Pack 1 Flashcards

Question

Inductive proximity sensors?

Answer 1

 coil inductance is greatly affected by the presence of ferromagnetic materials  here the proximity of a ferromagnetic plate is determined by measuring the inductance of a coi

Answer 2

the range of a transducer defines the limits between which the input vary. (the difference between the maximum and minimum values measured) The span is the maximum value of the input minus the minimum value. For example: a load cell for the measurement of forces might have a range of 0 to 50 kN and a span of 50 kN.

Answer 3

Is the extent to which the value indicated by a measurement system might be wrong. It is the summation of all the possible errors that might occur Accuracy of a temperature measuring device can be ± 10oc

Answer 4

variation that occurs when measuring the same part with the same instrument.

Answer 5

* It is broken down into three components: * Stability: the consistency of measurements over time. * Bias: a measure of the amount of output, when there is no input, or actual offset in measurement. * Linearity: a measure of the bias values through the expected range of measurements.

Answer 6

1. Repeatability (Equipment variation): variation in measurements under exact conditions. 2. Reproducibility (Appraiser variation): variation in the average of measurements when different operators measure the same part

Answer 7

Error is the difference between the result of the measurement and the true value of the quantity being measured. Error = measured value - true value

Answer 8

the sensitivity is the relationship indicating how much output you get per unit input, i.e. (the ratio of change in output to change in input).

Answer 9

Transducers can give different outputs from the same value being measured, according to whether that value has been reached by a continuously increasing change or a continuously decreasing change

Answer 10

Repeatability is used to indicate the difference in value between two successive measurements under the same environmental and operational conditions. Repetability = max-min.values/full range * 100

Answer 11

The stability of a transducer is its ability to give the same output when used to measure a constant input over a period of time. The term drift is often used to describe the change in output that occurs over time. The drift may be expressed as a percentage of the full range output. the term zero drift is used for the changes that occur in output when there is zero input.

Answer 12

the dead band or dead space of a transducer is the range of input values for which there is no output. The dead time is the length of time from the application of an input until the output begins to respond or change

Answer 13

The resolution is the smallest change in the input value that will produce an observable change in the output. Sometimes is defined as the number of measurements within the range from minimum to maximum * In position sensor the minimum displacement that can be measured

Answer 14

All the performance parameters discussed above are steady -state characteristics. The followings are from the dynamic characteristics of a measurement system: * Response time * Time constant * Overshoot * Band width

Answer 15

the response time also is called settling time. the response time can be defined as the time required for a change in input to become observable as a stable change in output.

Answer 16

The maximum overshot is the maximum peak value of the response curve measured from steady state value

Answer 17

bandwidth is one of the most important dynamic parameters of a transducer. Ideally, we would prefer a transducer to have the same output amplitude for any input signals with the same amplitude, independent of their frequencies. The bandwidth of a transducer is defined as the frequency range in which the amplification factor or the amplitude of the transfer function will not decay significantly

Answer 18

– Provide a signal that is continuous in both its magnitude and temporal or spatial content Most of the physical measurands are analog in nature Examples: Temperature, displacement, light intensity * Digital sensors Their output takes the form of discrete steps or states Digital signals are more repeatable, reliable and easier to transmit Examples: Shaft encoder, contact switch

Answer 19

Analog-to-digital converter (A/D or ADC) changes analogue voltage or current levels into digital information. * The input to an ADC is an analogue signal and the output is a binary number that represents the level of the input signal

Answer 20

– Number of input channels – Single-ended or differential input signals – Sampling rate (in samples per second) – Resolution (usually measured in bits of resolution) – Input range (specified in full-scale volts) – Noise and nonlinearity

Answer 21

 The higher the sampling rate, the better.

Answer 22

Most A-D converters use a comparator as part of the conversion process – A comparator compares 2 signals A and B * if A > B the comparator output is in one logic state (0, say) * if B > A then it is in the opposite state (1, say) – A comparator can be built using an op amp with no feedback

Answer 23

register is a small amount of storage available whose contents can be accessed more quickly than storage available elsewhere. they have also been implemented using individual flip-flopsflip-flop is an electronic circuit (a bistable multivibrator) that has two stable states and thereby is capable of serving as one bit of memory.

Answer 24

*Vref is a stable reference voltage as part of the converter circuit. *As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state. *The encoder generates a binary number based on the highest-order active input, ignoring all other active inputs. Unfortunately, it is the most component-intensive for any given number of output bits. It needs 2n -1 comparators, A fourbit version would require 15 comparators. Considering that eight bits is generally considered the minimum necessary for any practical ADC (255 comparators needed!)

Answer 25

Also known as the stair step-ramp, or simply counter ramp A/D converter When a conversion is required. A signal (conversion request) is sent to the converter and the counter is reset to zero As the counter counts up with each clock pulse, the DAC outputs a slightly higher (more positive) voltage. This voltage is compared against the input voltage by the comparator. If the input voltage is greater than the DAC output, the comparator's output will be high and the counter will continue counting normally. When DAC output exceeds the input voltage, causes the comparator's output to go low. A drawback of the counter-ramp converter is the length of time required to convert large voltagesThis method is useful for medium accuracy and slow speed applications

Answer 26

The successive approximation register is initialized so that the most significant bit (MSB) is equal to a digital 1. * * This code is fed into the DAC which then supplies the analog equivalent of this digital code (Vref/2) into the comparator circuit for comparison with the sampled input voltage. * If this analog voltage exceeds Vin the comparator causes the SAR to reset this bit; otherwise, the bit is left a 1. * Then the next bit is set to 1 and the same test is done, continuing this binary search until every bit in the SAR has been tested. * The resulting code is the digital approximation of the sampled input voltage and is finally output by the DAC at the end of the conversion

Answer 27

Instead of using a DAC with a ramped output, an op-amp circuit called an integrator to generate a saw tooth waveform is used, which is then compared against the analogue input by a comparator. 29The time it takes for the saw tooth waveform to exceed the input signal voltage level is measured by means of a digital counter clocked with a precisefrequency square wave (usually from a crystal oscillator). The single-slope ADC suffers all the disadvantages of the digital ramp ADC, The only good thing to say about this circuit is that it avoids the use of a DAC, which reduces circuit complexity

Answer 28

Resolution/complexity ratio: Single-slope integrating, dual-slope integrating, counter, tracking, successive approximation, flash. Speed: Flash, tracking, successive approximation, single-slope integrating & counter, dual-slope integrating. Step recovery: Flash, successive-approximation, single-slope integrating & counter, dual-slope integrating, tracking

Answer 29

Stepper motors are applied when: – Accurate positioning is required – Smooth rotation is needed * Advantages of stepper motors are – Low cost – Stable zero-speed – No feedback for position control or speed control – Most widely used, open loop positioning device – Compatible with modern digital controllers * Important properties of stepper motors are: – Small step angle, – High static torque, – No-cumulative position error, – Excellent dynamic behaviour by high ratio for torque/inertia. * Various implementations can provide: – Full Stepping Systems (200 Steps/Rev) – Half Stepping Systems (400 Steps/Rev)