Source: Thermal Control Circle
This article briefly introduces the principles and functions of the VM preprocessor (taking IPC704 as an example) to increase our understanding of the preprocessor.
The preamplifier is located between the TSI system's in-place probe and the cabinet fixture, and can be referred to as a signal conditioner, filtering and regulating the signal measured from the probe and transmitting it to the system for further processing.
1. Characteristics (characteristics):
(1) A high-pass filter and a low-pass filter are configured to filter out signals of abnormal frequencies and ensure that the system receives signals in the normal frequency range.
(2) It is divided into two-wire current output or three-wire voltage output, and the watt vibration signal of our unit usually adopts two-wire current output.
2. Brief introduction of principle:
The preamplifier converts the raw signal measured from the probe into a current or voltage signal that is further transmitted to the system via a two- or three-wire cable for further processing, the preamplifier is equipped with high-pass and low-pass filters, and an optional integrator to output a velocity signal, in addition, an RFI filter protects the input and output signals from RF interference and other electromagnetic interference.
The schematic diagram of the internal signal conditioning of the preamplifier is as follows:
其中，Symmetrical chargeamp：对称电荷放大器；HP and LP filters：高通和低通滤波器
Amplifier or integrator：放大器或积分器；Outputstage：输出级
(1) Preamplifier power supply requirements: voltage: 18-30V; Current: <25mA
(2) Transmission characteristics: The phase relationship between the input signal and several output signals is shown as follows
(3) Output characteristics:
(a) Two-wire current transmission:
基准电流：12 mA ± 0.5 mA；电气连接：+24 V =“+”, COM = “−”；
Maximum variation range: 5 mA peak/output sensitivity
(b) Three-wire voltage transmission:
基准电压：7.5 V ± 0.2 V；输出阻抗：750Ω（3线配置）;
Maximum variation range: 5 V peak/output sensitivity
Note: Three-wire voltage transmission without galvanic isolation is only suitable for sensors that are not sensitive to frame voltage, dynamic pressure sensors should be used in conjunction with GSI galvanic isolation.
(4) Filtration characteristics:
高通滤波器起始频率：0.5,1, 2, 5 or 10 Hz (all ± 20 %)
低通滤波器截止频率：200,500, 1 000, 2 000, 5 000, 10 000 or 20 000 Hz (all ± 10 %)
The diagram below illustrates the high-pass filter settings for different start frequencies.
(5) Electrical Connection:
The diagram below illustrates the respective electrical connections for the current output and voltage output of the preamplifier:
Source: SAIC Automation Center