Electronic Control Systems Week 4 – Signaling and Calibration EET273 Electronic Control Systems Week 4 – Signaling and Calibration

Lab 2 Recap Why use a sourcing switch vs. a sinking switch? Which type of switch do you imagine is more common? Think about what happens in the event of a ground fault.

Signaling and Calibration Reading: 13:1 – 13:7 4 – 20mA current signals Reading: 18:1 – 18:8, 18:11 Instrument calibration

4–20mA 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 4mA – 20mA 4mA lowest possible signal level 0% of scale 20mA highest signal level 100% of scale

4–20mA signaling – live zero vs. dead zero Because the lowest value of the range corresponds to a non-zero value, (4mA), 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 (4mA in this case), and a failed signal (0mA) “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 (~1MΩ) 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

4–20mA signaling Mapping a 50 - 250°C temperature scale to 4 – 20mA 50°C  4mA 250°C  20mA

4–20mA signaling example

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

Example of a 4-20mA 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 zero- biased. 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 4-20mA calculation

Reverse-acting 4-20mA calculation

Reverse-acting 4-20mA calculation

Converting a value to 4-20mA graphically

Calibration – calibrate vs. re-range 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 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