Choosing a PLC (Programmable Logic Controller)


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PLCs: https://www.digikey.com/products/en/industrial-controls-meters/controllers-programmable-logic-plc/814

Supply Power
This is one of the easiest ways to narrow down the selection of PLCs. Any PLC with dash in the Voltage - Supply filter means you will have to purchase a separate power supply module which can be found in the product details page of the individual PLC under the Associated Products section. This is going to be the voltage that is powering the PLC, not necessarily the same as the other devices you are controlling or the output of the PLC.

Operator Interface
The PLC will need some sort of input feeding it information so it can decide what to do based on your programming. This can be done using the inputs with simple push buttons, potentiometers, selector switches, etc. or you can get more advanced devices like a touchscreen HMI (Human Machine Interface) that need specific communication protocols. If you are installing this into an already built machine then you will want to check what kinds of I/Os and protocols it is currently using and accommodate them.

Communication Protocols
There are many different communication protocols used in the field today with more being added and improved all the time. Below is a link to some of the protocols we have listed in our system with a brief description on them.

I/O (Inputs and Outputs)
Inputs and outputs are talking about other devices that only require a single connection point such as a switch or simple sensor. This is different than connecting to complex devices that require a communication protocol.

Inputs can either be digital or analog. Digital inputs are typically switches like a start button or a limit switch indicating when a part reaches the end of a line. Analog inputs have a varied signal which takes measurements such as temperature or distance. The accuracy of analog inputs can vary depending on the ADC of the PLC. Be sure to verify that the output of the devices are within the PLCs input capabilities to avoid damage to either the PLC or device.

Outputs are used to either control a device or to send signals to other controllers. Each have advantages in specific situations, below is a quick description of each type.

Types of Outputs

Solid State
Typically found as a PNP or NPN transistor output. This will be important to note as it will decide whether the PLC is sourcing or sinking the power. These can switch faster than relay outputs, but they have lower current capabilities and are restricted to a specific voltage.

Relay
Each output will use multiple points of contact and are labeled as normally open (NO) or normally closed (NC). Most of the time they will be in pairs but they might also have a common contact with several outputs. Since the contacts are isolated from the PLC they can control devices that run at different voltages and higher currents.

PWM/Pulse
PWM (Pulse Width Modulation) outputs can vary and can be used to control certain devices like motors. The exact frequency and pulse width will be decided in the programming. Adding a smoothing capacitor to this output can serve as an analog voltage output, though it might not be as accurate as an analog output. Pulse and PWM are similar, the differences lie in the duty cycle and the shape of the waveform. PWM will have a square wave and an adjustable duty cycle, where a pulse output has a fixed duty cycle and either a trapezoidal or s-curve waveform.

Digital
Digital outputs provide a constant voltage when activated. The voltage is dependent on the PLC and typically can’t be changed. The current capabilities aren’t very high and they will usually go to indicator lights or relays which are controlling larger devices.

Analog
These outputs are either voltage or current based with ranges that can be found on the datasheet, typically 1 - 5V, 0 - 10V, 0 - 20 mA, or 4 - 20 mA. Most of these outputs are not designed to directly control a device but are used to send a signal to another controller for interpretation.

Programming
Another item to consider is how is the PLC going to be programmed. Most PLCs require specific software to connect it to a computer and program it. Taking a look at the software will give you an idea of what languages you can use in case you are more familiar or want a particular one. It will also give you an idea of the capabilities of it which can make certain tasks much easier. Price is another thing to look at, is it free, a one time purchase, or subscription based.


What is a PLC and what does a PLC do?