programming a timer for a dosing pump

My total programming knowledge is what I have learnt on the online tutorial and trying to mess with free downloads such as AB Picosoft6.
What I am trying to do is replace a 555 timer circuit which uses external pots (variable resistors) to control both the on and off times of a relay. For a PLC I dont mind the the on time being programmed in but I want the off time to be varied either with the pot feeding an analogue input or possibly use a digital encoder. Most plcs seem to offer plenty of timer and counter functions but I can get my head around how to program them together to produce the cycling output I am look for. Can anyone point me to some examples. Thanks

What make and model of PLC?

somthing simple

looking at AB Pico or Mitsubushi Alpha 2 or (see my fisrt post about 1 hour ago) have been recommended Micrologix 1000

If you go down the ML1000 route and need something cheap then you can pick up decent ML's on ebay for the same as a new Pico or Alpha, then again the Alph can also be found on ebay!

Im not a great fan of Mitsubishi, Just done some FX2N's and it was a backwards learning curve, i felt like i was slipping back to Windoze 3.11 !!

Just put this together for an ML1000 to give you a head start...

It doesnt use any analogue but if you add some rungs will give 16 different On Times depending on the Binary Sequence of Inputs 0 to 3. you could wire these to switches.

There are quite a few ways to do this, my method is only one of them.....

It is Untested and May Contain Errors!

oyster said:

My total programming knowledge is what I have learnt on the online tutorial and trying to mess with free downloads such as AB Picosoft6.
What I am trying to do is replace a 555 timer circuit which uses external pots (variable resistors) to control both the on and off times of a relay. For a PLC I dont mind the the on time being programmed in but I want the off time to be varied either with the pot feeding an analogue input or possibly use a digital encoder. Most plcs seem to offer plenty of timer and counter functions but I can get my head around how to program them together to produce the cycling output I am look for. Can anyone point me to some examples. Thanks

Click to expand...

Can you describe your application in more detail? I have done many CIP (Clean In Place) systems, where chemical is injected into the supply line solution and a conductivity meter measures the chemical strength of the solution in the return line. Depending on the chemical return line strength, a dose pump on the supply line injects chemical to bring the return solution up to a setpoint in chemical strength.

Because of the lag in time between the chemical injection and chemical measurement, a lag timer is required after each chemical dosing strokes to give time for the chemical to recirculate through the CIP lines.

I have always programmed CIP systems to do the following:

- chemical recirculation step, stroke dosing pump X times
- wait X seconds to allow initial dosing pulse to recirculate
- based on return chemical strength, stroke dosing pump X times
- wait X seconds for chemical to recirculate
- continue dosing X strokes until X seconds remain in CIP recirculation then shut off dosing pump

Ian

The Mitsibishi Alpha 2 is unlike the standard Mitsubishi PLCs. Is in the "Smart Relay" class. It is not programmed in gate logic. Pretty easy really.

The advantage of this little devil is you can program in alarms etc to be displayed on the LCD screen. The screen is larger than on other smart relays.

Another thing that would probably suit your application is that some of the inputs can be programmed as analogue inputs - not sure about pots though. You would have to check with them. You could also put a BCD thumwheel switch into it.

I think Omron still offer some PLCs with a timer adjusting module. If I remember correctly, the unit attaches to the shoe box PLC and has 4 pots to adjust 4 timers. They also used to have a PLC with pots built in. May be worth investigating.
I know the Omron ZEN, and probably most smart relays, has a compare function and a BCD thumwheel switch could be wired into 4 inputs and read and compared in the PLC.

I recommend using one of the smart relays with built in display. You should be able to change the timer presets via the display, and eliminate the need for an external pot.

Most of the smart relays, have free software and built in simulation. Your only cost is the cable. However, if you decide to use one of the brick PLC's, you may have to buy programming software, depends on brand and model of PLC.

I've recently migrated to the Teco SG2 programmable relay, low cost and so far reliable. They appear to be very similar to the LOGO, supports ladder or function blocks. Also, the DC powered units have built in analog inputs. You should could buy a test package for around $100.00 USD.

Or better yet download the free software and play around with the simulator and see if it meets you needs before buying anything.

Try this LINK