Optos aren't necessary or useful here. You could run +12 to a normally-closed switch, with the other side of the switch going to a resistive voltage divider, a hunky cap to ground, and into a pic pin. That mode is fail-safe, as either an open or a short in the switch circuit will look like a limit has been hit. The divider should be fairly low resistance, 1k or less, in case the switch gets wet. Something like that.
John
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!
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?
thanks for any help moray
Hello Moray,
There are others on this forum who are better qualified when it comes to micro controllers. But having split about 10 cords or so with a wood splitter myself I would certainly not do away with the usual hydraulic end switch features. Just imagine a 20-30 ton cylinder trying to continue past the end point because the controller hung. You'd have hoses blow, people injured or worse.
The other thing is when you hit a rather knotty piece of pine. Or what the pros call the "uglies". On a regular splitter I hear the engine cough and the wood release a weird creak, let go of the handle, reverse a bit and gently ease into it at another angle. If you'd let the ram continue you'd have that heavy piece flying all over the place.
Be careful.
Regards, Joerg
Use a micro PLC like the DL05 from
$110 for a 12-24vdc supply version. Your biggest issue would be power supply. I doubt this PLC would be happy on a vehicle type 12vdc supply, maybe you could make use of a laptop car charger as a cheap fairly robust switching supply for it.
There are many similar micro PLCs to choose from although a DC supply limits the range. You can find cheap PLCs on ebay but make sure you know what is required to program them.
moves,
power
relays
pull
We built gate and door controllers with 4000 CMOS with 10K resistors between the IC inputs and the outside world. The wires ran from the control board to doors & gates and we did not have failures. You can increase the resistor value if you like.
You need a 1K or so pullup to drive the actual microswitch contacts. I would not run +5V etc around - I would make ground the common to the switches.
Roger
The machine is for making kindling wood, and consists off one small ram to feed the wood the in, and the other main ram to split the wood. The machine will be powered by a tractor, and the maximum power off the main ram at
140bar is only 6.4ton. The machine is only designed to be fed with softwood, and has a pressure relive valve which will be set a lot lower than the tractor (this is more to do with being a hydraulic circuit problem workaround than being a safety measure though). We already have two normal splitters (one with a 3" ram and one with a 4" ram), which are used for normal logs, and the only thing the 4" ram can't handle is the "uglies" as you call them. But I don't think anything can split knotted elm.Having read through the other posts, I've realised that relying on using the PIC to stop the rams is not a good idea. As has been suggested, I'll use the limit switches to directly cut-out the relays, and have the PIC sense the change in status at the relay.
Thanks moray
Thanks for the suggestion. The other main choice for the PIC, was I may want to add other features in the future (such as have ram movements overlap, and programmed start/stop buttons). Also I'm trying to keep the circuit as simple as possible.
Other problem is, I'm in the UK, so the similar devices across here will cost the same in UKP than USD.
Thanks moray
Thanks to all those that have replied with comments and suggestions. I have replied to some of the individual posts, and am currently going to have another think about the circuit design. I'm sure I'll be back later to ask for some more advice,
thanks moray
Hear, Hear -
But the time the O.P. has gotten all components together, he will be out much of more money than the PLC costs!
Plus PLC's are debugged for that kind of application, there are Sensors for PLC's etc.
A Siemens LOGO might be the ticket for the job. Especially as you can program them directly with the local interface, no need to spend on a separate programmer.
- YD.
-- Remove HAT if replying by mail.
Which pretty much says a PLC is the way to go. You already have all the I/O interfacing and conditioning, program changes are a lot easier and you can add I/O modules as needed. You'll spend less money and time on this than trying to develop your own using PICs or similar microcontrollers. Unless you're doing it for the learning experience in which case I'd suggest trying to control something less powerful.
PLCs are usually powered by either AC or 24 Vdc but I think you can get them for 12 Vdc too.
Still cheaper than rolling your own.
- YD.
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