Wednesday, September 26, 2012

Nav Class, Day Two

Back at it today.  We had a long, chock full day of class.  So much so that at the end it was difficult to remember what we had done at the beginning.

I think we started out talking about analog inputs and outputs.  We'd done digital yesterday so moving on to analog was a natural progression.  We got a little derailed during this discussion while talking about scaling the values.  Analog values have a pre-scaling offset, a scale, and a post-scaling offset.  Coming to a good demonstration of that got confusing.  Suffice it to say if you are going to pick round numbers to make the math easy don't pick the wrong operation for those numbers lest you confuse your students who are following the math.

After the discussion we went to the lab and wired up two analog inputs: one from a pull string pot and one from a dial pot.  These were slightly more complicated than their digital cousins because they have an extra line: "the wiper."

From there we went back to the classroom to take about relay outputs.  These are different from digital outputs because they are a gate to pass something through rather than just sending a signal or not.  I got confused here because of my prior training with relays being mostly about SSRs.  I kept looking for how we were going to wire in a "control signal" but this doesn't have that as it is implicit in the I/O module.  It looks odd to have a "relay" with outwardly only one side of the thing, but I got my head wrapped around it.  Most of the relay discussion revolved around E-Stop setups.

Getting relays in our kit allowed us to wire in an E-Stop contactor in the lab.  And once we'd done that it meant we could wire in the motor drive and make things go (after doing a bunch of operations & indicators to flesh out the E-Stop functioning).  We had to set some parameters in the drive first, so that was new.  Then we wired up and assigned remote controls for the drive: go/stop, right/left, and speed.  I had a good trouble shooting moment here when we couldn't make the remote functions work: noticing that there wasn't just a parameter in the drive for how the signal would be sent, but also that there was one there to listen for the remote signal in the first place.

At which point we had a working motor, control buttons, and limit switches (did we wire the limit switches today or yesterday?  I don't remember).  We spent the rest of the day writing rules to make the rig do various things based on various inputs: one using three buttons and another doing most of the same operation with one button.  Through here I got to the point where I couldn't keep track of things in my head any more.  We had several SNAFUs with running in the wrong direction or using the wrong limit or having rules in conflict (some of which was exacerbated by muddy instructions) but in the end we got everything to work out (and finally finished something ahead of the other team - debugging is much harder through a translator I guess).

Half way through.

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