EET 273 Electronic Control Systems Week 5 Signaling

EET 273 Electronic Control Systems Week 5 – Signaling and Calibration

Signaling and Calibration • Reading: 13: 1 – 13: 7 • 4 – 20 m. A current signals • Reading: 18: 1 – 18: 8, 18: 11 • Instrument calibration

Lab Recap • Up to this point, we have looked at some of the basic building blocks of our motor control system (PWM board, motor, tach board) • Next, we need to understand what kind of signals we can use to send/receive signals to and from our PLC • Our PLC has • 2 voltage inputs (1 -5 V) • 2 current inputs (4 -20 m. A)

4– 20 m. A signaling • Most popular form of signal transmission in modern industrial systems • An analog signaling standard • An analog signal is “mapped” to a current range of 4 m. A – 20 m. A • 4 m. A • lowest possible signal level • 0% of scale • 20 m. A • highest signal level • 100% of scale

4– 20 m. A signaling – live zero vs. dead zero • Because the lowest value of the range corresponds to a non-zero value, (4 m. A), this type of signaling is referred to as “live zero” • Live zero signaling has the benefit of being able to discriminate between a true 0% value (4 m. A in this case), and a failed signal (0 m. A) • “Dead zero” signaling refers to a type of signaling where the lowest value of the range corresponds to zero signal level. • Dead zero signaling has the drawback of not being able to discriminate between a true 0% signal, and a failed signal

Why use current signaling? • Better noise rejection than voltage signaling • Voltage signaling requires high input impedance (~1 MΩ) at the receiver end • This makes the receiver much more sensitive to noise • Current signaling uses much lower input impedance (~250Ω), making them much more robust to noise • Voltage signaling is susceptible to voltage drops on the line, caused by: • High resistance in signal lines • Long cable runs • Current signaling is also not affected by voltage drops in the line

Voltage vs. Current signaling

4– 20 m. A signaling • Mapping a 50 - 250°C temperature scale to 4 – 20 m. A • 50°C 4 m. A • 250°C 20 m. A

4– 20 m. A signaling example

4– 20 m. A signaling example • Converting a 0% - 100% signal to the 4 – 20 m. A range: • Step 1: Convert to 0 -16 m. A range multiply by 16 m. A, divide by 100 • Step 2: Convert to 4 -20 m. A range add 4 m. A • General formula for convert a percentage to a 4 -20 m. A signal: • Example • x = 0% y = 4 m. A • x = 50% y = 12 m. A • x = 100% y = 20 m. A

Example of a 4 -20 m. A calculation

Solution

Example of a flow transmitter calculation

Solution

Solving using a linear equation • Use y = mx + b calculate slope, calculate y-intercept • For previous example:

Solving using a linear equation • This method is more useful when the measurement range is not zerobiased. Temperature is a good example of non-zero bias. • For a temperature transmitter with a 50 -140° range: • If we plug in y = 4 and x = 50: • Notice that the y-intercept (b) is not 4 in this case

Reverse-acting 4 -20 m. A calculation

Reverse-acting 4 -20 m. A calculation

Reverse-acting 4 -20 m. A calculation

Converting a value to 4 -20 m. A graphically

Calibration – calibrate vs. re-range • Calibrate: • check and adjust (if necessary) its response so the output accurately corresponds to its input throughout a specified range • This means exposing an instrument to a known quantity and comparing its output to the known quantity • Re-range: • Set the upper and lower range values so it responds with the desired sensitivity to changes in input. • Ranging an instrument involves setting the output range to which it responds to, calibration involves ensuring that the input maps correctly to that output range

Calibration • We can describe a linear relationship between the input/output of an instrument with a linear equation in the form y = mx + b • Calibration is simply matching our system’s behavior to this ideal equation • Two typical controls: • Zero – shifts the function vertically, the “b” • Adds or subtracts some quantity • Span – changes the slope of the function, the “m” • Multiplies or divides some quantity • A change in span typically produces a shift in the zero point, requiring a zero adjustment

Calibration Errors Zero shift Span shift

Calibration Errors – Linearity errors • Linearity error • The response of an instruments function is no longer a straight line • Cannot be fixed by a zero/span correction, because the response is no longer a linear function • Some instruments offer a “linearity” adjustment, which must be carefully adjusted according to the manufacturer instructions • Often the best you can is “split the error”, finding a happy medium between error high and low extremes

Calibration Errors – Hysteresis errors • Instrument responds differently to an increasing input compared to a decreasing input • This type of error can be detected by testing the instrument going up through the range, then down through the range • Typically caused by mechanical friction • Cannot be rectified through calibration, typically must replace the deflective component

Single Point Calibration • Most calibration errors are the result of multiple types of errors • Often, technicians perform a “single-point” calibration test of an instrument, as an indicator of calibration health • If the instrument passes the test, it is likely to be calibrated well • If the instrument fails the test, it needs to be calibrated