Core: LVDT

A TMA analyser will need to measure accurately both the temperature of the sample, and very small movements of a probe in contact with the surface of the sample. A typical analyser, as illustrated in Figure 11.20(a) and (b), uses a quartz probe containing a thermocouple for temperature measurement, and is coupled to the core of a linear variable differential transformer (LVDT). Small movements at the sample surface are transmitted to the core of the LVDT and converted into an electrical signal. In this way samples ranging from a few microns to centimetre thicknesses may be studied with sensitivity to movements of a few microns. For studying different mechanical properties the detailed construction of the probe will vary as is illustrated in Figure 11.20(c). 

Bourdon tube) is reflected in the position of the ferromagnetic core of the LVDT and is thus a function of the difference between the input and output voltages measured in terms of phase shift and amplitude ratio (see Section 7.8.4). 

The extension valve shaft, or control rod, is made of a metal suitable for acting as the movable core of a transformer. Moving the extension between the primary and secondary windings of a transformer causes the inductance between the two windings to vary, thereby varying the output voltage proportional to the position of the valve or control rod extension. Figure 6 illustrates a valve whose position is indicated by an LVDT. If the open and shut position is all that is desired, two small secondary coils could be utilized at each end of the extension s travel. 

An LVDT is suitable for measuring relatively short travel distances at high accuracy, in the range of microns to several centimeters. It is usually cylindrical in shape and is provided with a core that moves into the center bore of the cylinder. A magnetostrictive position transducer can have as high a resolution as 1 /on and a range from less than 25 mm to over 20 m. Typical error is 0.01% of range. 

Figure 5.1 Example TG design. As the specimen changes weight, its tendency to rise or fall is detected by the LVDT (see section 7.3). A current through the coil on the counterbalance side exerts a force on the magnetic core which acts to return the balance pan to a null position. The current required to maintain this position is considered proportional to the mass change of the specimen.

Figure 7.5 Operating principle of an LVDT. The dotted line represents the output of the left secondary, the dot-dashed line is the output of the right secondary. The solid line is their sum. The root mean squared amplitude of the solid line represents the core position.

Linear variable differential transformer (LVDT, Fig. 2) is a device that produces voltage proportional to the position of a core rod inside a cylinder body. It measures displacement or a position of an object relative to some predefined zero location. On tablet presses, LVDTs are used to measure punch displacement and in-die thickness. They generally have very high precision and accuracy, but there are numerous practical concerns regarding improper mounting or maintenance of such transducers on tablet presses. 

The specimens, which were placed in the hole of the bar, were compressed by the piston against the platen. Displacement in the axial direction was measured by a suitably mounted linear voltage displacement transducer (LVDT) whose core was attached to the piston (Figure 2). The calibrated output potentials of the LVDT and load cell were monitored by a fast responding dual channel recorder. 

The specimen was enclosed in a bellows vessel with silicone oil. This vessel was placed in a pressure vessel filled with another silicone oil and it was heated by an electric heater through the pressure vessel and outer oil jacket. The outer silicone oil was pressurized by a piston and an actuator. Thus at a given temperature, we could apply a hydrostatic pressure to the specimen through the bellows vessel. The volume change of the specimen was calculated from the displacement of the bellows measured using an LVDT (linear variable differential transformer). Inside the transformer, a pin was connected to an iron core and the pin was pushed by the bellows. At a translation of the bellows, the core, which was pressurized in a stainless steel pipe, also moved and the position of the core was detected by an LVDT coil from the outside of the pipe. 

A schematic of a commercial TMA instrument is shown in Fig. 16.33. The instrument consists of a dimensionally stable (with ternperamre) sample holder and measuring probe, a programmable furnace, a linear variable displacement transducer (LVDT) to measure the change in length, a means of applying force (load) to the sample via the probe (core rod, push rod), and a temperature sensor (usually a thermocouple). 

A three-layer particleboard was hand-formed in a metallic container with dimensions 220 mm x 220 mm x 80 mm. The total percentages of board mass were 20% for the upper face layer, 62% for the core layer and 18% for the bottom face layer. Boards were then pressed in a laboratory scale hot-press, controlled by a computer and equipped with a displacement sensor (LVDT), a load cell, thermocouples and pressure transducers. The mat was pressed at 195°C for 2.8 min to produce a board with a target density of 650 kg/m and with 17 mm thickness. 

The micrometer and dial gauge are simple mechanical devices. In many cases, one would like to have a continuous record of the thickness measurement. The LVDT (linear variable differential transformer) provides an electrical signal that can be used to monitor the thickness on a recorder. The LVDT is a device in which the displacement of an iron core changes the inductive coupling between primary and secondary coils see Fig. 4.20. 

Movement of the core produces an AC output signal that reflects the amount and direction of movement. If an LVDT is used in thickness measurement, one has to check the effect of temperature on the accuracy because the extrudate is generally at an elevated temperature that may not be constant. The LVDT can be quite accurate it can measure to an accuracy of about 1 pm. 

With the advent of accurate adhesive strain data via the KGR-1 extensometer, it became possible to verify the skin-doubler equations in Figure 1. To do this another extensometer was needed to measure the adhesive shear strain at the doubler tip. This measured value could then be compared to the calculated value. The second extensometer is identified as the KGR-2 and is shown in Figure 5. As before, the adhesive shear movement, Aa, at the doubler tip is made to move the core of an LVDT. The voltage change is fed to a recorder, which plots a curve of load vs. adhesive strain at the doubler tip. Tests were run on a skin-doubler specimen and the... 

For TMA, the length of the sample and the changes in length that occur during heating are measured by a linear variable differential transformer (LVDT). The movement of the transformer core produces an electrical signal, sensitive to direction, and this signal is transmitted to the data system. 

The displacement transducer shown in Fig. 5 is generally one of two types. A linear-variable differential transformer (LVDT) is effectively a transformer where the coupling to the secondary winding(s) is a function of the position of a movable magnetic core. A capacitive displacement transducer (CDT) is a capacitor where the... 

One of the more popular devices for cryogenic use is the linear variable differential transformer (LVDT). The LVDT (see Fig. 8.3) is a transformer whose core moves in response to the displacement input. The primary is wound in an ordinary manner, but the secondary is split and wound in such a fashion that there will be no output when the core is exactly in the central position. As the core is displaced from the center, a voltage is generated across the terminals of the secondary, which is directly proportional to the displacement of the core. 

As indicated in Figure 13, the sample is placed in a heater or furnace surrounded by a coolant or heat sink and insulation. For the measurement of linear thermal expansivity the bottom of the probe is perfectly flat, and the probe acts merely as a push rod. Connected to the upper end of the probe is an Ivdt and some mechanism for maintaining the probe-lvdt system vertical. The sample is heated at a controlled or programmed rate, and its change in linear dimension is measured by the displacement of the Ivdt core. The sample probe and outer tube are made of a low thermal-expansion material, usually fused quartz, and are... 

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