Principle of Operation
The transmission system consists of the following components:
- Optical fiber
The transmitter modulates the infrared light emitted by the transmitter diode with the conditioned signals of the available measuring values. This (amplitude-) modulated light is transmitted via the optical fiber to the receiver which converts these optical signals back to voltages. The attenuation of the optical fiber is compensated automatically (AC versions) or semi-automatically by means of a compensation switch (DC versions). An insufficient optical level (violation of the upper or lower limit of the dynamic range) is detected and indicated.
In order to ensure convenient measuring operations in expanded equipment to be tested or in the case of transmitters which are difficult to reach, the units can be fitted with an optional remote start feature via a second optical fiber. Transmitters of this kind are switched from the extremely energy-saving standby mode to the measuring mode by an optical pulse telegram. Standby times of several years are possible.
Besides just transmitting voltage signals of passive sensors, shunts, or dividers, the transmitter units optionally can also be equipped with circuits to process signals from active sensors and to provide their supply voltage.
Acoustic Partial Discharge Sensors
In order to use Power Diagnostix' acoustic partial discharge sensors AS75I and AS150I on high voltage potential, the transmitter can be equipped with the switchable 15 V/28 V supply circuit. Usually, to ensure the desired sensitivity, this option is combined with an input range of 10/20/50/100mV.
Low-Impedance-Piezo Pressure Measurement
So-called low-impedance piezoelectric pressure sensors or voltage-mode sensors supply a voltage in proportion to the pressure if the FET amplifier in the sensor is supplied from a constant-current source. Such a current source is available in the form of an option for the FOS1. It supplies 2 mA with a no-load voltage of typically 18 V. With the usual sensors, an open-circuit voltage of approx. 11 V is then obtained during operation, with a range of ±5 V for the measuring range of the sensor. This open-circuit voltage requires AC coupling. The lower limit frequency of 0.2 Hz, which is then in effect, differentiates (!) unipolar pressure processes.
High-Impedance-Piezo Pressure Measurement
Charge-mode or high-impedance piezoelectric sensors which are, for instance, specifically manufactured by Kistler supply a charge which is in proportion to the derivative of the pressure (q~p') and hence require a charge amplifier (integrator). Such a charge amplifier can be permanently integrated as a pre-amplifier into a transmitter. The charge amplifier and the measuring ranges are adjusted to a special sensor. The measuring input is connected to virtual ground potential during operation. When the linear control range is exceeded, the measuring input is protected against overvoltage conditions of up to ±40 V - higher overvoltage can cause destruction of the FET amplifier. The charge amplifier has a fixed, invariable gain which is adjusted to the specific sensor used. In conjunction with the internal scaling factor of the measuring-range switch, it is then possible to select four distinct pressure measuring ranges. The charge amplifier is adjusted in line with the nominal sensitivity of the sensor (e. g. Kistler 6203: -2 pC/bar or Kistler 601: -16 pC/bar). Fine-tuning of the specified sensor sensitivity in relation to its actual sensitivity should be done using a correction factor in the downstream evaluation system in order to maintain the 'anonymity' of the transmission system.
The use of transmitters for the transmission of measuring signals of potentiometric rotary position transducers or position encoders requires slightly more complex equipment. In order to ensure the error-free transmission of the output voltage of these 'potentiometers', DC adjustment is necessary before the connection of the supply voltage for the position transducer. For this purpose, the sequencing control system activates the transmitter and performs the necessary adjustments. Transmitters fitted with the option 'Displacement measurement' come with a 5 V source for the supply of the position transducers. This supply voltage is activated only 45 s after activation of the transmitter (via the remote starting unit), so that DC adjustment is possible beforehand.