1. Example D: Null measurements
3. MEASUREMENT METHODS Deflection, difference, and null methods
Example D: Null measurements
Let us first define some new terms that describe the interface of a measurement system:
transducer is any device that converts a physical signal of one type into a physical signal of another type,
measurement transducer is the transducer that does not destroy the information to be measured,
input transducer or sensor is the transducer that converts non-electrical signals into electrical signals,
output transducer or actuator is the transducer that converts electrical signals into non-electrical signals.
Reference: [1]

2. Input transducer (sensor)
3. MEASUREMENT METHODS Deflection, difference, and null methods
Example D: Null measurements
Input transducer (sensor)
Non-electrical signal
Sensor
Electrical signal ES
N-ES

3. Output transducer (actuator)
3. MEASUREMENT METHODS Deflection, difference, and null methods
Example D: Null measurements
Output transducer (actuator)
Electrical signal
Actuator
Non-electrical signal
N-ES ES

4. Measurement system interface
3. MEASUREMENT METHODS Deflection, difference, and null methods
Example D: Null measurements
Measurement system interface
Measurement System
Sensor
Actuator
Non-electrical signals
Non-electrical signals
Sensor
Actuator

5. Example D: Null measurements
3. MEASUREMENT METHODS Deflection, difference, and null methods
Example D: Null measurements
Our aim in this example is to eliminate temperature drift in the sensitivity of a dc magnetic field sensor with the help of a linear temperature-insensitive reciprocal actuator. Ha VS Vo
Hact
Sensor
Actuator Vs
Hact T1 T1 T2 T2 Ha Vo

6. Example D: Null measurements
3. MEASUREMENT METHODS Deflection, difference, and null methods
Example D: Null measurements
Sensor
Hact VS Ha
A= Vo
Null detector
Reference (Helmholtz coils)
VS  0 Io
Any ideas? VS
Hact=Ha T1
The sensor temperature-drift errors and nonlinearity are not important T1 T2
Hact 1=Hact 2 T2
DH=Ha -Hact
Vo 1=Vo 2 Vo
DH1=DH2 =0

7. Example E: Difference measurements
3. MEASUREMENT METHODS Deflection, difference, and null methods
Example E: Difference measurements
Reference (Helmholtz coils) Io
Hact
G AOL
1+AOL b
Hact =Ha _______ VS
Sensor Vo
VS > 0
A< Ha VS
Hact T1
The sensor temperature-drift errors and nonlinearity are important T1 T2
Hact 1
VS 2 T2
Hact 2
VS 1
DH=Ha -Hact
Vo 2
Vo 1 Vo
DH1 DH2