Those of you who have looked into battery monitoring for Arduino will know that the maximum voltage that the Arduino's ADC pins can handle is 5v.

Here's a simple design (5 solder points) that will take your servo battery (~6v) and your Arduino battery (~9v) and reduce the voltage from them to safely measurable levels. I wouldn't push this design past measuring two 9v sources as it gets a bit close to the maximum.

Assuming you are using the voltages specified, the power usage is 58.5 microWatts. If you use 10M resistors instead, you can reduce that by a factor of 10 (und so weiter...).

I will be making one of these for ABE and a half-one for the WRPC so that they can monitor their own battery levels.