glass melting kiln | solid state relay | n-mosfet | EAGLE | Roland mill | fabmodules
Assignment 9 has been the most serious preparation for my final project yet. As I reviewed in assignment 1, I'll be making a development unit for Mediated Matter's Living Glass project. In assignments 5 and 7, I modeled and fabricated some of custom mechanical parts for a glass printing system. For this assignment I've designed electronic components that can be used to take control over the kiln systems using a computer. The current kiln models are a mix of custom made and off the shelf systems that use 120VAC to power both the resistive heating elements as well as the industry standard LCD display and multibutton controller. They are an established program, one that includes a number of safety precautions that we are very interested in keeping, but the programming interface is built for a completely different time scale than that of the glass printing process. Essentially, all standard kilns are designed to be turned on and left on for weeks on end, while the glass printer really needs to adjust temperature by user input, on the minute time scale. Luckily, the controls are as simple as turnng the heating elements on, waiting some amount of time, and checking the thermocouple to see if it's at the requested temperature or not... repeat. And to run current through the elements, the controller throws two relays at 24V. To do this with user control, I've designed a microcontroller which can operate these two relays.
Both of these relays are run with 24V taken from a transformer that ouptuts that voltage from the 120V input. For the kiln company, this allows them to simply have one wall plug and one ouput plug for the kiln for everything. There is a second tranformer that transforms 24V to 5V for the logic board. The first relay is solid state, and will work under 5V, but the second relay is a mechanical safety relay with a 12V requirement. The pupurpose of this is that in the event the solid state relay fails, it may fail closed and if it does, the elements will stay on until you've burnt everything out, so the safety relay can be opened, and if it fails it is sure to fail open. This means my board needs to run two mosfets each controlling 24V coming from the tranformer and going to the relays.
I've used the template of the hello world board, with pins 7 and 8 as the addressable n-mosfets which go to a 2x2 header for two leads from the transformer's ground and two leads from the relays respective grounds. I've also added two LED circuits in parallel to signify which relays are activate at a given time. At the moment, the C code (generated from scratch, or from the Arduino IDE), can control the relays in any preset sequence. Next week I will include in the board an amp for a thermocouple, so that the code may be responsive to the inputs.
Finally, I've designed the device so that it can run on top of the standard controller. I've chosen to do this because the standard controller contains facilities for long sequences of predetermined temperature ramps holds, and delays which are very nice for raising the kiln temperature to the point where we can print. I may even include a togglethat switches control to the custom device when we need finer control, though this isn't totally necessary. The board's progression went from the simple adaptation of the 'hello world' led board, to one which had addressable pins for the relays in series with indicator leds, and finally the functional board with adressable mosfets and indicator leds in sseries.
