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  • PLC questions

    It has been suggested to me that a PLC might be the best way for me to control some of the functions on my sheep shearing machine.

    BUT I have no idea about PLC's.

    What I need to do is push one button and have one hydraulic cylinder extend fully. When it reaches its full extension I need another hydraulic cylinder to retract fully. Then two seconds later I need the first cylinder to retract again.

    I need my hands free to manage the sheep and don't want to try to find a foot pedal while I am persuading a sheep to lie still!

    I know some of you are very familar with PLC'S so any suggestions would be great.
    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  • #2
    mitsibushi fx series plc, works great see
    Last edited by boslab; 06-08-2011, 10:37 AM.


    • #3
      For such a simple control problem, a PLC is probably overkill. I would use a couple of time-controlled relays and some switches on the actuators.


      • #4
        You might want to look at a smart relay, they are actually little brother of the PLC.
        For limited logic, a few I/O it should work, they have added features such as real time clock etc.
        Very simple to program and no software or programing unit is needed.
        Many makes are avail, all relabeled Idec, Square D, Telemecanique, Siemens etc.
        Last edited by MaxHeadRoom; 06-08-2011, 11:07 AM.


        • #5
          Yes, Industrial PLC is overkill... Microswitches (industrial sealed with roller contacts for reliability), a timer relay and maybe a control relay or two are all you need. I'd add an emergency stop button that kills the pump pressure or whatever.

          I have an entire card cage full of AB 500 series cards... and even a conveyor scanner but... never have had a home application to justify the development software cost. I end up using a small handful of relays/timers. Need to move the AB stuff on before I get tempted.
          Last edited by lakeside53; 06-08-2011, 12:43 PM.


          • #6
            That is one advantage to the smart relay, they include timers, counters and if necessary logic tied to the real time clock.
            Saves a lot on wiring timers which will probably cost more than the whole Smart relay!


            • #7
              The problem is with PLCs is the software is stupid expensive. Automation Direct does have a limited version that works with theirs, so I am told.

              But you will be amazed what you can do with some DIN rail mount relays and timers.


              • #8
                I got a Siemens Logo! "news box" for a simple control system. Not too expensive and it includes a PLC, the programming cable and visual programming software for the PC. Back then it was 150 euro at Farnell.
                Available as 12V or 230V powered version.

                Never used a PLC before in my life and it was dead simple. Finally added all kind of features and had a 100 block diagram in there.....
                Still very easy to use though.



                • #9
                  Thanks for the information.

                  I am trying to get information on the practical side of actually using one of the Siemens Logo units. Google has lots of info and I am reading and reading.

                  The smart relay sounds like a good solution.
                  How to become a millionaire: Start out with 10 million and take up machining as a hobby!


                  • #10
                    Hi Black Forest,
                    Mitshubishi have a Smart Relay called Alpha 2, it uses a simple programme and its Bolean which means its easy to use. The software is free, its a pick and place type software and you can write the programme on your PC then by use of the cable download it to the Alpha.
                    You just need to get a NO Pushbutton for the Start Command and Reed Switches on the Hydraulic Rams, that will tell the Alpha when its at End of Stroke. The Alpha has Timer Programmes inbuilt that can do the two second timeout.
                    I would suggest an Emergency Stop that will kill the Hydraulic Pump if you get into any trouble and wire it directly to the Pump Contactor not the Alpha.



                    • #11
                      If you can get them there, you might check out the Automation Direct "click" series of micro PLC's. Sort of like smart relays but a bit more power and flexibility, and cheap. Most are well under 100 bucks and the programming software is free.

                      I've used them a few times, very easy to use and worked well. One example, 8 inputs, 6 outputs, $69.00, is here:


                      Lots of other configurations available.

                      just a thought...


                      • #12
                        Cycle timer

                        One of the spot welder "blueprints" had a cycle timer that would do all that..

                        BUT.. you need confirmation of each step.. more like a motor driven drum timer, which all these young guys have never saw before, since a small $75 brick plc (relay replacer) will do without all the mechanical bullcrap..

