Safety barrier (isolator) signal and field connection
The safety barrier (isolator) and the signal connected to the field are isolated from the grid and control system, and are self-contained and floating with the ground. Even if it is connected to the ground potential or other unequal potentials, it will only float at this potential, and there will be no loop and current generation between the two, which will not affect normal operation.
The signals connected to the safety barrier (isolator) and the field are mainly voltage, current, millivolt, and resistance.
Voltage input signal, the circuit is high impedance; current input signal, the circuit is low impedance; no special requirements are imposed on the connecting wire.
The temperature of the cold junction of the thermocouple is to compensate the temperature of the end of the wire. When the temperature of the compensation component is consistent with the temperature of the end of the compensation wire, the temperature difference between the hot and cold ends of the thermocouple can be effectively compensated, and the absolute temperature of the hot end of the thermocouple is correctly converted to the millivolt value.
The thermal resistance is connected from the field to the input end of the converter in a three-wire manner. The wire diameter, material and length of the three wires are exactly the same, which can effectively offset the conversion error caused by the lead resistance.
Output signal is connected to the system
The safety gate (isolator) standard output signal 4~20mA can be accurately transmitted from a long distance, but when connecting with the system AI card, it is necessary to pay attention to the sampling method of the card to 4~20mA signal and the ground connection mode.
Unconnected two-wire connection
The sampling resistor inside the AI channel (such as 50Ω in Figure 2) uses a 4-wire sampling method, using two lines to form a current loop with the output of the external barrier (isolator), and the other two are internal and AI channel amplifiers. Form a signal voltage loop. No current is used in the voltage loop, so the 4~20mA signal current does not flow into the signal ground of the AI channel. This method can simply separate the signal transmission from the signal sampling without increasing the ground potential of the AI channel. The internal signal grounds of multiple AI channels may be common or may be separated. There is no specific requirement as to whether the input signal of the AI channel is differential.
The feature of the non-common two-wire connection is that the output of each safety barrier (isolator) forms a separate loop with the AI channel, and the signal current does not flow into the signal ground of the AI. Therefore, no ground potential error is generated between the multiple signals. This connection method requires that the output signals of each channel must be isolated; the safety barrier (isolator) itself has the path isolation property, which can easily meet this requirement.
Common ground connection
The common ground single-wire connection is characterized in that each output of the safety barrier (isolator) is connected to the AI channel only by one wire, and the ground wire is common. When used in batches, the wiring inside the disk can be reduced, as shown in Fig. 2. However, since all 4~20mA signal anodes are connected together, the ground line resistance will rise to the ground potential in addition to the external ground. In addition to increasing the copper bar to reduce the ground resistance, the input signal of the AI channel is required to be differential. of. This in turn increases construction costs and difficulty, offsetting the benefits of reducing on-board wiring. This method is suitable for the connection between the non-isolated multi-channel capture card and the system between the outputs, so it is not recommended to use it in the connection of the barrier (isolator) to the system.