Sunday, 6 January 2013

Magnetic Bed Testing..

Just performed a quick and dirty test on the magnetic bed, while holding a smallish piece of steel to the bed and applying 12volts to each of the coils the piece of steel was held to the bed, enough that i could turn the bed upside down and it stayed stuck to the bed. then i tried moving the block laterally and it moved really easy :( so i thought how much power do i need to get that to stay stuck to the bed and not move sideways, so i turned up the power supply to 24volts and the coils drew 6.3amps. this held the bit of metal nice and tight :). I wouldn't mind  running the coils off 24volts but the amount of heat that was generated was beyond ridiculous.... I then thought i would test out the 10% duty Cycle test, and the results are promising, if i fire up the coil pack with 24volts i get good grip, then if i quickly turn the voltage down to 5volts the coils continue to hold the piece of metal nice and tight but after a few seconds the coils relax and the piece of metal is then allowed to move.. Time to make a PWM controller that can switch 24+volts at about 10amps and see what happens :)

Will post a further update once the controller is done and tested.

If i was to break the bed up into 3 sections of 20 hexagonal cells each section would have a effective resistance of  1/( (1/22) * 20)  =  1.1 ohms

Each section of bed would draw @12volts 12/1.1= 10.1 amps of current at 100% duty cycle
but this could be cut back to 2.5 amps once running @25% duty cycle, this could be done with a capacitor charge pump circuit of some sort or i could break each section down further into two this would give me a sectional current draw of 5.05amps @ 100% duty cycle and 1.25amps @25% duty cycle.

if was to run the coils at 24volts (like the good test showed is required) the sectional current draw would be doubled. with 3 sections each drawing 20amps @24volts ( and each sub section drawing 10amps) if i was to turn them all on at once i would need 60Amps of current at 24volts, this is unreasonable. but if i stagger fire each subsection @100% Duty cycle then cut back to 25% duty cycle i would need 10amps then 2.5amps per sub section.

the current profile would look like this
 @ 10% Duty Cycle
@ 25% Duty Cycle

ie each coil is hit with 10 amps and then its dropped back to 10Amps @ 25% Duty Cycle.
at 25% duty cycle the overlap turns out to be quite high.. i would need a PSU that spits out 25Amps @24 volts.. this is closer to reality but still a bit too far off i think.. i guess i will have to see what ebay has to say about the price of fish.. if i can find a 24V psu that is cheaper than further complicating the controller i wil do that other wise i will have to break down the coils into even smaller groups..

actually i have made an error.. the curve will look like this

the previous chart didn't take in to account the fact that we will be turning off cells after they have been driven @25% this makes things much more manageable. I can probably stagger out firing order a little more and make the curve a little flatter but this will do for now.. 15Amps @ 24Volts can be had really cheaply thanks to Ebay. just found a buy it now 24volts 20amp PSU Delivered for $66Au Bargain. this PSU allows me to fire two sub sections at once, basically this will allow for upto 33% duty cycle and if managed correctly 100% startup/inrush current can be achieved, the plan will be to turn each coil on for 90% duty cycle for the first pulse and then .1s after that, as the cycle steps thru the first time each coil is energized it will actually be on for 100% for one cycle, 90% + 10% then after that they will be turned on for 10% of the time rotating between the 6 coil subsections i can manage 16.6% duty cycle without overlap. if overlap is used i can get 33.2% duty cycle before my PSU will start to complain.

worst case is that i end up with a nice 20Amp PSU for my printer. I was running the steppers off 24volts but got to tidying things up and switched back to 12volts..

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