Encoder Question

Hi all,

Is it possible to connect a simple 24vdc incremental rotary encoder to a standard Seimens S7-300 digital input module, use the pulses to increase a counter in the program, have the counter reset with the 0 pulse, and turn on and off an output when the counter is at a certain value. What I am trying to do is make a cam controller. Is this possible or should I use a high speed counter module like a FM350-1?

I have that is 5-26vdc line driver out (7272), differential, bidirection with index (AA BB ZZ), and a 1000ppr. The shaft turns between 60 and 70 rpm.

Thanks

Lob a little math at it.

70 rev/min * 1 min/60 sec * 1000 pulse /rev
Thats 1167 pulses per second, or 1 pulse every 0.86 milliseconds. You need to be able to process each edge to be able to count a pulse. That gets you to 0.43 milliseconds.

So your hardware needs to have a minimum on->off and off->on of less than 0.43 milliseconds. Your processor needs to be able to service this input every 0.43 milliseconds.

What say you?

Keith

Wow, that seems really fast. My cpu is a siemens s7-300, 6ES7 315-2EH13-0AB0. It has the following processing times:

for bit operations, min. 0.1 μs
for word operations, min. 0.2 μs
for fixed point arithmetic, min. 2 μs​

for floating point arithmetic, min. 3 μs

So, in theory it should work but is it too close to be reliable or is this just fine? Or should I just go ahead and splurge and get a high speed counter module? I have not worked with processing times and all this is new to me. Thanks for the help.

Thanks,

OK, a little about processing time.

You won't be putting in just one instruction. You will be putting in hundreds. Those times will start adding up.

There is overhead in the processor scan. The PLC needs to perform various housekeeping and data handling tasks every scan. This time value can be determined but it is not listed as part of the instruction scan times.

Tpyical digital input points may not be able to respond fast enough to see the change. This is a hardware limitation of the input module you select.

Long story short you are better off using a counter card. As I remember the S7-300 counters aren't all that expensive.

Keith

After looking at and getting to understand the processing times a counter card would be much better. Are they faily easy to install and incorporate into the system? I have been doing some reading up on them and it looks complicated, or am I just reading too much into it?

So would a leeser ppr work, say 360ppr encoder, could it be used in the way I asked. That would would 2.3 msec which would be plenty of time wouldn't it?

Again, thanks for the help and I will use the counter module, mrt19

Stick with the FM350 it will be much easier in the long run.

Nick

That is what I will do, thanks for the help, mrt19