STI LVDT Displacement Sensors

Transcription

1 STI LVDT Displacement Sensors

2 The LVDT Still the most reliable and widely used displacement transducer available today. The best performance to cost ratio of any of its rival products in today s market. Suitable for use in hostile environments where most alternatives would fail. Almost unlimited applications. Inherently simple and rugged design. Proven to perform to specification for decades without maintenance.

3 What We ll Cover What is an LVDT displacement transducer? How does it work? The range of LVDT transducers. Competitive Technologies & Competitors. Specific applications.

4 What is an LVDT Displacement Transducer? LVDT stands for Linear Variable Differential Transformer. People often say LVDT when they mean differential-inductance transducers or DC LVDTs. LVDT transducers are used to accurately measure displacement, deviation and position.

5 How does the LVDT work? The excitation is applied to the primary coil (P) in the form of an ac voltage. STI normally use 0.5V to 5kHz r.m.s. The output of the transducer is measured from the two secondary coils (S1 & S2). The secondary coils are connected in opposition. In the central position, the voltage induced into the secondary coils is equal but because the coils are connected in opposition, it cancels out resulting in a zero output.

6 LVDT Output (Non Phase- Sensitive) Using non-phase sensitive Voltage detection. Voltage Null If the output voltage is measured with an r.m.s. Voltmeter, the output rises in both halves of the coil and a measured voltage could be one of two positions. -FS Armature Position +FS Note that the curve does not meet the horizontal axis. This is due to the residual output.

7 LVDT Output (Phase-Sensitive) Using phase sensitive Voltage detection Voltage Null If phase sensitive demodulation is used, the sign of the output enables one to detect which half of the coil the armature is in. -FS Position +FS Today's signal conditioning uses phase sensitive demodulation as it eliminates the residual output and enables the user to know which half of the coil the armature is in.

8 LVDT Simulation

9 LVDT Linear Range Electrical Zero Position Armature IN +V Linear Range -V Armature OUT If we consider the output curve over the mechanical range of a typical STI LVDT, you will note that the ends start to turn. This is because the mechanical range is larger than the Linear Range. When calibrating an LVDT, it is essential that the electrical zero point is used as a reference and that the transducer is used over ±F.S. centerd around the electrical zero position. If the output is not centered about electrical zero, one of the full-scale positions will be outside the linear range and may therefore cause a linearity error.

10 Ranges Available Because LVDTs are inherently center-zero devices, it is normal to express their range plus/minus the half stroke. For example a ±1 transducer has a total measurement range of 2. Our short stroke LVDT range are typically 0.32 or 3/8 diameter and have measurement ranges from ±0.01 to ±0.5. Our long stroke units are mostly 13/16 diameter and have ranges from ±0.5 to ±18.5.

11 Types Available Unguided transducers have an armature that is separate from the body. They should be installed such that the armature does not touch the inner tube. Having done this, they have absolutely friction free movement. Captive transducers have a PTFE bearing that guides the armature in the inner tube. Spring return transducers are captive transducers with a spring that pushes the armature outward.

12 LVDT Cut-through Front-end Bearing & Cap (if fitted) Coils Armature tube or Inner Tube. (Spring fits in here when fitted.) Armature or core Case Push rod End thread / Probe tip (if fitted) Rear-end Bearing (if fitted) Cable Exit Internal Amplifier (if fitted) Rear-end Bearing & Cap (if fitted)

13 STI Offers a Broad Range of LVDTs

14 STI LVDT Designations

15 DC to DC LVDTs Typical Internal Amp Schematic SUPPLY DUAL REGULATOR SINEWAVE OSCILLATOR SINEWAVE OSCILLATOR OUTPUT The oscillator and demodulator are built into the transducer. This gives all of the advantages of the LVDT technology with a DC to DC interface. The supply and output voltages vary with model but ±10V to ±20V and 6 to 40V are covered, as well as 4-20mA 2-wire. Outputs from ±2V to ±5V and 0 to 10V are possible.

