High Speed XY Pen Plotter with Acorn Controller

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bsteele1123
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High Speed XY Pen Plotter with Acorn Controller

Post by bsteele1123 »

I am looking for some advice for setting up a high speed pen plotter with the ACORN controller. The XY movement is very straightforward and no issues there. In this case it is a couple of servos that drive cables that drive XY stages. It is capable of very rapid movements. I plan on creating the print files and NC code for it in Fusion360 if possible.

Now the interesting part will be the Z movement. For this application the Pen is either up or down but the pen pressure is something that is controlled via Arduino and PWM output plus an input that specifies direction--these control a voice coil. The z axis is capable of moving quite fast and can move up and down 40 times per second so in programming I need a method that can operate it this quickly.

I am looking for advice on the best way to set Z pen movement and pressure control in Acorn. The main pieces are to either supply the Arduino with PWM or DC level that specifies the pen force and also be able to supply a direction signal that can change quickly. Of course the movement is not instant but the dwell time before resuming XY movement would be 20-25ms so relatively short.

So how to set this up in ACORN and Fusion360. Like a laser engraver where PWM controls laser intensity and ON/OFF. Like a spindle where DC or PWM is like spindle speed that varies and a Z direction output controls up or down? It is sort of like a mix between something like a laser and something like an engraver. Other?

Maybe someone out there has done something similar?
Richards
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by Richards »

bsteele1123 wrote: Sat Oct 23, 2021 1:39 pm For this application the Pen is either up or down but the pen pressure is something that is controlled via Arduino and PWM output plus an input that specifies direction--these control a voice coil. The z axis is capable of moving quite fast and can move up and down 40 times per second so in programming I need a method that can operate it this quickly.
If I were doing this, I would have the Acorn only send an Up or Down signal to the Arduino. I would let the Arduino handle the pressure independent of the Acorn. In other words, if the Z-axis is Down, the Arduino would keep the pen in proper contact with the media. An Arduino is capable of doing complex tasks if you provide the sensors required.
-Mike Richards
bsteele1123
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by bsteele1123 »

I am curious the best way to create this operation somewhat automatically when generating NC code from Fusion360. This is why I though I might include the PWM output as well. So for example I would set this up like a router and set it up with some nominal but very small z movement (G1 Zxx) for the router the Arduino/pen would then just look at the Direction output from the acorn to know whether to move up or down. Then potentially have the pen force controlled by spindle speed PWM output. So when I generate NC code from Fusion360 I could configure a spindle speed that has some equivalent pen force? This way most necessary parameters could be generated by Fusion360 but the Arduino would take care of any calibration and setup for generate the correct force and direction from PWM output and Direction output from the Acorn.
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by Richards »

I use two different methods with I design a process control device. One method is Top Down. That means that I start with a list of requirements that the process control device must handle and then I build/buy all the parts required to handle those requirements. The other method is Bottom Up. That means that I must work with the parts/devices that exist in the device and then find a way to make them do whatever is required.

If I needed to control pen pressure/height on a plotter, I would start by finding/using an existing electrical/mechanical device that controls pressure/height and then I would design whatever was required to make that part work. The only signal that I would need from the Acorn would be a Start/On signal. The only signal that I would send to the Acorn would be an Enabled/Working/Functioning return signal.

If Fusion360 has the means to control pressure/height, than I would install whatever hardware Fusion360 talks to.

My D-size HP plotter uses gravity and a weak spring to keep the pen in contact with the plotter paper. It lifts the pen until it needs to put some ink on the paper and then it drops the pen. Gravity and the weak spring do the rest. The pen Up/Down is a simple On/Off solenoid type operator.
-Mike Richards
bsteele1123
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by bsteele1123 »

In this case I have an application that is top down for hardware--I can choose what I want to physically create pen pressure, calibrate pen pressure, etc. I am engineering those pieces to work extremely well.

The software and to some extent CNC controller are more bottom up when it comes to actually plotting with a desired pressure. I am effectively trying to hack a way for this pen device to interpret already accepted lines of NC/G code into a pressure that I want using fusion 360. So the closest analog to this in fusion 360 would not necessarily be plotting but for instance--routing/engraving. In that case the pen itself might have a small tool diameter that I assign. The setup would also have a spindle RPM that I configure for the process. In this case the spindle speed--which may output as a PWM signal would then be interpreted by the Pen mechanism/arduino as a pressure--or more precisely a force to apply using a precision voice coil and VCA controller.
miker
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by miker »

That is a very interesting project. Unfortunately for me, it is far above my pay grade. I'll be following your progress.
bsteele1123
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by bsteele1123 »

I did get this working in CNC router mode. The Spindle speed is used to set the pen pressure and the Z axis direction output is used to control when the pen moves up and down. The speed it flows through the Z axis commands are set based on the lowest pressure setting. The voice coil controller worked with a few different options from Copley and A-M-C that control DC motor current via PWM input and direction input. It works quite well, the pen can move up and down within 12ms and force is controlled within 1 gram between 30-140 grams. Fun figuring out how to get it to work with the ACORN.
Topaze47
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by Topaze47 »

Wow, neat project! I built a pen plotter way back in the stone ages using variable reluctance 4 phase steppers. My version moved the pen for the X axis and moved the paper for Y; the major problem I had was bashing the points of my plotter pens flat after only a few plots. I solved the problem by milling a groove in the platen under the path of the pen and adding a fixed stop to prevent the pen from traveling too far down. That way, I could hammer the pen up and down as fast as I liked!

Your way is a lot more elegant, though...

Video of HP pen plotter @
bsteele1123
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by bsteele1123 »

That is a nice quick plotter. What is moving the pen? In my case since the pen pressure is controlled by a voice coil there is no specific stop needed since the plotter will only apply a fixed amount of force when the pen is down. Was your plotter done with an ACORN also or some other controller?
Topaze47
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Re: High Speed XY Pen Plotter with Acorn Controller

Post by Topaze47 »

Actually, the video is of an HP pen plotter from the 90's, but my plotter worked in a similar manner, just slower. The problem with a voice-coil pen mechanism is that for the pen control to be fast, the voice coil has to accelerate the pen downward very rapidly and then bring it almost to a stop just above the paper surface, and then apply the 'hold-down' force. This requires position feedback as well; if you don't stop the pen just before the paper surface, you will flatten the tips of your pens within a plot or two...
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