Designing The Testing Setup and Building a Controller for Testing the Air Cylinder And Taking the Measurements.

Patents, Prototypes, Manufacturing, and Marketing New Inventions

Homebrew cylinder testing with home made fixtures and electronic controller.




 

 

 

 

 

Development testing proceedures to test things like life expectancy from our pneumatic cylinders.

Continued from the previous page Prototype Development


Testing setup and procedure for a prototype high-speed, long-stroke air cylinder for cutting asparagus spears with a mechanical asparagus harvesting machine.

As determined in the previous page, I need to obtain the following information about my air cylinder.

  • Stroke extend time
  • Stroke retract time
  • Time blade spends in soil
  • Number of strokes until failure
  • The effects of Lubrication
  • The effects of different blade mount masses

Testing Constraints

Unfortunately I do not have a well equipped lab or shop... only my garage.  I will need to build the testing apparatus and measuring equipment myself from my junk box.  I have a pretty nice junk box in many respects. I think all inventors should have a well stocked junk box for crude prototyping purposes. 


Securing the cylinder

With such a high speed travel and heavy impact at the end of the retract stroke, the cylinder wants to move all over the place when it's cycling.

I have a large antique vise that was my grandfathers. It came from a blacksmith shop.  Big heavy thing probably weighs about 50 pounds. The vise was bolted to my work bench which was originally a kitchen counter and cabinets or something. It's quite heavy.

I first tried clamping the cylinder with the vise.  When I tried firing the cylinder it started moving my workbench around.  I did not want my workbench moving around, so I built a stand out of a couple of 2 x 6 boards which I bolted the vice to. Then I fastened a plastic bin to the board behind the vise. In the plastic bin I placed two heavy injection molds I happened to have laying around.

Then I used coat hanger wire to fasten a 2x4 to two steel pipes that are embedded in the concrete in my garage to prevent a car from hitting the furnace.  I secured the vise-stand assembly to the 2x4 with several pieces of wire.

Now it would move a little with each stroke but it would not travel.
 


Designing the electronic controller

If I am gong to do a life test and measure the speed of the blade, then I will need a controller that will periodically fire the air cylinder for a long period of time.  I will also need to be able to vary the length of the pulse that causes the solenoid on the air valve to stroke the cylinder and the period of time that will elapse between firings.

My air compressor is one that I purchased from Home Depot or Lowe's; I forget where I got it.  But hey, it isn't a commercial air compressor, and doesn't put out all that much air.  It turns on at 120 psi and off at 145 psi.  It puts out something like 6 CFM at 145 psi.

I have to limit the firing to about 1 cycle every 15 or 20 seconds so the air compressor can keep up.  It will take a long time to accumulate 650,000 strokes...

To cycle the cylinder every 15 to 20 seconds I built a controller by using a microcontroller chip, a computer on a chip.  I used a Microchip 12F675 controller, two potentiometers, a couple of resistors and a Darlington power transistor.

Microchip controllers are very easy to use.  You supply the chip with 5 volts, connect up a couple of external parts, program it with a simple programmer, and you have a useful circuit.  The 12F675 is an 8 pin chip and has built into it a voltage comparator, several timers and counters, the processor itself, and an analog to digital converter.  It also has a built in clock circuit with several speeds available.  I use the internal 4 Mhz clock.

I don't program the chips in machine language but use a basic compiler. Programming in basic is much easier. For the controller I simply programmed the chip to produce a pulse with a variable "off" time and a variable "on" time.  The two pots control the on and off times allowing me to produce a pulse that depending on the position of a pot is from 1 millisecond to 100 milliseconds long in 1 millisecond increments.  The second pot controls the time between pulses from 1 second to 30 seconds.

I used one pin as an output to drive a Darlington power transistor which in turn supplies 12 volts to the air valve solenoid.

The air valve, an Allenair brand directional control valve, sends the air to the cylinder ports causing it to extend.  The valve remains in the open position until the pulse to the solenoid stops, at which time the valve reverses the flow and returns the cylinder to its retracted position. 

Continued on the next page: Measuring the stroke period and average cylinder speed

 

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