On 10/08/2012 01:30 PM, Shane Hathaway wrote: > The large current requirement (10A) and the need for a middle position > make the circuit interesting and difficult to achieve using low cost > analog components. An analog H bridge would work well if you only > needed 2 positions. You can't dump that much current through an > ordinary op amp. (Audio amplifiers use big, expensive op amps that > easily surpass the cost of an Arduino.) PWM is the usual solution for > handling that much current.
> However, can you achieve the middle position using a simple mechanical > spring? That would allow you to use a very simple H bridge. > > It would help a lot to understand the application better. What kind of > resting position do you want? When the power is lost, do you want it to > revert somewhere, or hold its position firmly/loosely? Okay, so here's what I have. I have a linear actuator, which is essentially a screw driven by a motor. And the actuator has a built-in potentiometer to give you position. The motor uses very little or lots of amps depending on the load. I won't be having much load, so the amp draw is very low, like under 1 amp at 12 VDC, most likely. The actuator, being a screw, cannot move when power is not on. So I don't need to hold it actively. It is also very slow (compared to a servo), so PWM is not required. Full on voltage or no voltage is sufficient. As I said before, I don't need super-accurate positioning, so I don't need the actuator to seek back and forth around the target point. When it gets there it can just stop, even if it's too far or too close by a small amount. This actuator is controlling a hydraulic valve assembly so it only needs three positions: extended, middle, and retracted. It will always be in one of these positions; there's no resting position. Nick's idea of using limit switches is a good idea. I am confident a simple circuit can do it all without that kind of extra mechanical stuff though. Really I'm not worried about amp draw because I can use the result of the op amps to drive a relay to switch the higher amps if I need to. /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
