Following is a report of my progress regarding the viscometer. Having spent several months on this project and having gone through four mechanical iterations and umpteen electronic iterations of design, it’d be a good idea to go through and describe where I am in the project, and where I have to go in order to follow the project to completion.
Magnet Drive (click image to see full size)
This unit is the main magnet drive. It is responsible for the pulse width modulation control of the four H-Bridges below it. Each H-Bridge is fully optically isolated, can happily push 15A at ~3KHz with any duty cycle. The H-Bridges were also designed in such a way that they don’t produce much power supply noise and very little inductive hiccup. The Board on the top has a microcontroller that controls the four H-Bridges and accepts input from the main control board and the dip switch set on-board for debug purposes.
The unit has been fully tested (waveform, heat and max current) and works, no more modification or re-design is required.
Input Totem (click image to see full size)
This unit was built to stem problems arising from EMF and Static discharge. Earlier versions of the project would suffer from random shutdowns, errant signals and noise. I wasn’t entirely sure for some time why certain events would occur for no reason but even pressing a switch sometimes was enough to roast a microcontroller. I recently tracked it down to long input lines picking up noise and also generating power dips when, say a switch, is activated. This unit isolates the inputs from the faceplate and debounces the signal in order to remove potentially hazardous voltages from making it to the mainboard. This unit also dissipates large transient voltages through the use two voltage suppressors per input line.
Currently this unit works and has been tested. It does require some additional programmatic tweaking.
Main Motherboard (Click on image to see full size)
This unit comprises of a larger PIC18F microcontroller with a 4 digit LED display, this is the primary control board. This board is responsible for controlling the Magnet drive, The LCD Front Panel Display, The Debug Display (The one on the board), calculating the results of data, outputting said result and it also handles the filtered input. This is basically the ‘brain’ of the machine and this particular part of the project has gone through the greatest number of iterations, probably 8 or so.
This unit, circuit wise, works and has been tested. It still requires programming as the entire unit has been redesigned and functions in a different fashion than the previous versions.
General overview (Click on image to see full size)
This is a general overview of what the internals are going to look like when assembled. Of course, these are not the final positions of each unit and a couple of units are missing from this picture (namely the +5v isolated supply and the Serial Out board)
The Current Design (click on image to see full size)
This is the latest design of the unit. Several design changes have been made in relation to the previous 6 designs (3 were made). Following is a list of changes.
- Made the magnet units smaller but longer, opting for size in heat-sink area.
- Magnets are closer together towards the paddle
- Sensor unit no longer consists of two reed switches and now consists of one hall effect sensor.
- Timing is found by center point activation rather than activation at extents of travel.
- Wiring is changed from Single solid-core Teflon coated wire to shielded, multi-lead stranded wire for easier assembly and maintenance. Also the shielding provides protection for outside interference and electrical problems.
- Top and bottom plates are solid instead of multi-piece in order to simplify manufacturing
- Paddle is shorter and has less area, PWM will ensure proper shear rates on paddle.
The previous designs had the following flaws:
- Distance to paddle from magnets was too great, creating a ‘dead’ spot in the center of travel
- Making a magnet larger does not mean greater field intensity at larger ranges. Each previous iteration was based on the false assumption of larger=stronger.
- Reed switches were problematic. Either moisture or improper magnetic application proved to create intermittent contact on the switches making for a difficult to track problem.
- Previous units were susceptible to heat problems. Smaller magnets, lower current and warm-up cycle solved the issue. Plastic sometimes melted under large loads
- Units were difficult to assemble, thus, difficult to maintain in an industry setting.
- Units had lots of nooks and cranny s making it difficult to clean.
- Previous paddles were either too larger or were of the incorrect length at both the signal and Thrust sides.
As far as this part of the unit is concerned, I’m not far from completion, aside from machining.
What is left to do?
So far, not much is left to do on a surface level. I have the following items to complete.
- Finish design for unit (Decide on arrangement and potting of sensor)
- Machine parts for prototype
- Finish some programming on the input totem.
- Start testing with dummy board
- Once tests on mechanical portion are done, main board needs programming
- Program main board (in no particular order)
- Confirm and program communication scheme
- Derive algorithms for appropriate sampling and error correction
- Program debug display
- Program for parallel LCD display
- Create initial setup and calibration routines
- Create run-time user interface and system.
- Test, test, test.
That’s the list thus far, and completed as per my knowledge.
It should also be noted that I’ve built an automated rotary paddle type viscometer (rudimentary prototype) that is functional and can easily be adapted, with appropriate components, for in-line use. I’ve also produced a working vibratory viscometer, it however doesn’t follow non-Newtonian fluids well.
Hopefully this was informative, thanks for reading.