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AD598AD 데이터시트(PDF) 10 Page - Analog Devices |
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AD598AD 데이터시트(HTML) 10 Page - Analog Devices |
10 / 16 page PROVING RING-WEIGH SCALE Figure 20 shows an elastic member (steel proving ring) com- bined with an LVDT to provide a means of measuring very small loads. Figure 19 shows the electrical circuit details. The advantage of using a Proving Ring in combination with an LVDT is that no friction is involved between the core and the coils of the LVDT. This means that weights can be measured without confusion from frictional forces. This is especially im- portant for very low full-scale weight applications. EXC 1 EXC 2 LEV 1 LEV 2 FREQ 1 FREQ 2 B1 FILT B2 FILT OFFSET 1 OFFSET 2 SIG REF SIG OUT FEEDBACK OUT FILT A1 FILT A2 FILT AD598 C3 C4 SCHAEVITZ HR050 LVDT 6.8 µF 0.1 µF 0.1 µF 6.8 µF –15V SIGNAL REFERENCE 15V + –VS R L VOUT +VS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 15 17 18 19 20 1 µF 634k 10k 0.33 µF 0.1 µF C2 0.1 µF C1 0.015 µF VA VA VB VB Figure 19. Proving Ring-Weigh Scale Circuit FORCE/LOAD PROVING RING LVDT CORE Figure 20. Proving Ring-Weigh Scale Cross Section Although it is recognized that this type of measurement system may best be applied to weigh very small weights, this circuit was designed to give a full-scale output of 10 V for a 500 lb weight, using a Morehouse Instruments model 5BT Proving Ring. The LVDT is a Schaevitz type HR050 ( ±50 mil full scale). Although this LVDT provides ±50 mil full scale, the value of R2 was cal- culated for d = ±30 mil and V OUT equal to 10 V as in Step 9 of the design procedures. The 1 µF capacitor provides extra filtering, which reduces noise induced by mechanical vibrations. The other circuit values were calculated in the usual manner using the design procedures. This weigh-scale can be designed to measure tare weight simply by putting in an offset voltage by selecting either R3 or R4 (as shown in Figures 7 and 12). Tare weight is the weight of a con- tainer that is deducted from the gross weight to obtain the net weight. The value of R3 or R4 can be calculated using one of two sepa- rate methods. First, a potentiometer may be connected between Pins 18 and 19 of the AD598, with the wiper connected to –VSUPPLY. This gives a small offset of either polarity; and the value can be calculated using Step 10 of the design procedures. For a large offset in one direction, replace either R3 or R4 with a potentiometer with its wiper connected to –VSUPPLY. The resolution of this weigh-scale was checked by placing a 100 gram weight on the scale and observing the AD598 output sig- nal deflection on an oscilloscope. The deflection was 4.8 mV. The smallest signal deflection which could be measured on the oscilloscope was 450 µV which corresponds to a 10 gram weight. This 450 µV signal corresponds to an LVDT displace- ment of 1.32 microinches which is equivalent to one tenth of the wave length of blue light. The Proving Ring used in this circuit has a temperature coeffi- cient of 250 ppm/ °C due to Young’s Modulus of steel. By put- ting a resistor with a temperature coefficient in place of R2 it is possible to temperature compensate the weigh-scale. Since the steel of the Proving Ring gets softer at higher temperatures, the deflection for a given force is larger, so a resistor with a negative temperature coefficient is required. SYNCHRONOUS OPERATION OF MULTIPLE LVDTS In many applications, such as multiple gaging measurement, a large number of LVDTs are used in close physical proximity. If these LVDTs are operated at similar carrier frequencies, stray magnetic coupling could cause beat notes to be generated. The resulting beat notes would interfere with the accuracy of mea- surements made under these conditions. To avoid this situation all the LVDTs are operated synchronously. The circuit shown in Figure 21 has one master oscillator and any number of slaves. The master AD598 oscillator has its fre- quency and amplitude programmed in the usual manner via R1 and C2 using Steps 6 and 7 in the design procedures. The slave AD598s all have Pins 6 and 7 connected together to disable their internal oscillators. Pins 4 and 5 of each slave are con- nected to Pins 2 and 3 of the master via 15 k Ω resistors, thus setting the amplitudes of the slaves equal to the amplitude of the master. If a different amplitude is required the 15 k Ω resistor values should be changed. Note that the amplitude scales lin- early with the resistor value. The 15 k Ω value was selected be- cause it matches the nominal value of resistors internal to the circuit. Tolerances of 20% between the slave amplitudes arise due to differing internal resistors values, but this does not affect the operation of the circuit. Note that each LVDT primary is driven from its own power am- plifier and thus the thermal load is shared between the AD598s. There is virtually no limit on the number of slaves in this circuit, since each slave presents a 30 k Ω load to the master AD598 power amplifier. For a very large number of slaves (say 100 or more) one may need to consider the maximum output current drawn from the master AD598 power amplifier. REV. A –10– AD598–Applications |
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