Ladder Logic Help/Advice needed!

Hi All,
I was in the design stage of a new stand-alone machine and need some inputs/advice from all you experienced and knowledgeable industry folks.

Some essential information
a) Machine is an assembly machine for medical devices.
b) Web travels a defined distance>Stops>Punching/inserting/Die cutting ops carried out/Web travels again.
c) Cycle time will be 2~3 parts per second depending on centre distances between parts.
d) Motion is done using a stepper motor coupled to a friction drive
e) Mechanical operation are pneumatic driven (24VDC Sol Valves)
f) Total of about 13 stations starting from right to left, machine is about 3m long, 1m wide and 1 m tall – all stations are mounted on waist level for ease of use.
g) Past machines have been Mitsubishi A1/Siemens S7200 based.

In my previous programs, we used feedback reed switches to operate & control the logic. It has worked well but I was hoping to improve things to a time-based format. I hope that it will be easier to fine tune the machine and get better responses.

For a start, I have a rough plan chalked-out.

1. On startup, First scan initializes values, loads a recipe and ensures all valves are at rest.
2. Start a 1ms timer.
3. Always on bit moves current value of timer to a D register.
4. From the HMI recipe, as an example, Cylinder 1 has a delay of 5 ms, and then a duration of 25 ms (duration when the air is ON).
5. Comapres the D value of Delay and SolEnd and keeps the sol coil energized for that time.

Will this work accurately? I mean, with most of my PLC scan times in the region of 20 ms, will a timer of 1ms be inaccurate? Will it miss time occasionally due to scan time?

How do you guys go about designing such machines?

Appreciate your inputs.

Cheers
Xsailor

I doubt if swapping from the positive indication of a reed and limit switch system to a time based (I think it is there so I will perform the task) system will be an improvement. In my opinion it will be a huge backward step.
Regards Alan Case

When you go to a time-based system, you have to start making a lot of assumptions. Like, you must assume that:

1) The machine is still running EXACTLY the same speed as when you originally programmed it. This is not just motor speed, but also valve speed in your case.

2) The machine ran at an exactly constant speed during the entire operation. This would also include air CFM.

3) The trigger to cause the next step actually happened.



We are actually in the process of working through some machines that have been time-based --- and have ALWAYS been problems. We're putting in sensors to resolve those issues. I agree with Alan - this is a huge step backwards in reliability.

Every time you remove a sensor, you are removing the verification of a real-world condition. At least as far as the valves are concerned, this will cause you problems in the future. Pressure varies, flow controls get adjusted, pistons wear--pneumatics is a very unpredictable beast.

To get the fine tuning you want, you could put on-delay timers on some or all of your sensors. Setting all the timer presets to zero would essentially run the machine at maximum safe speed.

Noted. Thanks Gents.

However, I was under the impression that Motion Controller usually work on time bases and deliver very accurate results.

Please do guide me on the scan time query...will the timer keep good time?

Xsailor said:

Will this work accurately? I mean, with most of my PLC scan times in the region of 20 ms, will a timer of 1ms be inaccurate? Will it miss time occasionally due to scan time?

How do you guys go about designing such machines?

Click to expand...

Please do guide me on the scan time query...will the timer keep good time?

Click to expand...

No !!

Will it miss time occasionally due to scan time?

Click to expand...

Yes !!

Removing the sensors is equivalent to making your PLC controller like a blind man. He may be able to do the job, but not very fast and not very efficiently. Put in as many sensors as the budget will allow. Use your timer as an alarm timer, to determine how long to wait after each motion is energized and the corresponding sensor does not see the event happen. The PLC timer will work well for that function, and your machine will have a built-in troubleshooting guide for your maintenance people. They will thank you for it.