Not really a retrofit (yet), just wanted to try out EMC. HP recorder was closest thing to an XYZ table I could find in my storage. And the Z-axis is of course a pen holder solenoid.
X- and Y-axes can be easily controlled by feeding voltage to built in banana (lab) sockets, with simple DAC (This one obviously has integrated PID).
Plotter has an active area of about 260 mm x 195 mm. There are 3 different sensitivity levels: 5 mV/cm, 50 mV/cm and 500mV/cm.
This means I have following options for DAC range:
X-axis
5mV/cm * 26 cm = 130 mV
50mV/cm * 26 cm = 1.3V
500mV/cm * 26 cm = 13V
Y-axis
5mV/cm * 19,5 cm = 97.5 mV
50mV/cm * 19,5 cm = 975 mV
500mV/cm * 19,5 cm = 9.75 V
I have an old ISA-I/O card with 0...5V or 0...10V DAC ranges so I can easily get started. Card has 12-bit DAC's with enough speed. This means I can get resolution of 5V/2^12 = 1.2 mV/step. If I want to have full X-Y-range and use 5V reference, this leads to resolution of :
50mV/cm = 1.2 mV / x => x = 1.20 mV * cm / 50.mV = 0.24 mm
which is nice for testing. Even though I could just set reference voltage to 1.3V and get resolution of 260mm / 2^12 = 0.06 mm.
Let's see what kind of feed rated this resolution provides. If we move control servos using linear ramp and 1 kHz DAC, speed will be:
1000 steps/s * 0.24 mm / step = 240 mm/s. Is more than enough.
1000 steps/s * 0.06 mm / step = 60 mm/s. Still fast enough.
Z-axis does have only a switch, so some modifications are needed. Maybe relay or just an opto-isolator with FET. I just have to find control pins.
When pen is switched down, pins 3 and 6 in J2 are shorted. Pin 2 is also ground for PSU on-line indicator LED and pin 1 is anode.
(continues...)
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