Working on new viscometer head

  Well, I’ve been designing some new stuff since having a little wind of inspiration. This head weighs in at about 700g and provisions have been made to allow an attachment to a handle and therefore a hand-held version could be made. Here are some images.
 

Line Drawing
Line Drawing

Fully Assembled visc head
Fully Assembled visc head

Viscometer head opened
Viscometer head opened

Viscometer head from the back
Viscometer head from the back

 
  I’m going to use these design concepts on the next in-process version. I’m going to design the external case for the electrical portion and for possible battery storage. Since the motor only draws 100ma at 24v or 200ma with a 12v motor, a hand held version is definitely possible. This version will use a PIC16F767 for control and using a smaller LCD display to display info.
  Now, on to developing some other devices. I’ve wasted far too much time in the last many months, time to get busy.

3 point quadratic regression formula

While developing the in-process stormer viscometer, one of my goals is to allow the end user to calibrate the device with 3 fluids of known viscosity. With a bit of help from mathematica, I’ve found the formula and for whoever wants it, it’s posted, also for my own future edification. This formula is meant to convert 3 data points of the form {X1,Y1},{X2,Y2},{X3,Y3} into a form of ax^2+bx+c=y. This is probably the fastest way to do this kind of operation on a microcontroller. Here’s an image of the formula

Reverse Quadratic from data
Reverse Quadratic from data

also, here’s a dirty FreeBasic program using it. regress-3point.bas

Also, for fun I decided to do a cubic version. This is in the form of 4 data points {X1,Y1},{X2,Y2},… converting to ax^3+bx^2+cx+d=y

Cubic equation inverse
Cubic equation inverse

You gotta love Mathematica!!!

New Viscometer board design and physical revamp.

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
LCD out
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.

Hopefully this will be the final hurrah!

Viscometer Ver.(Eleventy Billion Jillion) a la rotational

Well, this has a been a long few days of work. I have created the ideal rotary viscometer in both terms of price and repeatability. Now, this idea is nothing new but I’m simply happy that it works. I guess it’d be prudent to go in to how it works…

How it works:

The concept is simple. you have a shaft that is separated by a spring, in this case two plastic cups connected by a ball bearing with rods sticking out one side. This shaft is driven by a small gear motor and a set of paddles is attached to the other end. While this is rotating, the difference in the driven and the resisted side is measured with some form of instrument be it a hall effect sensor or a slot type optical transducer.


Viscometer and stand
Viscometer and stand


What I used:

Well, as far as materials are concerned, for the prototype I used blocks of polyethylene to support both the motor and the shafts. The material has pretty low friction properties at low loads and thus I used it as the bearing on the bottom. The actual difference mechanism is made from a high density plastic. I would probably use this material again as it’s light, rugged and easy to machine. For the real unit, I may still use this plastic as it is more than strong enough and it’s easy to machine.

As for the drive I used a design similar to the older designs with PWM motor output, serial output, keyboard input, LCD out and 2 channels of input. This is all controlled with a PIC18F2620 which is more than enough for it’s needs.

The motor itself is a Hsiang Neng gearmotor running at 12 volts. It’s a piece of shit but that’s not important at this stage of the game.


Motor and sensor
Motor and sensor


So, finally, a desired feature list:

1. RS-485 Out
2. KU and Cp out
3. Multi-fluid calibration, this feature allows the user to select fluids of any KU value and calibrate the unit by entering them in.
4. Easy to use menus. Too often have I seen automation stuff that’s unintuitive. This hould be easy for the operator to understand and easy for the people to use.
5. Speed selection, so that you can use under-powered motors.

Anyways, here’s a gallery of some of the pictures of what could be a DIY stormer viscometer.

Rotary Strain Gage update

Well, it’s been a little while since the last update, I suppose I should have stated something sooner. So far, the device seems to work but now I have mechanical problems. Friction on the top is preventing the load from being sensed at the paddles. Plus, the strain gages, I believe, are placed improperly. While it looks as though one side gets compressed and the other side gets stretched, I am thinking that this is not the case.
Unfortunately what that means is that the strain gages are counteracting each other, thus the voltage difference is too small to detect effectively.

Gonna design a new one with only two strain gages.


Working on yet another stormer viscometer board. :(

Well, For the last week or so I’ve been developing a new viscometer board. Sloppy as it is, it’s a jump ahead from the last board in terms of overall inputs allowed. I used the 68PLCC package PIC18F6680 in this one because it had a sufficient number of IO and because it looks cool in it’s socket. 🙂

Anyways, this one has the following features:

  • Several analog channels for temperature measurement.
  • Several analog channels for use in a torque sensor set-up.
  • Some analog channels.
  • built in RS-232 Output.
  • built in RS-485 Output.
  • Provisions for PICKit2 hookup.
  • TVS’s on all input lines
  • Separate PIC12F683 for PWM output
  • Debug Serial Output.

All in all, it’s gone smoothly, I’ve double checked a number of the traces, let’s hope all of them are well when they come back from AP Circuits later on. 🙂

Yet another iteration of the stormer viscometer board
Yet another iteration of the stormer viscometer board