Wednesday, 18 April 2012

Thermocouple Experiment and an Update

I know that many of you will be anxious to see pictures and video footage of the tests I did on the prototype Shear Coaxial Injector. I am working on that post and editing the video footage to ensure the salient points are presented.

I am also still working on reinstating my workshop. In the meantime I decided to try to make some progress on the development of the sensor suite and data acquisition system for the completed engine.

At the start of this month I received some 0-60 bar (0-870 psi) pressure sensors and some ruggedised K type thermocouples. The pressure sensors are Parker ASIC Performer types. These units are 1-6 Volt output. This is accessed by means of a micro DIN connector. The thread on the business end is 1/4 inch BSPP. Here is a photograph of one of the units:-

I intend to use these devices to measure fuel, oxidiser, pressurant and chamber pressure on the test bed. I haven't trialled one yet as I will have to get one plumbed into my Enerpac, which will have to wait until the workshop has been set up.

The K type thermocouples came from Rachel McGill of McGill Motorsports, in Kirkcaldy, Fife. The units are designed for use in performance cars to give an indication of exhaust gas temperature. In the Thunderchild Engine they will be embedded in the injector and the chamber flanges, to try to get an idea of chamber wall and injector face temperature. Mounting is by means of a stainless steel adaptor threaded to 1/8 inch NPT at one end. When assembled, the opposite end of the adapter looks similar to a JIC or AN fitting. The resemblance is superficial however, as the barrel nut hides a brass ferrule. This arrangement grips the thermocouple and allows it to be set at any depth in the measurement port. This will be easy to integrate into the Thunderchild chamber flanges and injector plate. Here is an exploded photograph of the thermocouple assembly:-

I have done a lot of work with microcontrollers over the years, primarily with the BASIC Stamp and PIC devices. Whilst these are capable systems, I have decided to move over to the mbed NXP LPC1768 device. This is a 40 pin DIP packaged device based on an ARM 32 bit Cortex core. It is a highly capable device intended for rapid prototyping and supports a wide variety of analogue and digital input and outputs as well as all the popular communications protocols, i.e. USB, RS232, CAN, Ethernet, SPI and I2C. The unit is very well supported via an excellent website. A Handbook gives detailed information and programming examples whereas a Cookbook allows users to share code and other project information. The user friendly compiler is online based, which allows the user to develop projects anywhere, on any machine.

Communication with the device is via a virtual RS232 port over a USB connection. The unit looks like a USB memory stick to the host computer, and firmware is loaded onto the device from the compiler by dragging and dropping, as one would do to save any file type to a memory stick. More information can be found at:-

I interfaced the thermocouple to my mbed unit using a MAX6675 Thermocouple to Digital converter IC. This is a fantastic little device that applies the cold junction compensation on-chip and then transmits the measured temperature using the SPI protocol. The device is only available in an SO package and so I got mine on a break out board from AdaFruit:-

Here is a photograph of the breadboard with the mbed microcontroller and the MAX6675 break out board connected:-

Here is a close up of the MAX6675 break out board:-

The mbed is programmed in C. I am much more familiar with PBASIC and I am finding C quite a trial to learn; compared to the average C program, PBASIC looks like a lost work of Shakespeare!

Fortunately the mbed website includes a wealth of example programs and I downloaded the "Hello World" example for the MAX6675. This reads the temperature from the thermocouple and then constantly transmits this to the host computer via the built in pseudo RS232 port. I used TeraTerm to display the results. This is a fine, workmanlike freeware terminal emulator:-

Here is a shot of TeraTerm with the displayed temperature of my kitchen!

I am going to continue setting up the workshop this week, and working on the blog post detailing the results of the Shear Coaxial Injector experiments. This will be followed by more on injection - how and why I decided to move towards swirl coaxial types.

Keep watching.