16 LVDT or DC-DC LVDT? How to decide

17 Advantages of LVDT + Separate Amplifier Transducer has a higher max temperature and is smaller than a DC to DC unit. Amplifier can be calibrated over a precise measurement range. Electrical damage to the system can normally be repaired (we can usually fix our external amplifiers). Better bandwidth, lower noise, technically the best approach. BUT.. More expensive than a DC LVDT. Transducer requires calibration with the amplifier. Perceived as a less simple than a DC transducer.

18 Advantages of DC LVDTs Simple self-contained package. The amplifier linearity error is included in the total linearity error. Cannot be accidentally / intentionally adjusted. BUT.. Cannot be adjusted! Not suited to radiation or higher temperature applications. Incorrect connections can irreparably damage the transducer. More susceptible to shock and vibration.

19 Competitive Technologies: Linear Potentiometers (Pots) Wiper V+ 0v A resistive track with a wiper. Applying a potential difference to the ends of the track and measuring the voltage at the wiper position allows one to measure position. Advantages over LVDTs Pots are normally cheaper. Pots are normally shorter for the same stroke. Advantages of LVDTs LVDTs have no contact with the sensing element, which means that there is no wear. Pots do have contact and can quickly wear, especially if there is vibration. LVDTs can easily be made dust and water proof, even the standard versions usually have a much better IP rating than potentiometers. Vibration of the transducer does not cause signal drop-out, unlike pots where the wiper can leave contact with the track under vibration.

20 Advantages over LVDTs Competitive Technologies: Encoders A device that counts pulses on a linear track, as they pass a detector. Encoders are normally shorter for the same stroke. Encoders have an easy digital interface. Encoders can have extremely high accuracy. Advantages of LVDTs LVDTs have a better analog frequency response. LVDT transducers are usually more rugged. An LVDT knows where it is as soon as it is turned on, unlike incremental encoders that need to make constant reference to a known position.

21 Competitive Technologies Magnetostrictive Wave Guide Position Magnet The interaction between the current pulse and the magnet causes an ultrasonic wave to travel along the wave guide. Electric Current Pulse A detector converts the ultrasonic wave to an electrical pulse.

22 Advantages over LVDTs Competitive Technologies Magnetostrictive Magnetostrictive sensors can have very long strokes. Magnetostrictive sensors are very suited to use in hydraulic cylinders. Magnetostrictive sensors are more accurate in longer strokes. Magnetostrictive sensors are very short for their stroke. Advantages of LVDTs LVDTs are less susceptible to shock & vibration. LVDTs are less susceptible to stray magnetic fields. The LVDT signal conditioning can be remote from the sensor allowing higher temperatures and high radiation etc. Magnetostrictive sensors are not suited to short strokes of ±4 or less (which accounts for the large majority of our business).

23 Competitive Technologies VRVT & Fastar Use electronic techniques to measure the characteristics of an inductive coil by measuring changes to inductance or reluctance etc. Advantages over LVDTs VRVT & Fastar have a shorter stroke to length ratio. Advantages of LVDTs LVDTs are less expensive. LVDTs have a smaller diameter. LVDTs are more rugged and do not wear. LVDTs last longer.

24 Competitive Technologies Capacitive Linear Sensors Use a capacitive technique to measure the position of a slider. Advantages over LVDTs Capacitive Linear Sensors can be more accurate. Advantages of LVDTs LVDTs are less expensive. LVDTs are less susceptible to environmental conditions. LVDTs are far more rugged.

25 Applications Research 1 We have supplied LVDTs that are used to measure the size of apples. Testing of component movement at high temperature. Long term creep testing of special alloys in large labs. Civil Engineering for building/bridge design verification and investigation. STI transducers have been used in a steam chamber to measure the effects on wooden beams of differing temperatures and humidities.

26 Applications Research 2 Transducers have been used to measure the sway of large sea-weed fronds that protect coastlines from erosion.

27 Applications Research 3 Measurement of sample deformation during explosion.

28 Applications Research 4 Materials testing, transducers are used to measure the deflection of samples, often in conjunction with a load cell. In this particular application, Zinc is being tested at 200 C, requiring a special option on the displacement transducer fitted to the the loading frame.

