Pack 1 Flashcards
What are some mechanical elements?
Structure,mechanism,thermofluids etc
Mechanical elements may include static/dyanmic characteristics
A mechanical element interacts with its environment w puppose
Mechanical elements require power to produce motion/force
Types of sensors?
Displacement/position/proximity
What do displacement sensors measure?
Position/Velociy/Stress/Force etc
What are some displacement sensor types?
Strain gages/potentiometers/transformers optical motion etc
What is the strain gauge?
The electrical strain gauge is a metal wire, which is a wafer like that gauge the strain of an applience these are usually aligned with the direction of stress for stress analyisis.
What are some strain gages?
Metal-wire,metal-foi,semiconductor
Behaviour of a strain gage?
They’re made up of a preloaded resistance wire connected like a MOARA, and initially the resistances are equal the excitation voltage enters at Vs 2,3 and the output will be at 1,4. R1R3 = R2R4
What are the thermal effects of a strain gage?
Temp changes the resistance gage, the composition approches are using dummy gages/special compensated gages. Balance at R1/R2 = R3/R4 after temp R1+<>Rt/R2 = R3/R4 - not balanced you will need to compensate with the model
How to compensate?
Add an unstrained gage and both will be effected by the temperature = balance
called dummy gauge
What is a wireless signal transmission?
Miniature transmitter broadcasts from strain gauges to a receiver unit
* digital data encoding reduces noise
How do potentiometer work
Like a voltage divider and t read it you will need an analog - to - digital. It is used to sense position
What are resistive displacement sensors?
A resistance with a potentiometer may be used to measure linear ot rotational displacements. The output voltage at the contact is proportional to the displacement.
What are the different types of potentiometer?
Turn counting, linear motion, multi turn potentiometer possible errors are non-linearity, contact wear or noise
Causes and solution for noise in potentiometer?
Bouncing of the slider, dirt and wear - solution is having two diiifferent sliders
What are digital position transducers?
The position is indicated by a series of pulses - each pulse representing a unit of unit and may be linear or rotary
What do optical encoders do?
Provide a digi output as a result of angular displacement:
- Usually three concentric tracks with three sensors
are used:
- One slot on inner track to locate the 0o
angle
- Two tracks with n slots on each, half slot width offset to
detect the direction of rotation and angle
- 60 slots on 1 revolution result 360/60 = 6o
resolution
What do tachometer encoders do
Pulses conted to indicate displacement can’t tell direction of motion and there’s no indication of origin
What do incremental encoders do?
2/3 set of tracks and detectors. One track is a quareter of cycle behind the other.
Shows direction and magnitude but no origin
What is an incremental optical encoder
The index signal can be used to do several tasks in the
system.
- It can be used to reset or preset the position counter
and/or generated an interrupt signal to the system
controller
How to calculate distance travelled for optical encoders
– Example:
* Given: Encoder stripes = 128
* Given: Wheel diameter = 6”
* Given: counted 85 pulses
= 12.52 inches
1 revolution 6 inches 85 pulses
128 pulses 1 revolution
what about speed?
Revs/sec
What is an absolute encoder?
Several tracks
* Each track has a light source and detector
* Each position has a unique binary code
* The absolute position is determined from the
combination of values in each track
* Used in machine tools, CMMs, robots etc.
for each encoder position, there is a different binary output so that the shaft position is absolutely determined.
When is optical absolute optical encodiing used
Used when loss of refference is not possible,
A gray code - only changes a bit at a time
The info is transfered in parallel form
Linear/Incremental encoding - when is it used?
Uses a signle line that alternates black and white - two offset sensors produce signals and detect motion in each direct.
Pulses can be countet to determine the absolute position
What is an LCDT linear variable differential transformer?
- Consists of three coils symmetrically spaced along
an insulated tube - It has one primary coil, two secondary coils
- An AC voltage is applied to the primary coil and the
induced voltage in the secondary coils are
compared - No mechanical wear ensures a long life Complete
electrical isolation
Capacitive displacement
Capacitive sensors can sense anything. The object should have a dielectric
different from that of air, or it must be conductive; the only conditions for the
detection by capacitive sensing
Variable distance (d) sensors operate over a range of a few millimeters
* Cross-sensitivity to temperature and humidity (specially if the dielectric is
air)
* Capacitive sensors are also commonly used to measure pressure
* “Condenser” microphones measure changes in air pressure of incoming
sound waves
* A moving object could be attached to the dielectric or the plates to
generate capacitance changes
Inductive proximity sensors?
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
What is the range/span
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.
What is accuracy?
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
What is precision
variation that occurs when measuring the same part with the same
instrument.
How is accuracy of a measurment taken?
- 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.
What is the precision of a measurement?
- Repeatability (Equipment variation):
variation in measurements under exact
conditions. - Reproducibility (Appraiser variation):
variation in the average of measurements
when different operators measure the
same part
What is the error?
Error is the difference between the result of the
measurement and the true value of the quantity being
measured.
Error = measured value - true value
What is sensitivity?
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).
What is hysteresis error?
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
How to get and what is repetability
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
What is stability?
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.
What is dead band/time
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
What is a resolutin?
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
What are dynamic characteristics?
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
What is a response time?
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.
What is overshoot?
The maximum overshot is the maximum peak value of the response
curve measured from steady state value
What is bandwith?
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
Analog and digital sensors?
– 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
Analog to digital converter?
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
What is the most sginifigant criteria when selecting A/D hardware?
– 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
Sampling rate A/D converter
The higher the sampling rate, the better.
Comparators?
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
What is a register and a flip flo?
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.
What is a flash adc?
*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!)
What is a digital ramp ADC?
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
Successive approximation converter?
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
SIgnle slope ADC
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
Practical Considerations of ADCs
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
Stepper motors?
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)
