Fast PLCs for Sensor Monitoring & Relay Triggering

[opmal7]

I'm working on a project uses a PLC to monitor an analog input from a sensor. Once the sensor's signal exceeds a certain value, the PLC sends a command to a relay to fire a solenoid.

I've hooked the system up to an o-scope, and I'm seeing a delay of about 75ms from the time the sensor reaches the point to when the relay triggers. I am using solid state relays with 5ms delay, and the sensor operates at 500 Hz. I am using a Siemen's PLC, which I believe is causing most of the delay. From reading the PLC literature, it sounds like the PLC scans the input, stores the value to a memory location, and then looks to that memory location to compare the values. This whole process takes about 65ms.

I would like to reduce the delay as much as possible. Are there any super fast PLC's on the market that would operate faster? I would expect to find most of the delay in the sensor, relay, and solenoid and am surprised that the PLC is what is slowing me down. You would think that with computers being so fast nowadays the computing part should be the quickest.

I'm looking for suggestions on fast PLC's, but am open to other ideas that might reduce the delay in this type of system.

 

[berkeman]

I'm working on a project uses a PLC to monitor an analog input from a sensor. Once the sensor's signal exceeds a certain value, the PLC sends a command to a relay to fire a solenoid.

I've hooked the system up to an o-scope, and I'm seeing a delay of about 75ms from the time the sensor reaches the point to when the relay triggers. I am using solid state relays with 5ms delay, and the sensor operates at 500 Hz. I am using a Siemen's PLC, which I believe is causing most of the delay. From reading the PLC literature, it sounds like the PLC scans the input, stores the value to a memory location, and then looks to that memory location to compare the values. This whole process takes about 65ms.

I would like to reduce the delay as much as possible. Are there any super fast PLC's on the market that would operate faster? I would expect to find most of the delay in the sensor, relay, and solenoid and am surprised that the PLC is what is slowing me down. You would think that with computers being so fast nowadays the computing part should be the quickest.

I'm looking for suggestions on fast PLC's, but am open to other ideas that might reduce the delay in this type of system.

Yikes, that is a long delay. I'd start by calling the Customer Support folks at the company that makes the PLC. They may have some other options for PLC units with (much) faster processing delay.

 

[dlgoff]

From reading the PLC literature, it sounds like the PLC scans the input, stores the value to a memory location, and then looks to that memory location to compare the values. This whole process takes about 65ms.

I'm wondering what all else your PLC is doing?

It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs.[5] This scan time may be a few milliseconds for a small program or on a fast processor, but older PLCs running very large programs could take much longer (say, up to 100 ms) to execute the program.

Programmable logic controller Scan time