Re: Circuit Design Thoghts wanted
From: Paul Hovnanian P.E. (Paul_at_Hovnanian.com)
Date: 02/24/05
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Date: Wed, 23 Feb 2005 20:44:48 -0800
Moray Cuthill wrote:
>
> I'm looking for a few suggestions for a circuit to control a hydraulic
> machine (automatic wood splitter) I'm working on. As the mechanics off the
> machine are nearly complete, I'm needing to start thinking about the
> electrical controls for it. I've worked on and designed circuits in the
> past, but am looking for someone to check my ideas, and I'm open to any
> possible suggestions.
>
> The machine consists off 2 double acting rams, with each direction
> controlled by solenoid valves (ie 4 solenoid valves). Basically, one ram
> moves, activates it's limit switch, then the next one in the sequence moves,
> and so on.....
>
> My initial toughts are to use a PIC (already have programmer + a couple
> 16f84s kicking around), but the problems are with the interfacing and power
> supply.
>
> The power supply will be from a 12v lead acid battery (will vary from 11V
> upto 16V depending on charge off battery, and if it's being charged while
> the machine is in use).
>
> To power the hydraulic solenoids, I was planning on using relays with 12V
> coils, which will be switched via transistors driven by the PIC. The relays
> will have diodes placed across the coil, to eliminate voltage spikes when
> released.
>
> My main concern lies with the limit switches. Do I use them to directly pull
> the voltage off the PIC pins used for input, high or low? Or would it be
> better to use some form off optoisolator to isolate them from the pic?
If the limit switches aren't used for anything else (direct control of
solenoids, motors, etc.) then optoisolators are overkill.
Pull up vs pull down probably won't make much difference at low speeds
of the sort you are building. Either way, the PIC input should not be
left open in either state. Provide a low impedance to a high level or
ground.
In some cases, you might want to make a decision on pull up vs pull down
based upon the most likely circuit failure modes and the system behavior
in the event of an input failure.
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ Parity on, dudes!
- Next message: Don Bruder: "Re: Need to build a remote controlled momentary switch"
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