Measuring stroke time and piston rod speed with optical switches and Solid State Laser Modules From My Junk Box.

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I found in my junk a couple of solid state lasers and a couple of photo transistors. That should do the trick.






Measuring Stroke Time and Piston Rod Speed With Optical Switches Using Laser Modules and Photo Transistors

Continued from the previous page: Test Procedure and Fixture Design

Measuring Stroke Time and Piston Rod Speed With Optical Switches

I decided to use optical detection to get my speed measurements.  I would put one detector at the retracted position of the piston rod so that when the rod extends about 1/16 of an inch it breaks the beam. I could use that pulse to start a counter.

The second detector would be mounted in such a way as to detect the end of stroke, and by using the pulse produced by the second detector to shut off the counter I could get my measurements.  I grabbed my Mouser catalog began looking at photo-electric switches.

Upon reading the specs, I found that most of the optical switches had response times of about 30 milliseconds... not good enough.  I need an accuracy of better than 1 millisecond.  Besides, I didn't want to spend several hundred dollars.  Time to look in my junk box and see if there was something I could come up with that would work better. 

I found in my junk a couple of solid state lasers and a couple of photo transistors. That should do the trick. I scrounged up some scrap metal and mounted the lasers and photo transistors with a gap of about 2 inches between the laser and the photo transistor.

The lasers produce very small diameter beams, about 1/16 inch in diameter.  The photo transistors respond within microseconds to the beam assuring an accurate timing measurement.

I first tried mounting the optical switches on the test stand I had built for the cylinder, but the impact of the cylinder firing caused too much vibration and made the optical switches come out of alignment on nearly every stroke.  I instead tried mounting the switches on their own separate stands which solved the problem.

 With this arrangement I would be able to obtain all the information I needed.  I could measure the time from retract to full extension by placing one switch next to the retracted piston rod position, and one switch at the position of the piston rod end when fully extended.  By putting the switches an inch apart I could measure the speed at any point on the stroke and by putting both at the end of stroke position I could measure the hesitation time at the end of the stroke when the blade would be in the soil.

Now I need a counter that I can control with the optical switches... back to the inventers junk box. 

Counting the pulses

For counting the output pulses from the optical switches I again used a 12F675 chip. I programmed the chip to count pulses after one input pin gets a pulse, and to stop counting after a pulse from a second input pin.  The chip counts at a rate of 1 count per millisecond.  The output is sent to a LCD display which shows the count in milliseconds.  When the chip gets the second pulse and stops counting, it then waits for a reset pulse on a third pin supplied by a push button.  Until it is reset the chip won't do anything when additional pulses come in from the optical switches.

That was necessary because for one thing, when the cylinder retracts and hits the spring cylinder in the rear it bounces a bit.  The bouncing would cause multiple pulses to come from the optical switch that starts the counting. This way my data is preserved until I hit the reset switch.

Continued on the next page:  Air Pressure Supply Problems - Testing Prototypes


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