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Friday, April 27, 2012

CryoBUG Controller

Well I've been busy over this last week or so. Besides trying to track down a source of cheap R-600a (Isobutane), I have also been designing an electronic controller for the more finalized version of the CryoBUG.

Here is a list of some of the things I wanted to incorporate into this controller.
  1. Toggle compressor power ON and OFF by pressing momentary push button switch.
  2. Have a bi-color led back light in the switch for "ON" as well as "FAULT" indication.
  3. When experiencing a fault such as an Over Pressure condition, to shut-down the compressor and require user intervention in order to reactivate it.
  4. Have the fault indication retained even after the condition that caused the fault has cleared (fault memory).
  5. Provide redundancy in case the logic circuits fail, so that a fault can still shut-down the compressor.
  6. Provide a remote means of both monitoring and control of the CryoBUG functions.
  7. Utilize a short circuit proof control transformer and a sealed compressor relay for hydrocarbon refrigerant safety.
  8. Maintain low EMF and RF emissions by the use of low speed CMOS switching circuits and linear voltage regulation.
And here is the circuit design that incorporates all of these requirements...

Click on Image to Enlarge

And here is a small video showing the bread boarded prototype in action...



Just a little more background on the remote aspect. This design incorporates a fully isolated interface for remote monitoring and control of all the CryoBUG functions. And in the anticipated end user application, there will be several of these units providing water vapor cryopumping for small vacuum coating chambers on an automated process cycle. It is desirable to have all machinery, pumps, ect. controlled remotely by the central computer control system of this vacuum coating system.

Here are the functions provided by the CryoBUG remote interface:
  1. Control: Compressor ON/OFF (universal 6v-24v AC/DC optically isolated input)
  2. Status: Compressor Running (contact closure)
  3. Status: At set point temperature "Ready to Coat" (contact closure)
  4. Status: Unit Fault (contact closure)
(Edit 4/29/12 Rev 1.1: the remote control pin-out has changed as well as some functionality)
If it is desirable to disable the manual On/Off switch via the remote, then holding the remote in an active high state will do so. With this inverted logic, it would be necessary to momentarily remove the remote On/Off signal (bring it Low) in order to toggle power remotely, and then immediately restore the signal (bring it High). However If simultaneous manual and remote operation is desired, then use normal logic for remote power control (momentary High to toggle power).

As for temperature monitoring and set point, I decided that it would be best to utilize an independent process controller such as the Solo SL4824-RR 1/32 DIN from Automation Direct.

These PID controllers are getting to be very inexpensive for all that they are capable of doing. It just wouldn't be practical for me to try and duplicate this functionality into my CryoBUG controller. Although there are some cheap temperature displays available from China, but most of them are not able to measure much below -40°C.

So in my design I routed the relay contact of the process controller through my board, and make it available as a status contact on my remote connector. The user simply enters a set point on the process controller where they wish to be alerted when the CryoBUG has achieved that temperature. This "Ready" signal let's the vacuum coating system know that water vapor cryopumping is active, and that the coating process can begin.

At this stage, the controller design looks pretty solid, so the next step will be to lay out a printed circuit board and get a few fabricated. I'm using the ExpressPCB software to design my board, and will compare their cost to Futurlec's to see which fabricator I wish to use. As far as I know, these are the only two choices available when using that software package. On the plus side, it is extremely easy to use, so much so, that it's truly a pleasure to work with. Also the ExpressPCB software works great on my Linux machines under Wine.

As for other parts of my CryoBUG's development. I'm still waiting for some R-600a to arrive from England, so further charge related tests are temporarily on hold.

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