HPR4748: Programmable Logic Controls - Episode 5

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01 Introduction

This is the fifth episode in an 8 part series.

02

In the previous two episodes we looked at the Allen Bradley PLC 2 and the Siemens S5 series of PLCs.

In those episodes we mentioned I/O modules, but didn't really go into any detail on them.

Instead we looked at the CPU units and focused on things from the programmer's perspective.

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In this episode we will look at the I/O modules, what they are, how they work, and the purpose that they serve in the system.

In addition to this, I will discuss machine safety systems and also claims made by certain people about cyber security and industrial safety.

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04 What is I/O?

First though, we must address the question of what I/O is.

While the CPU module may run the program, in order for the program to do anything useful it must interact with the real world outside the PLC.

It must sense the state of the machine it is controlling through inputs, and it must affect the operation of the machine through outputs.

05 A Simple Example

Let's take a very simple example to try to make the above a bit more tangible.

Assume that we wish to press two parts together, such as pressing a bearing into a housing.

This is a very common industrial process.

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Let us assume we have the following items which connect to the inputs.

* An emergency stop circuit.

* A safety system to protect the operator.

* A part presence sensor used to detect that the housing was correctly placed in the tooling.

* Another part presence sensor used to detect that the bearing was correctly placed in the tooling.

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* A sensor which indicates the hydraulic cylinder is fully retracted.

* Another sensor which indicates the hydraulic cylinder is fully extended and the part was fully pressed home.

* A button which the operator uses to initiate the press cycle.

* Another button which the operator can use to abort the press cycle in the event of a fault.

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We also have the following items which connect to the outputs

* A hydraulic valve with two solenoids (down and up).

* A pilot light which tells the operator that the cycle was completed successfully and the part is good.

* A pilot light which tells the operator that the cycle was not completed successfully and the part is bad.

This gives us a total of 8 inputs and 4 outputs.

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A typical very small PLC would be suitable for this application.

The PLC program would look at the state of the inputs, and apply the necessary logic to turn the outputs on or off at the appropriate time.

As you can see, what makes a PLC useful is its ability to control I/O.

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Small PLCs typically have a limited amount of I/O built into the same package as the CPU and can be used as is right out of the box.

Medium to large PLCs will interface to racks of I/O, and the number of I/O points can be in the dozens to hundreds and even the thousands.

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11 Types of I/O

The above example is very simple.

There are in fact many types of I/O.

I will outline the most common ones here.

12 Digital I/O

The first is "digital" I/O.

This is overwhelmingly the most common type.

Digital I/O are basic on-off signals.

A digital input will detect if a switch is on or off.

A digital output will turn a relay or valve on or off.

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Digital I/O comes in three main types categorized by the type of electronic device used.

These are

* AC

* Relay