LVDT: 3

Linear variable differential transformer, used to measure displacement. Consists of a primary coil winding that is wounded onto a hollow cylindrical rod; the coil is attached to an AC voltage source which is located in the centre of the rod. It also consists of two secondary coil windings that have the same number of coil turns at equal distance from either side of the primary coil. These two secondary coils are connected to each other in series. The hollow tube contains a ferromagnetic core that is free to move within the tube (can be attached to a specimen to measure its displacement. This core can induce a voltage in the two secondary coils, the net EMF of the coils:

Enet=Ecoil1−Ecoil2

With zero displacement (null position), but secondary coils induce the same magnitude EMF, therefore 0 overall EMF. Advantage: linear relation (need to use a PSD and low pass filter), cheap, long life, high resolution, operates within large temp range (-600 to 600 C), frictionless measurement, no need for amplification, polarity can be determined

Disadvantage: nearby equipment can also induce EMF within the coils (can experience residual voltage at null position), difficult to fit within small devices.Uses: CNC and lathe machines, aircraft jet engines, creep measurements, bridges to measure the displacement due to temperature change in seasons.

DRAW DIAGRAM WITH EMF INDUCED

Why and HOW output of LVDT into dc

Easier to interpret output (a line on the oscilloscope proportional to displacement). Will receive a proportional voltage to that of displacement, instead of reading peak to peak voltage of an AC signal. To convert into DC you need a phase sensitive detector and a low pass filter.

Semiconductor Strain GaugeSemiconductor is attached to a backing that is attached onto the specimen. This type of strain gauge is also very common due to its cost efficiency and the repeatability and the achievement of a wide range of data.

This type of strain gauge uses a semi conductive material such as silicon or germanium, which usually have very high gauge factors (between 100 and 300). The reason behind the high gauge factor is due to the Piezo-resistive effect that it experiences. This will in turn have a greater change in the resistance for a small deflection, achieving a better precision and is able to measure much smaller strain changes. It is also inexpensive, can be manufactured to be very small due to its high gauge factor.

Figure 4: Semiconductor strain gauge

Strain gauge factor it is more sensitive to temperature and for high levels of strain the strain gauge value does not act linearly.

Measurement of torque: 5

Torque measured by measuring the angular displacement or surface strain.

T=12πGr3θ

Shear sensitivity = θT

= 2

πG r3

One strain gauge will measure the strain in the direction it is placed in, so using two strain gauges 45 degrees to each other.

Operating principles of tachogenerator

A device that measure the rotational speed of a shaft and converts it into a voltage. Without load resistance it is able to produce a voltage proportional to the rotational speed. Coils are place in the tachogenerator and as the magnetic material interacts with the coil, an EMF is produced that is usually proportional to that of rotational speed.

Resistance vs temp for thermistor vs metallic resistance thermometer: 5

Dynamic response of transducer +derive expression: 3

This aint fucking dynamics

Example of angular velocity transducer: 3

Why transducer no susceptible to temp variation

Operation of optical absolute shaft encoder: 4

Draw Wheatstone bridge to provide temp compensation for strain measurement

Have one as R(1+x) other as R(1+y)(1+x) on same side. Y cancels out.

Non-contact way to measure temperature: 4

Thermometer: mercury expands and displays the tempChange in pressure: capillary tube up to 60m long, pressure inside increase cause dial to move Peltier effect: a peltier cooler, the change in temp will cause a voltage to run through circuit. Temperature sensitive gauges can cause change in voltage to the Wheatstone bridge.

Charge amplifier derive output voltage + sketch: 2

2 marks each: Non-linearity Sensitivity Resolution Precision Bias

Charge amplifier instead of voltage amplifier: 2

Charge amplifier > voltage amplifier, since capacitance effects are diminished in connecting leads. Voltage amplifier consumes capacitance and voltage and reduces the output. Therefore a less effective amplifier.

Low pass filter + derive (Q3 2007)

Derive sensitivity of strain

V0/e or V0/Vs

Operation of piezoelectric accelerometer (Piezoelectric force transducer)

When acceleration is detected using this device, the mass inside the accelerometer causes the piezoelectric material to either contract or expand. It is know that piezoelectric materials such as quartz crystals exhibit a phenomenon where it produces an electrical current when it changes shape. This change in shape and using Newton’s second law of motion, will output an electrical signal that is proportional to the acceleration applied. Using these 2 points, the output signal can be interpreted to an acceleration value.

How Poisson effect changes sensitivity of strain

Have one as R(1+x) and other as R(1-vx), you find that V0/Vs is ¼ (1+v) eG.

Non-contact method to measure displacement: 2

Variable inductance displacement transducer (JJ lab). Ferromagnetic E piece. Coils around centre E piece. Ferromagnetic plate that displaces. AC source. V=L di/dt, L=N^2 /Rm. N=number of turns of coil, Rm= reluctance of coil. Can measure the voltage that is proportional to the displacement.

Ultrasound. Laser. GPS. 3D camera

Load-cell: 2

A load cell is a transducer that is used to convert a force into an electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge measures the deformation (strain) as an electrical signal, because the strain changes the effective electrical resistance of the wire. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration. The electrical signal output is typically in the order of a few millivolts and requires amplification by an instrumentation amplifier before it can be used. The output of the transducer can be scaled to calculate the force applied to the transducer. The various types of load cells that exist include Hydraulic load cells, Pneumatic load cells and Strain gauge load cells.