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