                        So.. I'd suggest a micro-brick plc, mitsubushi? Seems automation direct had one that was affordable with free software..

                        programming, I'd do Cycles, a internal relay for each sequential completed step. Just like that drum timer I used to use in the 70s..

                        DOING YOUR Programming, Do a Input-output-logic list on paper, then do a first program page of "every contact and relay, output relay, input you need and Label it.. that way as you go through the program inserting the Logic contacts the "name" pops up as you insert it and you can double check yourself. at the end, delete that first page, or leave it and put some contact in it so it can never turn on..

                        The brick smart relays have solved a lot of control problems..
                        Excuse me, I farted.


                        • #13
                          AB is phasing out their Micro-controllers and there are some very good deals to be had in that product line

                          Idec used to have a very competitively priced line of stand alone micro-controllers as well and the software was very competitive as well - $120 US last time I purchased it

                          That being said the advantages over hardwired logic in product development are enormous. Additionally if this is a product that you will be marketing - the possibilities for client customization are endless. Who would have known the "Bean-Counters" would have went ape-sh$t as they did over "Cycle Times". In your case knowing how long it takes the average sheep to be sheared, or knowing WHO is shearing the most sheep per shift

                          Micro controllers require more time then hard-wire logic to initially set up and program. But once the initial development is done changes are but a couple of clicks of the mouse. Additionally performance optimization can be performed almost effortlessly


                          • #14
                            So---lets do a walk thru. Sheep is driven down chute by helper untill it can go no farther. When sheep stops, (hopefully in correct "fore/aft" position" to be aligned with lifting crate), does a bar drop down behind the sheep to keep it from backing out of position? You then energize the "squeeze" cylinder to come over and squeeze the sheep to prevent it from trying to leap out of the lift crate when it starts to move. I assume that when the squeeze cylinder has reached is limit of travel, that that is when you want the lift cylinder to lift and tilt the crate. Then when the crate is fully tipped, and you have a grip on the back wool of the sheep, you want the squeeze cylinder to retract so you can yank the sheep out the top of the lift crate and down into the shearing cradle. As you are sticking the sheeps legs into the restraints (which lock mechanically without you being required to do anything except manage the now panic stricken sheep), the cradle lift cylinder should be automatically retracting back to "loading" position.
                            You hold the sheeps neck down with left hand and shear with your right. Then when sheep is sheared on one side, you energize the rotate cylinder which turns the leg restraints thru an arc, thus turning the sheep (which is lyeing on its back on some low friction rollers in the bed of the shearing cradle---so you can access the other side of the sheep). Do you completely rotate the sheep thru 180 degrees in one blast, or do you stop part way thru the rotation to shear its belly? Then when the sheep is fully sheared, you swing the rotate cylinder back about half way, stop it, manually release the mechanical lock on the leg restraints, and lift the sheep down onto the "escape" floor.-----Have I got that about right?----Brian
                            Brian Rupnow


                            • #15
                              Originally posted by alanganes
                              If you can get them there, you might check out the Automation Direct "click" series of micro PLC's.
                              I second this. Although I haven't used this particular PLC it has quite a reputation of being a cheap but powerful little controller, and the software is free. The Unit is expandable as well in case you end up scrapping that particular operation down the road and want to use it for something else.

                              I personally would not consider timer relays because you might find when you are actually using the machine that you need extra time values between operations, or you need some more interlock sensors, etc. Rewiring timers and relays gets old.

                              Look up a PLC book by Hugh Jack. Although I program ladder in a slightly different manner than him it is a good read for the beginner.

                              Unless your cylinder extensions are very repeatable and nothing can break I would seriously consider end of travel limit switches. These can be wired into the PLC and programmed to interlock with the sequence sucht hat nothing breaks if a cylinder jams or goes too slow. Having said that, if nothing is critical and things move fairly slow you can often get away with time delays in the PLC software.

                              I'd be happy to give you pointers on the program once you get it.