08 Measuring Systems and Sensors Flashcards
Application of Position and Angle Measurement
- Position-, angle-, and speed control in feedback controls
- Detection of actual position:
o Control devices
o Filling level
o Calibration of work pieces
o Surface Scanning
Requirements of Position and Angle Measurement (6)
- High resolution and accuracy
- Absolute and relative position detection
- Robust construction (e.g. temp. variation)
- Flexible assembly
- Durability
- Universal Data interface
Direct Data Acquisition
- Drive System and transducer are uncoupled
- Direct comparison of measured and referenced value
- Derivation of the speed
o Rotary positioners: indirectly via the motor encoder
o Direct linear drives: via position change
High accuracy requirements demand direct measurement systems, because they are more accurate than indirect measurement systems
Direct Data Acquisition - Error Factors
o Temperature drift
o Pitch errors in the measuring scale
o Separation and angle deviation between measuring head and scale
o Joint points over the measuring scale
Indirect Data Acquisition
- Drive System and transducer are identical
- Conversion of the measured value in another physical measured value (e.g. position of the slide by the rotation angle of the ball screw)
- Speed and position for the control loops are indirectly derived by the encoder
- Specific error factors at the movement transformation (e.g. torsion or compression of the spindle)
Indirect Data Acquisition - Error Factors
o Spindle elastic deformation
o Spindle pitch errors
o Backlash
o Spindle wear
o Transducer error
o Temperature drift
Principles of Data Acquisition: Digital vs. Anlogue Measurement
A measurement is called analog, if the measurand (input value) is related to a signal (output value), in a distinct and pointwise continuous representation of the measured.
o In theory, analog measuring systems have an unlimited resolution.
A measurement is called digital, if the measurand is assigned to a signal, which supplies a quantized mapping of the measurand with a fixed step size.
o The resolution of digital measuring systems is given by the quantization resolution.
Principles of Data Acquisition: Incremental vs. Absolute Measurement
Incremental measurements count and interpret the periods of a periodic input signal.
o Relative measurement method, as the counting process can be started at any time and any position. -> Reference point is necessary, to achieve a repeatable measurement value
-> The reference point has to be reacquired at every startup of a system
Absolute measurements enable an a priori measurement
o It is possible to measure the absolute position of a slide or a turntable, directly after powering up the machine, and without acquiring any reference point
Main Components of photoelectric Measuring Systems
o Scale or graduated disk
o Scanning unit:
Consists of a light source (almost exclusively LED), optics (condenser), scanning grating (scanning plate) and photodetector
Photodetector receives light of modulated brightness, when there is a relative motion between the scanning unit and the scale
Receiver circuit converts the light intensity into electrical signals which can be evaluated for displacement measurement
photoelectric Measuring Systems
Scanning is carried out by incident light or by transmitted light
photoelectric Measuring Systems - Incident Light Method
Scale has altering reflecting and non-reflecting zones
photoelectric Measuring Systems - Transmitted light method
Scale consists of transparent and opaque zones
Disadvantage of all incremental measuring methods
Incremental measuring systems require a known starting position from which counting of the increments can begin for left and right travel.
o On linear scales, several reference marks can be applied which have different distances to each other -> After traversing two such distance-coded reference marks, the evaluation unit determines the absolute position on the scale by counting the increments lying between the marks.
Code measuring systems (digital-absolute)
Each path element is assigned a unique numerical value
o Binary-coded scales: Distance of the finest graduation decreases by a factor of 2 with each additional track
o Dual number can be read at any point along the path when all tracks are scanned simultaneously
o Number of code tracks increases with the length of the scale -> with the number of code tracks the resolution of the system increases
o If scanning line detects a high number of segment changes, mechanical tolerances can cause jitter at the transition -> Lead to short-term false evaluations
-> Different coding options can be used to prevent these misinterpretations (e.g. Gray code)
Rotary Measurement Systems
based on the same measuring principles as linear measuring systems.
- Instead of a scale, partial disks (incremental measuring system) or code disks (absolute measuring system) are used.
