Encoder output on an HP 7550 Plus plotter

mojorizing

Attached is a pic of the pen height encoder output from the pen holder of an HP 7550 Plus Plotter. It's not what I expected - what I expected was a continuous stream of pulses with a shift in phase as I move the pen up and down. What I see is two distinct pulse streams that are'nt identical and no pulses are generated if the pen isn't moving up or down.

Can someone fill me in on what encoder is being used here?

Kevin

Duane Degn

It looks like a quadrature encoder to me.

Channel 2 looks like it's 1/4 out of phase with channel 1 which would be expected wtih a quadrature encoder.

Gadgetman

Looks pretty identical to me, as long as you realise they're phase-shifted from each other, and that the difference in pulse shapes at the beginning is because of the 'ramp up' (the motor is accellerating).

But this being a HP, I also expect there to be at least one endstop switch.
(I could open my own 7550a to take a look, but... It's stored safely in the attic, and I'm a bit lazy)

mojorizing

Thanks for the replies.

Looking at the output some more, it is a quadrature. It was the acceleration and change in direction that I wasn't familiar with.

I'm trying to use the plotter to print to make PCB's, but one feature with the plotter is when the pen is up, the X and Y acceleration is maxed out to transverse real estate quickly - the board losses it's register when it gets shot out of the plotter. A software solution was offered by Mr Carpenter in Circuit Cellar #202 that broke down the path to minimize pen up conditions. I thinking of actually intercepting the pen up encoder
and telling the plotter the pen is down when it's actually up, otherwise this plotter hasn't been seeing much use.

software from Mr. Carpenter can be found at:

http://www.mmnt.net/db/0/0/ftp.circuitcellar.com/pub/Circuit_Cellar/2007/202