Efficiently running RC motor with NXT

[storkken]

Hey there. This is my first time being on this site, or any blog for that matter... straight to the point, I've built an air compressor with four of the mini pumps using one of the high RPM Technic RC motor's. My plan was to connect it to my NXT via the converter cable from lego (#8528) and then use mindsensor's pressure sensor to relay the PSI back to the NXT; the NXT would then simply relay to the RC motor to run whenever the PSI was below the desired amount. Programming the NXT to do so was relatively easy and I had my system up and running within a few minutes. Problem: I would like the RC motor to run at it's maximum RPM, like it does when I connect it to my power functions battery box, however, it does not, it runs at a much lower RPM that is not satisfactory. My question is whether or not this issue is fixable. I'm not sure if it's a software problem where the NXT is simply just not relaying to the RC motor to run at it's max power (I have it's power at max on the program but I'm not sure if maybe that's a drawback of using stock lego mindsensors program?). Or, if it's a voltage issues, which I kind of doubt. I thought of possibly buying an infrared link and thereby powering the RC motor with the power functions battery box instead. But, I figured I'd give this a shot and see if anyone here has any advice before buying the link simply due to the fact that the new EV3 will have built in infrared. Anyhow, any help would be greatly appreciated and hopefully I didn't drag this on to long haha. -cheers

[jwiger]

I strongly recommend reading this (maybe even saving the link) http://www.philohome.com/motors/motorcomp.htm The problem is the RC motor draws over twice the current of the NXT motors. So you are having a voltage drop issue, in fact it's quite risky to run the RC motor directly from the NXT as you could damage the motor driver circuits. Even the original IR receiver has trouble keeping up with the RC motors. the V2 (version 2) IR receiver that comes with the rock crawler does a better job. I would reccomend a dedicated battery box then use an NXT motor to throw a switch. If you don't mind hacking some cables you could build a relay box controlled by the NXT to switch the RC motor. There may be some other off the shelf solutions out there though.

[storkken]

Had no idea

I strongly recommend reading this (maybe even saving the link) http://www.philohome.com/motors/motorcomp.htm The problem is the RC motor draws over twice the current of the NXT motors. So you are having a voltage drop issue, in fact it's quite risky to run the RC motor directly from the NXT as you could damage the motor driver circuits. Even the original IR receiver has trouble keeping up with the RC motors. the V2 (version 2) IR receiver that comes with the rock crawler does a better job. I would reccomend a dedicated battery box then use an NXT motor to throw a switch. If you don't mind hacking some cables you could build a relay box controlled by the NXT to switch the RC motor. There may be some other off the shelf solutions out there though.

I ran my set up again today, this time with a fully charged NXT, and the rpm was what I thought it should be, though now I'm somewhat concerned with what you've told me, these motor's aren't particularly cheap. I ran my set up for quite awhile and it seemed that the RC motor was running fine but it's hard for me to know for sure.

[jwiger]

Hopefully someone with more knowledge on the motor driver circuits of the NXT can chime in on here....

[hassenplug]

My thoughts on this. Are you using the NXT for anything beyond turning off the power when the required pressure is reached?

Here's a very nice trick for controlling the compressor pressure...

Instead of the pressure sensor, put another [small] piston with a rubber band, to keep it compressed, and only connect the hose that would make the piston extend. Then connect a polarity switch so when the piston extends, it turns off the switch and when it's contracted, the switch is on.

When you turn the power on, the motor will run until there is enough pressure to flip the switch. You can change the pressure, by changing the rubber band.

You may be able to find some examples of "LEGO Compressors" on line.

Hope that helps.
Steve