-> Transilluminated with the aid of semiconductor light sources and scanning plate and read out on the back by photodiodes
Rotary Measurement Systems: Multiturn absolute encoders
Several scanning groups can be accommodated in one housing in rotary angle encoders, to increase the measuring range
-> Connected to each other via precision gears (Backlash must be eliminated to ensure that the measuring accuracy remains constant over the entire measuring range)
Incremental Measuring Systems: Reference Marks
- Distance-coded reference marks
- Start reference marks
Distance-coded reference marks
o Marks have different distances
-> System only has to find two marks to know the exact position
o After passing two marks, the absolute position can be determined from their distance
o Only possible with linear scales
Start reference mark
o Reference mark only exists once
o After switching on the measuring system, the reference mark bust be approached
o Absolute position only by adding up the increments
Position Detection
o Decoding of digital position-information
o Problem: Sampling is limited by physical boundaries -> Jitter at position transition
o Decoding of transition can lead to errors -> Pseudo-random or gray encoded transitions are used
o Amount of codelines rises with length and resolution of the system
Absolute Photoelectric position detection
- Allows to determine the position without travelling to a reference point
- The sampling device uses 5 (example) photodetectors aligned in the sampling line A -> Read all five line values simultaneously
- Resolution of the system is determined by the size of the finest line division
Double Sampling for dual encoded transducer
- Linearly arranged photo detector set-up produce some incorrect readings especially if the device is taking samples exactly at the transition between two coded values -> One method to avoid such failures it the implementation of a double sampling scheme
- V order of the detectors enables a special sampling method that eliminates the acquisition uncertainty caused by sampling on the transitions
o Takes the values of the detectors following the order from the least significant to the most significant bit.
o If the sensor read value is equal to logit “0” (-> no light) the value of the next samplingline will take the value of the right side of photo detector (vice versa)
Gray Code
- Can be used to prevent incorrect evaluations
- Only one bit (one code line) change at the transition of one value to the other.
Advantages of Optical Measurement systems
- High resolution and accuracy
- Application of special measure-supports enable thermos invariance
- Measure length is customizable
Disadvantages of Optical Measurement Systems
- High acquisition costs
- Absolute systems need highly accurate treatment -> Therefore often short linear system or rotary encoders are offered
- Generally long distances are measured with incremental systems and therefore need reference marks
- Dust and dirt can influence the systems negatively, so that sealing air is required
Electromagnetic Induction Principle
o Altering current flows through conductor
o Changing of magnetic field induces a voltage in the conductor
o Amplitude of voltage depends on magnetic flux density
Inductosyn Coil Configuration: Application in Measuring Systems
o Increase of magnetic flux density by usage of coils instead of single conductors
o Direction detection can be realized by the application of a second conductor level (shifted by 90°)
o Position of the moving coil relative to the lineal coil is being modulated on the carrier signal
Linear- vs Round-Inductosyn
Linear-Inductosyn: Used to measure linear deplacements
Round-Inductosyn: Used for angular measurements
Advantages and Disadvantages of Inductosyn
Advantages
- Passive scale possible
- Variable scale length
- Easy system integration
Disadvantages
- Active component necessary
Inductive Principles for rotary decoders: Resolver
Electromagnetic goniometer, which can be used as a position measuring system in combination with a ball screw
o Cyclic absolute measuring system
o Rotor represents field winding
o Two perpendicular measuring coils applied to the stator (-> Shifted 90°)
o Resulting voltages represent rotor position
Advantages of the resolver measuring system
o Small design
o High precision
o Extreme robustness
Magnetic Position Measurement System
- Magnetic position measuring systems consist of a magnetic scale bar
o The scale bar contains periodic fields with alternating magnetic north (N) and south (S) poles
