21. ANALOG INPUTS AND OUTPUTS

Topics:

• Analog inputs and outputs

• Sampling issues; aliasing, quantization error, resolution

• Analog I/O with a PLC

Objectives:

• To understand the basics of conversion to and from analog values.

• Be able to use analog I/O on a PLC.

An analog value is continuous, not discrete, as shown in Figure 21.1 Logical and Continuous Values. In the previous chapters, techniques were discussed for designing logical control systems that had inputs and outputs that could only be on or off. These systems are less common than the logical control systems, but they are very important. In this chapter we will examine analog inputs and outputs so that we may design continuous control systems in a later chapter.

Figure 21.1 Logical and Continuous Values

Typical analog inputs and outputs for PLCs are listed below. Actuators and sensors that can be used with analog inputs and outputs will be discussed in later chapters.

Inputs:

• oven temperature

• fluid pressure

• fluid flow rate

Outputs:

• fluid valve position

• motor position

• motor velocity

This chapter will focus on the general principles behind digital-to-analog (D/A) and analog-to-digital (A/D) conversion. The chapter will show how to output and input analog values with a PLC.