Pressure Transducer

One one my main reasons for doing this blog is to document the control rebuild on my injection molding machine. Well, I'm going to try to make some improvements where I can. One of my problems is that I don't really know what improvements need to be made! While reading through a document Dow put out, Injection Molding Processing Guide, I realized that my machine did every step in its sequence based on time. However, according to Dow, new machines did at least one step based on the injection pressure.  This is the part that was the most informative:

 Hydraulic Pressure
The injection molding process is generally divided into
two stages. The first is injection (or fill), and the second
is packing (or hold). During the first – or filling – stage,
it is suggested that you set your machine pressure
near its maximum setting and control the speed of the
ram with velocity controls. The machine will only use
whatever pressure is necessary to move the ram at the
set speed. This technique will help produce consistent
parts because the cavity is filled at a uniform rate, despite
differences in viscosity due to temperature fluctuations
and other factors.
It is very important to switch from the first to
second stage when the part is 90 to 95 percent
full. If you do not switch from first to second
stage pressure before the cavity is full, the high
pressure of the first stage pump may cause the
part to flash and may cause damage to the mold.
It is suggested to switch from first to second stage
based on either the position of the ram or the cavity
pressure. Switch-over based on the hydraulic pressure
and time is suggested if ram position or cavity pressure
controls are not available.

After reading what Dow had to say, I decided to adapt this structure to my machine. The remainder of this blog post is documenting how I did it.

My controller has a 10 bit analog to digital converter which it converts a voltage from a 0-5V to a number between 0 and 1023.

The pressure sensors in this range are very expensive; some of them I found were over $250. I found one on ebay that was less than optimal for my application. It was a 0-5V transducer that read from 0-400 Bar. I got it anyway because the price was right.

When I hooked it up to my machine I got 2.1V at 2500 psi. Using the transducer like this, I get a resolution of about 6psi. Here is a little chart showing how it works.

Pressure (psi)	voltage	10 bit      number
100	0.08	17
300	0.25	52
1700	1.43	292
2500	2.10	430

Note: My DAC could measure between 0 and 2.1 volts if I set a refrence point for it to 2.1 volts. I could get over double the resolution if I did this but I'm going to save that for later. This resolution might work just fine and I may want other sensors that may use the full range.
With this pressure transducer in place, I can use it to switch from injection to holding pressure when it hits the pressure I think the cavity is 90% full. 


http://arduino.cc/en/Reference/AnalogRead
http://arduino.cc/en/Tutorial/AnalogInputPins
http://arduino.cc/en/Reference/AnalogReference?from=Reference.AREF