o The scale core is fabricated using a hard magnetic material with a high coercivity coefficient, assuring a long life of the magnetic pattern -> Still sensitive to mechanical shocks and collisions with metallic objects - Magnetic measuring systems are insensitive to dirt, oil or grease Due to this and small size it is possible to integrate them directly into linear guidings
- Measuring system in combination with an interpolation unit can reach measuring resolutions up to 1 micrometer
- Guide rail can also contain a reference mark
Advantages of Magnetic/Inductive Systems
- Low acquisition costs
- Resistant against non-magnetic pollution
- Integration in guide rail is possible
- Simple production of measure scales (stamping, partial magnetization)
Disadvantages of magnetic/inductive systems
- Referencing is required (direct acquisition)
- Low resolution compared to optical systems -> Interpolation necessary
- Metallic scales are highly temp. variant
Current Measurement - Application
o Feedback of current control in Motion-Control-Systems
o In-Process diagnosis: Tool wear, collision detection, adaption of machining process
o Condition monitoring: Oscillation diagnosis, wear of bearings
Current Measurement - Requirements
o Galvanic isolation form high voltages
o High bandwidth
o AC/DC measuring
Direct current measuring with shunt-resistance
Application of low resistance -> Low heating for high currents
Voltage drop over resistance (typ. <100mV) is directly proportional to the current
Data logging via A/D converter
Advantages of Direct current measuring with shunt-resistance
- Very high bandwidth
- Low price
Disadvantages of Direct current measuring with shunt-resistance
- Dissipation equals RI^2 -> Only for low currents
- Galvanic isolation has to be applied seperately
Direct Current measurement with current transformers
Construction similar to transformer
Alternate currents induces via N1 a magnetic flux in N2 -> While constant change of flux, a current proportional voltage can be measured at N2
Advantages of Direct Current measurement with current transformers
- Low costs
- Galvanic isolation
Disadvantages of Direct Current measurement with current transformers
- Only for AC
- Bandwidth is limited by conductor material
- Influence through external magn. fields
Indirect Current Measuring with closed loop transducers
The magnetic flux induced by lP is compensated by the coil applied with the current lS
Therefore a hall sensor detects the magnetic flux and transforms the result with amplifier A into the current lS
LS effects voltage drop VM at RM; This voltage is proportional to lP
Advantages of Indirect Current Measuring with closed loop transducers
- Galvanic Isolation
- High bandwidth and accuracy
- AC and DC Measurement
Disadvantages of Indirect Current Measuring with closed loop transducers
- Phase shift between lS and VM
- External voltage supply necessary
Acceleration/Oscillation Measurement - Application
o Increase of control precision
o Motion detection
o Frame construction surveying (modal analysis)
o In-Process diagnosis -> Chatter detection
o Condition monitoring -> Oscillation diagnosis
Acceleration/Oscillation Measurement - Requirements
o High dynamic
o High bandwidth
o Single sensitivity in measuring direction
o Robust, thermo-invariant
Acceleration/Oscillation Measurement - Indirect piezo-electrical measurement
Seismic mass is applied to measuring object via spring/damper combination
Acceleration causes a force that results in a displacement which can be measured
Acceleration/Oscillation Measurement - Indirect piezo-electrical measurement:
Piezoelectric Evaluation
spring/damper combination is represented by a piezoelectric material
Forces on piezoelectric materials produce a electrical charge on their surface -> this charge can be measured as a force proportional voltage
Evaluation unit in sensor calculates a force proportional signal
Acceleration/Oscillation Measurement - Indirect piezo-electrical measurement: Advantages
- Highly thermo-invariant if separate evaluation units are used (-269°C to +750°C)
- High dynamic
- Therefore ideal for dynamic events (e.g. shocks)
- Robust construction/casing
Acceleration/Oscillation Measurement - Indirect piezo-electrical measurement: Disadvantages
- Evaluation unit limits temperature range (-50°C to +120°C)
- Static events can not be detected. Therefore an evaluation of the internal resistance is required (piezo-resistive effect)
- Different frequency ranges require different sensors
- High efforts for signal conditioning
Acceleration/Oscillation Measurement - Indirect Capacitve Measurement
