Analogue Inputs and Processing Lecture Number 6 Analogue Inputs and Processing

In this session… In this session, you will learn about analogue inputs and how they are processed in the PLC

Learning outcomes At the end of this session you should be able to: Understand the difference between and analogue input and digital input Understand how to develop and utilise scaling equations

Analogue Inputs and Sampling

Digital to Analogue Conversion R2R Ladder The R2R Ladder is a common system used to generate an analogue output from a Digital value.

Digital to Analogue Conversion R2R Ladder The output of the R2R Ladder is given by this equation. The device is called a Digital to Analogue Converter (DAC).

Analogue to Digital Converter An Analogue to Digital Converter (ADC) is used to take an analogue value and convert it to a number which represents the original analogue quantity. This process is called Quantisation.

Analogue to Digital Converter The system below is one way this can be achieved.

Analogue to Digital Converter This is an example of a 4 Bit Ramp Compare ADC

Sampling An analogue signal has an infinite number of discrete states.  

Sampling In order to represent the whole signal in a computer we need to take ‘samples’ of the signal  

Sampling The sampling rate and resolution on the ADC greatly affects the quality of the sampled signal.  

Analogue to Digital Converter ADC’s and DAC’s are categorised mainly in terms of the word length of the binary output. Commonly word lengths of: 8 bit 12 bit 14 bit 16 bit are used

Quantisation The process of converting a signal from analogue to digital is called Quantisation. This formula can be used to determine the data value that represents the quantity being measured.

Quantisation Example If a 12 bit ADC is used to measure the weight of product in a process, given the following data: Weight Range : 0-1000kg ADC : 12bit If the current weight is 280kg, the digital value generated is calculated as follows:

Quantisation Example Now remember how a ADC works. The value is basically the count in a counter. Therefore, we cannot have decimal places in the quantised result. The rounding process depends on the type of ADC. Rounding down, the ADC value is: For this system 1146 represents 280kg

Quantisation Exercise Determine the digital value for a system that measures flow of liquid in a process given the following data: Flow Rate Range : 0 – 350 lmin-1 ADC : 8 bit Current Flow Rate : 245 lmin-1

Quantisation Exercise What calculation would you use to calculate the flow rate in a computer?

Quantisation Resolution Since the minimum change in the data is 1 bit (1 lsb). The resolution of the system can be defined as:

Quantisation Resolution Exercise Determine the Resolution of both a 8 bit and 14 bit ADC.

Industrial Standards Signals -10 ~ +10V 0 ~ 10V 4 ~ 20mA - Preferred Standard 0 ~20mA

Summary You should now have an understanding of how to implement analogue inputs on the PLC.