10 Sensors, Safety, Analog and Troubleshooting
We now look outside the control panel at devices and methods of design that affect the design of the PLC and the overall project. Included in this chapter are sensors, a separate study of safety of the machine and the operator, machine vision, bar-code and RFID, analog signal acquisition, troubleshooting techniques and diagnostic circuits. We end the chapter with a short discussion of other ‘stuff’ and some articles on design and opinions about machine safety from some experts.
SENSORS
An important area is the discussion of sensors and sensor types. Many times the sensor and the application of the sensor are as critical to the success of a project as the PLC program. Web- sights such as www.sensorsmag.com, www.flowcontrolnetwork.com and www.globalspec.com provide search engines for information about various types of industrial sensor types.
A good website for an organization dedicated to instrumentation and sensor advancement is the International Society of Automation’s website .
Allen-Bradley lists a number of types of sensors in their website:
- Automatic Float Switches
- Connection Systems Drum Switches
- Encoders and Resolvers
- Limit Switches
- Liquid Level Switches P
- hotoelectric Sensors
- Pressure Controls – General Industrial
- Pressure Controls – Traditional Machine Tool Proximity Sensors
- Rotating Cam Limit Switches
- Speed Switches
- Temperature Control
- Ultrasonic Sensors
Siemens likewise contains similar sensor types to Allen-Bradley and can offer sensors for most industrial applications. The following page(s) list only one type of Siemens sensor (for pressure). The list is rather long and shows the diverse nature of the sensor industry with each type of sensing requiring a great deal of investigation in order to provide the best possible device for the application at a competitive price.
Importance of Sensors
Sensors are important in the overall design of any control system and the engineer/technologist must be aware of their importance to have a successful control system. Cost is always a factor as well as safety and maintainability. Something as simple as level measurement is not always simple. With the example of sensing level in a tank, it can be seen that the methods and costs vary over a wide range. It is also important to consider whether temperature, humidity or other physical change will upset the device. Also to be considered are changes in density, composition, moisture and other physical characteristics. Accuracy and repeatability are also considerations. The factor that the material may or may not have to actually touch the sensor may need to be considered as well. Some powders are especially difficult to handle and sense.
If the powder cakes or is chunky or granular, the sensor may require special techniques for sensing.
Devices such as tanks or silos with level control are an example of the sensors described that are chunky or granular. Level sensing in a tank may inv