Well, things have been proceeding apace, and I’m feeling pretty confident that I’ll have both versions done fairly soon. For debugging the device I’ve decided to create a kind of terminal, this allows one to directly access commands and see log information from a terminal program. This is handy for seeing data over time and for various other uses.
The viscometer is nearing full completion! YAY! To mark this occasion to mark the end of a long two years, I am placing a small gallery of the almost finished product. The only thing missing is the outside cover which consists of a 4″ od aluminum tube. Also, the springs I’ve ordered have also not arrived as yet, however, for now, the elastics will suffice. Here are some of the features and facts:
Can be used in-lab or in-process
Selectable RPM with a tight tolerance on RPM +/-0.5RPM
User calibration routines. This allows the end user to calibrate with 3 fluids of known viscosity
16 key keypad, used for calibration and settings, also for running special tests
Can be used as a laboratory gel-timer
Can be used for custom tests besides stormer viscometry
Low power consumption <100ma or <2.4W
RS-485 Serial output
Control electronics have complete galvanic isolation
24VDC supply required
repeatability (requires further testing) +/- 1.5%
Modbus protocol (not yet implemented)
Here’s the Gallery!
Yes, a few too many pictures, oh well.
This thing took me quite a while and what I learned from it was immeasurable. Thankfully now that everything works as expected I can focus on my other projects without having this thing hanging over my head. Here’s to completion!
As an aside, here’s an interesting document on viscosity. here
Since my design was accepted in terms of moving ahead on a prototype, I’ve been working first on the board design. I have decided to abandon the PIC18F2620 in favour of the 18F4685. The reason for this change over is due to the fact that the 18f2620 doesn’t have enough I/O to handle the addition of two analog channels and four I/O for RS-485 communication.
This post is more for my own edification and to help me sort out my thoughts on this issue. I suppose for the sake of following my train of thought while sitting here, I’ll outline the specifications, as I think of them.
Overall feature set:
PWM output for 6-24V DC motor.
RS-485 Out – Rec enabled
16 key keypad in
2 temperature sensors
LED indicators for power/error
Serial out for RS-232
Input for external reflection sensor
2 inputs for timing sensors
So, thats 1+4+6+8+2+2+1+1+2 = 27 inputs
I found some nice Molex headers that are single row, .100 pitch and is latched. Typically I use the friction based header and housings but it needs to be secure inside the housing and thus I’m trying out the new set. Also I’m going to use vertical out terminal blocks in order to save space inside the unit but not necessarily on the board.
I’m also considering adding an RS-232 port along side the board.