Measurement systems KONC 2004 Mechatronics Basics 17 11

Measurement systems KON-C 2004 Mechatronics Basics 17. 11. 2016 Tapio Lantela

Lecture topics Analog-to-digital conversion Frequency response Electromagnetic compatibility

Measurement system Communication Sensors Actuators Process Interface Signal processing Input Controller Output Controlling Computer

Target - Accurate data collection without distortion or (too much) loss of information Requirements - Amplitude linearity Phase linearity Sufficient frequency response Sufficient resolution

Data acquisition 18. 2. 2021 5

Accuracy vs. precision vs. resolution Accuracy – The ability to produce a response or a reading that is close to the true value Precision – Random variation is minimized Resolution - Smallest detectable change

reading Amplitude linearity nonlinearity Calibration curve quantity 18. 2. 2021 7

Sensor Range Sensitivity Linearity Frequency response Zero offset Resolution 18. 2. 2021 8

Amplifying & compensating Often weak sensor signal must be amplified before filtering or converting to digital Also other analog signal conditioning can be done - Removing DC offset - Temperature compensating e. g. with temperature dependent resistors Operational amplifiers used for amplification, summing, filtering etc. - Specialized chips such as differential amplifiers 18. 2. 2021 9

Analog data storage Mechanical - Vinyl records Optical - Chart recorder Magnetic - C-cassette - VHS tapes

Digital data acquisition p. 322 -327.

Sampling frequency/rate - Usually constant Momentary or averaged measurements Reconstruction by interpolating

Quantization is mapping the analog continuous signal to discrete values https: //en. wikipedia. org/wiki/Quantization_(signal_processing) 18. 2. 2021 13

AD converter range and resolution http: //www. analog. com/media/en/training-seminars/design-handbooks/Data-Conversion. Handbook/Chapter 2. pdf 18. 2. 2021 14

Aliasing Too low sampling frequency causes aliasing - Minimum (Nyquist) frequency at least twice as high as the highest frequency component in the input signal

Sampling frequency and aliasing (Auslander & Kempf, Mechatronics – Mechanical System Interfacing)

Fourier transform Decomposing a signal into frequency components Signal in time domain Signal in frequency domain p. 101 -104.

Fourier transform 18. 2. 2021 18

Discrete Fourier transform Returns the amplitude of bands (or bins) of frequencies as complex numbers - 0 Hz = DC offset - Magnitude = absolute value of complex number - Phase = angle of complex number FFT – Fast Fourier transform algorithm - Matlab function fft(data) 18. 2. 2021 19

Frequency bandwidth A range of frequencies where the signal is attenuated less than 3 d. B - -3 d. B = 50% signal power - -3 d. B = 70% signal amplitude

Low pass filter http: //www. electronics-tutorials. ws/filter_2. html Passband = low frequencies Stopband = high frequencies Cutoff frequency - Amplitude -3 d. B i. e. gain ~0. 7 Example: RC filter - 1 st order low pass filter - Simple to implement, not very effective - 1 resistor, 1 capacitor 18. 2. 2021 21

Filter types Four main types - Low pass High pass Band stop Filtering affects both amplitude and phase

Phase linearity 18. 2. 2021 23

Signal processing devices 1/2 Analog circuits - operational amplifiers (op amps) for e. g. addition, subtraction integration and differentiation - signal filtering before AD-conversion - simultaneous sample and hold Digital circuits - programmable array logic (PAL) controllers programmable logic arrays (PLA) field-programmable gate array (FPGA) application specific integrated circuit (ASIC)

Signal processing devices 2/2 Programmable logic controllers PLC - industrial control applications DAQ boards - data acquisition modules for computers Microcontrollers and DSPs - compact computers for embedded systems - high level programming - DSPs (digital signal processor) for high speed floating point calculations Single board computers or any PC

Electromagnetic compatibility 18. 2. 2021 26

EMC Directive Article 4 (1) To ensure that the electromagnetic disturbances produced by equipment does not affect the correct functioning of other apparatus as well as radio and telecommunications networks, related equipment and electricity distribution networks. (2) To ensure that equipment has an adequate level of intrinsic immunity to electromagnetic disturbances to enable them to operate as intended. 18. 2. 2021 27

Disturbance coupling mechanisms Conductive Capacitive Inductive Radiative 18. 2. 2021 28

Inductive coupling 18. 2. 2021 29

Plate capacitor https: //en. wikipedia. org/wiki/Capacitor 18. 2. 2021 30

Capacitive coupling Changing voltage in a conductor is coupled via electric field - Stray capacitance between conductors Increasing factors - High frequency Close proximity Length of parallel cables High impedance target Unshielded conductors http: //www. electrical-installation. org/enwiki/Coupling_mechanisms_and_counter-measures 18. 2. 2021 31

Conductive coupling Disturbances conduct from device to device in the circuit Frequencies from 0 to 10 MHz Power supply Sensor 3 mΩ 0 V 0. 1 m. A 3 mΩ 1. 23 m. V Amplifier 10 m. A 3 mΩ 2. 24 m. V Controller 400 m. A 3. 66 m. V 18. 2. 2021 32

Conductive coupling Disturbances conduct from device to device in the circuit Frequencies from 0 to 10 MHz Power supply Controller 3 mΩ 0 V 400 m. A 3 mΩ 1. 23 m. V Amplifier 10 m. A 3 mΩ 1. 26 m. V Sensor 0. 1 m. A 1. 26 m. V 18. 2. 2021 33

Radiative coupling Source emits electromagnetic radiation, target acts as an antenna Frequencies usually over 15 MHz Distance larger than wavelength 18. 2. 2021 35

Digital signal frequency content Rule of thumb: Signal rising edge contains significant frequencies up to 0, 4/rise time 18. 2. 2021 36

Preventing disturbances Minimize loop area Cable shielding Conductive enclosures Ferrites Star grounding Galvanic isolation 18. 2. 2021 37

Signal transmission Voltage Differential signaling - Single ended - Differential Current - For long distances - 4 -20 m. A standard Digital - Most immune to noise - Compressed data https: //en. wikipedia. org/wiki/Differential_signaling

Example – measurement system for a hydraulic valve 6 pressure sensors, current and voltage sensors Data acquisition - 8 channels – 16 -bit resolution (1 bit for sign) - 100 k. Hz sampling frequency • Only in short bursts, otherwise too much data 4 -20 m. A signal from pressure sensors - Magnetoresistive wheatstone bridge with temperature compensation - Differential amplifier and current transmitter -10 to 10 V inputs for voltage and current Optoisolator for analog output 18. 2. 2021 39

Summary Basics - Accuracy vs resolution - Frequency decomposition - Frequency response Measurement systems - Analog to digital conversion - Filtering , aliasing Disturbance coupling mechanisms - Conductive, capacitive, inductive, (radiative) - Prevention 18. 2. 2021 40