Posts Tagged ‘Load Cell interface’

Racing sim update

Friday, March 26th, 2010

Ok, as promised here is the details of the electronics.  Most of you would be interested in the load cell interface so i’ll start with that.  The interface is truly simple, the main chip, an INA122PA is only about $7 from RS and a little more from Farnell.  There are other equivalents but this is the one i used and is also one of the cheapest.  The pinout is very basic for our purposes.  You have a power supply (coming from the Mjoy) to power the chip and the load cell.  The load cell applies it’s voltage to 2 pins and the chip simply amplifies the voltage from the load cell.  The gain (how much it amplifies the voltage) is set by the resistor accross pins 1 and 8.  The capacitor is a simple 0.1uf filtering type. The cell puts out 2mv/v so at 5v excitation we get 10mv at full scale.  To get 0-5v we need to multiply that by 500.  For this chip that means a resistor of about 400 ohms.  Less resistance will give more gain.  Since my load cell is a 100kg item i have increased the gain on mine to about 600.

That’s about it!  Hook it up inline instead of a pot and you’re done.  Just make sure you get the wires around the right way…

lc_interface Load Cell interface

As for the main Mjoy board, I modified a layout i found around the net.  I have not removed any ‘branding’ so if one wanted to seek out the original designer i’m sure they could.  The only real change i made was to add pads to be able to vertical mount standard diodes for the buttons rather than use SMD types.  Most other changes were to add some distance between tracks etc.  Print, transfer, etch, drill and solder etc.  I made up my own paralell port programmer interface to program the chip in place with the provided header.

One thing i would like to add here is that originally i tried to use an AtMega88 as i was told they were basically the same.  Unfortunately they are not.  There are too many dfferences to allow the standard hex file (for the AtMega8) to work.  Someone has ported the code to the ’88 but i have not been able to get this to work.  I presume it has something to do with fuse settings on the chip but even though i have set them right, it still does not work for me. I ended up having to stick with an AtMega8 at about 5 times the price of the ’88.

mjoy_pcb p3210025_sml1

PCB Layout files (Sprint 5.0 format, free reader available to be able to print them http://www.abacom-online.de/uk/html/dateien/demos/viewlayout50.exe)

NB – The Mjoy PCB has incorrect axis lables, they are backwards so it should read X,Y,R,T,Z,Rx.  Not really important other than the fact that the XYRT axes are 10bit and the other 2 are only 8bit. Earlier versions of Mjoy only have 2 10bit axes (X and Y)

pcb-mjoy-v14
load_cell_board

Please let me know if you download these files, it’s nice to know someone else is finding these useful.

I’ve also started work on my H Shifter for the sim.  I’m trying to keep the design simple but at the same time i want something robust and somewhat realistic.  Combining the ideas of some others, i have come up with my own design which should allow for this.

Shown below is the basis of the shifter (not complete obviously).  Between the 2 horizontal bars (the ones with holes in them) will go a block of delrin which supports 2 standard skate bearings.  I have not yet finished the profile on the horizontal bars, there will be another dip in the middle. The delrin will freely slide over the 1/2″ shaft and will be followed by a spring and then a retaining clip.  This will force the block/bearings down on the ‘ramps’.  This should give a ‘clunk’ into gear and also allow for a little freeplay between gears in the ‘y axis’ when viewed from above.  The centering mechanism will be another single spring around the shaft that sits on the rod end and is secured again by a retainer.  The compression of the spring will try to keep it centered.  From there it is only a matter of making a gate plate and hooking up the switches for the gears.  I intend to mount the switches in the base of the unit to keep them safe.

p3250031_sml p3250030_sml