Connection bridges form a spring-damper system
Plate-like mass inside the sensor forms two plate capacitors with the covering and base plate of the sensor
Plate is deflected during acceleration
Capacity depends on the plate distance -> accelerations cause change in capacitance
If the capacity is part of a resonant circuit, the frequency f0 of the resonant circuit changes while accelerating
Electric evaluator in the sensor calculates the associated acceleration and returns the related current value
Acceleration/Oscillation Measurement - Indirect Capacitve Measurement: Advantages
- Static and dynamic accelerations measurable
- Use of simple evaluation devices
- Low-level frequency measurable
Acceleration/Oscillation Measurement - Indirect Capacitve Measurement: Disadvantages
- Evaluation device restrict the range of temperature
- Dynamic range in midfield
- Comparatively large design (necessary in order to achieve relevant changes in capacitance)
Force/Torque Measurement - Application
o Feedback for control loops
o Acquisition of drive torques and forces for process control
o Surveillance of highly stressed workpieces
o Support for startup
o Weight acquisition
Force/Torque Measurement - Requirements
o High accuracy
o Low temperature drift
o High flexibility
Force/Torque Measurement: Processing of Actuating Current - Advantages
- “Recycling” of information
- No external sensors required
Force/Torque Measurement: Processing of Actuating Current- Disadvantages
- Sensitive concerning changes of model parameters
- Bandwidth depends on control frequency of inverter
- Repercussions of mechanical components (inertia, friction…) lead to very indirect measurement
Hooke’s Calculation
Metals elastically deform proportional to the loading force
Strain Gauges
Used to detect material strain on machine parts
When strain gauge is stretched, it reduces the cross-section of the wire -> Resistance is increased
Evaluation of deformation via measuring the resistance variation in the conductor
Sensitivity can be increased by application of conductor coils or semiconductors instead of single conductors
Semiconductors can effect (piezo-resistive effect) negative k-factors
Advantages of Strain Gauges
- Possible application in critical positions
- Static and dynamic impacts can be measured
- High accuracy
Disadvantages of Strain Gauges
- Application typically with adhesives -> Reuse impossible
- Assembly requires qualified personal and prepared surfaces
- Lateral contraction must be avoided
- External amplifier required -> Supply lines are length limited
- Absolute acquisition requires calibration after application
- Thermo-variance and drift require expensive evaluation systems
Temperature Measurement - Application
o Feedback for process control (e.g. cooling system)
o Compensation of thermally induced expansion and displacement
o Safety shut down in case of overheating (semiconductors in inverter, strains in drive)
o Indirect acquisition of friction
o Calorimetric measurement
Temperature Measurement - Requirements
o High absolute accuracy
o Insensitive concerning electromagnetic disturbance (e.g. measurement of coil temperature)
o Calculable reaction rate
Temperature Measurement - Principle (Thermal Element and Metal-/Semiconductor thermometer)
Thermal element
Use of Seebeck-effect
* Used in satellites to provide them with energy (Nuclear decay heat is used in thermal generators to generate electricity)
* Advantage: No moving parts
* The inversion of the Seebeck-effect is known as the Peltier-effect -> Used fo cooling of CPUs
Temperature difference effects voltage between the contact points of two different (material) conductors
Characteristic function of voltage and temperature is deposited in evaluation unit
Metal-/Semiconductor-resistance
Electrical resistance of metal- and semiconductor elements depends on temperature
For semi-conductors positive (PTC) and negative (NTC) temperature coefficients are possible
Coefficients can be looked up in datasheet
For continuous voltage supply (self-heating) a temperature dependent voltage can be measured
Temperature Measurement - Quartz Thermometer
o Resonant frequency of the thickness oscillation of a quartz plate is thermo-variant
o An oscillating quartz plate near its resonant frequency is represented by an inductance L1, a capacity C1 and a dynamic resistance R1
o Together with the capacity of the supply-lines and -ports C0 the resonant frequency can be determined (R1 can be neglected)
o Very accurate but technically complex -> Mainly useful for calibrating other temperature sensors