29 Applications Industrial Control Systems 1 Valve control Anywhere there is a control valve LVDT transducers can measure the position. Particularly important is main steam control valves on power station turbines. STI has literally hundreds of transducer in operation in this application. Sluice position Submersible transducers are suited to measuring the position of sluice gates in water management and sewage applications. Paper industry Slice indication The chemicals in paper stock can be corrosive. STI has both standard and special transducers in application on paper making machines, mainly with Valmet and Sandusky Walmsley (formerly Beloit).

30 Applications Industrial Control Systems 2 Roll gap measurement In order to maintain an even thickness of rolled products the roll gap is often measured at both ends. Oil flow control valves, located on the seabed. Actuator Position LVDT transducers are frequently fitted in parallel with hydraulic actuators. They are particularly suited due to their very low wear ensuring that they can be used for many millions of cycles. Dancer position In order to maintain constant tension in strip product a tension roll is used. Again due to the fact that there is constant movement, LVDT transducers are perfect for this application.

31 Applications Industrial Control Systems 3 Slitter Measures the position of cutter heads that slice rolled sheet materials. Tube straighteners and strip levellers Measures the position of rollers used to straighten rolled strips of material prior to stamping. Winders LVDTs can be used to measure the size of rolls of product, for example to signal to a control system when a roll is up to its maximum size.

32 Applications Machine Tools Production equipment used in the manufacture of machined, pressed or stamped parts. Frequent applications in bush insertion where the insertion force and position of the bush must be measured to ensure the correct fit. Test jigs to measure roundness, flatness etc. of machines parts for quality analysis. Test jigs to measure the position of components with respect to each other, such as the height of a switch contact for example.

33 Applications Aerospace 1 STI displacement products are mainly used for component production rather than on aircraft. We do have transducers on some aircraft as customer qualifies them as part of their system. Measurement of actuator response The transducer measures the position of flight surface actuators for maintenance purposes. The actuators must have a certain speed of actuation. Helicopter Rotor Analysis The transducers are used on helicopters to measure the pitch angle of the rotor blades. Engine housing measurement Transducers are used to measure the movement of engine housings as they heat up to test the design.

34 Applications Aerospace 2 Flight simulators We have transducers installed on flight simulators to measure the position of operator controls. Turbine blade measurement Normally sampled at 50+ points on their surface. Engine nozzle aperture control. Transducers used to measure the aperture movements of the engine thrust aperture. Windscreen tests Our transducers have been used to measure the deflection of aircraft windscreens during a simulated impact with a bird. Wing tests Testing of aircraft wings to measure deflection vs. load. Transducers are used to measure the position of multiple points on the wings and control surfaces.

35 Applications Civil / Structural Engineering STI transducers have been used to measure the propagation of cracks in tunnels, dams and buildings. STI sensors have been used to measure movement in bridges due to traffic flow and wind. We have applications to measure the deflection of the floors of buildings when tests are carried out prior to the installation of large pieces of equipment. When lifting large structures (such as bridges to replace the bearings) it is necessary to ensure an even lift. LVDT transducers are ideal for this. Soil tests are often carried out in the lab, prior to building on land. A sample of the ground is subjected to compressive tests, LVDTs are ideal for measuring the position.

36 Applications Sub-Sea Cable Installation Submersible displacement transducers are used to measure angle and position of various parts.

37 Applications- Soil sample testing Submersible displacement transducers Measure the Soil Expansion as water is added.

38 Applications Automotive Engineering Body torsion measurement. Suspension component testing. Gauging of precision manufactured components. Set-up of engine components such as the diesel injectors. Testing of seats, doors, pedals and handles etc. for simulation of extended life. Vehicle testing. Our transducers are used in vehicle safety test centers that measure the effectiveness of the brakes. Windscreen profile measurement to test that windows are the correct size prior to installation. Numerous special purpose machines for component manufacture and testing.

39 Applications Power Generation Turbine run-out. Our transducers have been used to measure the position of turbines to ensure that the bearings are not wearing. They need to be PY sensors as the environment is very harsh. Main steam valve position. The main steam valve controls the flow of steam into the turbine. It constantly adjusts to maintain a constant rotation speed on the turbine. The LVDT is ideally suited to the high temperatures, dirt and continual movement that this application presents. Bypass valves. When the bypass valve opens, the transducer can experience a temperature of 200 C.

40 Custom Engineered Solutions