Sunday, June 20, 2021

Solenoid No. 2 - Testing

 Looking around for some different Solenoids, and I found this one marked 6V 350mA:  https://www.aliexpress.com/item/32777233179.html  Here is a photo of it in case the link expires:

The solenoid centre pin moves around 13mm and the coil has a resistance of around 3.23 ohms. It has two handy M3 mounting holes on the opposite side of the label above.

I wanted a method of being able to quantify different solenoids, so I hit upon building an adjustable ramp.  It took a day to design up and laser cut it out of 3mm MDF wood. This is what it looked like in the DesignSpark Mechanical CAD program:


And the finished result:

I am using a ESP32 with a variable resistor to set the pulse length (6mto 150 mSec), along with a OLED screen, so I can see what the pulse length is set too.

The pulse output is fed into a AO3400, 30V 2A MOSFET but it can be intermittently pulsed up to 8A.  During one of the tests, I forgot about the MOSFET voltage rating, and wondered why it momentarily fired the solenoid by it's self when I turned the voltage past 30V - followed a short time later with a burnt electronics smell a few seconds later!  It didn't take long to replace the SOT-23 sized AO3400 MOSFET.  The driver circuitry is mounted on a small piece of vero board:

To fire the solenoid, I am using a small XL6019 boost power supply board.  This will be powered from the Alphabot2 Li-ion 6-8V batteries and can generate up to 37.5V from the batteries:

As the original home made solenoid driver can only go up to 29V, I made a new one that can handle up to 60V using a final 100A capacity HY4008 MOSFET.  To drive the final MOSFET, I use a combination of a N-MOS to drive a P-MOS MOSFETs.  These can switch the Li-ion 6-8V battery voltage, which is just enough to drive the HY4008 MOSFET gate.  Ideally it should be 10V or 15V so that it's driven or switched on a bit harder, however, due to the 25-30 mSecond switching times, non of them get even warm.  I did however add 3 x 1000 uF capacitors to help supplement the driver voltage pulse.  The new board:


Lots of spare room to add on the high voltage power supply board.

From a 6V input, it can be wound up to a 37V output, but it can't supply much current, so I have mounted a (supposedly), 18F, 100V Chinese super capacitor across the output:


When the super capacitor is in place, the firing voltage is quite steady, however it is quite a large and impracticable size for mounting on a small Alphabot2 robot.  I have identified a few different smaller 10mF, 63V or 15mF 35V capacitors that could be tried out at a later date.

I am attempting to drive a 42mm diameter golf ball that weighs around 45.6gm up a ramp that is set to a slope of around 16 degrees.

Now for some graphed results.  But first, some notes on testing.  I ran 5-6 tests per record.  These usually resulted in a range of distances that might vary by 10mm, or up to 30mm.  Ideally I should have recorded each distance, but I took the fast approach and just recorded the minimum and maximum distance from the 5-6 solenoid firings for each setting.  To graph these numbers, I then picked an average between the min/max numbers.  This is not quite accurate, as maybe most of the distances might have been around the high number.  Anyway, the graphs do tell a story about the solenoid and each of the inputs.

1) How does the solenoid firing pulse length effect the ball distance?



I tested various pulse lengths using either a 6V or 12V input voltage for the Boost PSU and MOSFET gate voltage (Vgs).

This is kind of what you would expect, in that once the firing pulse is long enough for the solenoid to fully move, then using a long pulse is just wasting battery power.

2) How does the initial distance between the solenoid and the ball effect the ball distance?


Be default with no spacer, the ball rests about 1-2mm away from the end of the solenoid pin.  The spacer is just a piece of 3mm MDF, so that puts the pin around 4-5mm from the ball.  This gives the solenoid a bit of extra time to build up energy before contacting the ball and start pushing it along the rails.

3) What is the effect of voltage that is fed into the solenoid?


This gets a bit complicated, as I have done 4 tests (with or without the 3mm spacer and with a 6V or 12V input voltage), for each output voltage data point.

Basically though, higher the voltage the more current your going to drive through the solenoid, so the faster it's going to move.  Testing with both 6V and 12V input allows me to verify that the MOSFET will work ok down at the worst case input voltage, ie: both Li-ion batteries are nearly flat at 3V each.

When the boost power supply board is turned up to the maximum boost voltage of 37.5V, there is no difference to using the 3mm spacer.

It would be interesting to try some higher voltages, however, I don't currently have a better boost or higher power supply available.


As an side note, this afternoon I tested this solenoid out with both a rubber super ball:  it only went about 50% of the distance compared with a golf ball (the rubber absorbs a lot of the solenoid pulse); and a ping-pong ball: due to it being much lighter than the solenoid central firing pin, it flew up the ramp, hit the end and launched into the air.  I had to turn the pulse length down to 3mSeconds before it would stay on the ramp.

No comments:

Post a Comment