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

2. 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.

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

4. 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

5. 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

6. 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

7. Voltage vs. Current signaling

8. 4–20mA signaling Mapping a 50 - 250°C temperature scale to 4 – 20mA
50°C  4mA
250°C  20mA

9. 4–20mA signaling example

10. 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

11. Example of a 4-20mA calculation

12. Solution

13. Example of a flow transmitter calculation

14. Solution

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

16. 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 ° range:
If we plug in y = 4 and x = 50:
Notice that the y-intercept (b) is not 4 in this case

17. Reverse-acting 4-20mA calculation

18. Reverse-acting 4-20mA calculation

19. Reverse-acting 4-20mA calculation

20. Converting a value to 4-20mA graphically

21. 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

22. 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

23. Calibration Errors
Zero shift 
 Span shift

24. 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

25. 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

26. 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