Production process monitoring and control for a variety of automation instrument, computer and the corresponding actuator. In the process of both the weak signals to the millivolt level of small signal, and dozens of v large signal, and even up to several kilovolts, hundreds of ampere of signals to deal with.

Tell from the frequency, a dc low-frequency range, there is also a high frequency/pulse peak. Mutual interference between the equipment instrument became a must to solve the problems in the system debugging.

Besides electromagnetic shielding, solve various equipment instrument "to", or point of reference for the signal potential difference, will become the important subject.

Because of the different equipment, instrument signal to send each other each other, there is a signal that reference point. In other words, to make a complete transmission signal, idealized situation is all the equipment instrument signal to have a common reference point, namely a "ground".

Further, all equipment instrument signal potential difference between the reference point of "zero". But in a real environment, it is almost impossible, in addition to the various equipment inside this instrument "and" the connection between the resulting voltage drop is resistance, there are all kinds of instrument equipment in different environment of disturbance, and wire contact withstand wind rain led to the decrease of the quality of the contact, and many other factors, there are differences in between the "ground".

In diagram 1, for example.

Shown in figure, there are two field device 1 #, 2 # instrument signal to PLC and PLC to two sets of equipment on site 3 #, 4 # instrument signal. Signals are assumed to be the 0 to 10 VDC. Ideally PLC and two field device 1 #, 2 # instrument "and" potential fully equal, and no interference in the process of transmission.

So from the point of PLC input, receives the correct.

But as I mentioned earlier, the two field devices usually have a "to" potential difference.

Device, for example, 1 # "to" "to" with potential with PLC, 2 # device is higher than their "and" potential, so 1 # device to the PLC signal is 0-10 v, and 2 # equipment to PLC, the error is generated. At the same time of 1 #, 2 # equipment "to" line in the PLC rendezvous, would impose on the PLC to line voltage, may damage the PLC local "to" line.

At the same time shows the wrong data. The resulting problems appear frequently in the field debugging.

For example, some large building materials company production line PLC monitoring system using the AB - external hand operator a domestic manufacturer. AB - PLC of each data acquisition boards is composed of eight channels, eight channel Shared A 12 bit A/D, analog after transformation by 12 light coupling isolator with host isolation. No isolation between its eight channel input, that when the input signal, each channel separately entered into the acquisition boards are normal. But at the same time two or more than two external input signal, show jumping about digital fault cannot be ruled out.

And like a space a department use K type thermocouple as each point temperature sensor test engine. With the above similar, only test a point is normal. But to the host access when two or more than two temperature signals, display the temperature value of obvious errors. In both cases after the use of isolator, normal.

Isolator can play the role, is that it has the completely isolated input/output on the electrical characteristics. In other words, there is no common between input/output "to", external signal either 0 to 10 v, or with the common-mode interference voltage of 0 to 10 v after isolation of 0 to 10 v.

The newly established after isolation "ground" and external devices and instruments "to" it doesn't matter. It is for this reason, also has realized the input to the PLC host multiple isolation between external equipment instrument signal, that is not "to" relationship between them.

Talked above the input signal and the PLC signal isolation, same PLC to the external device output signal has a similar problem. Obviously using isolator can solve a problem.

This kind of voltage isolator and voltage/current/voltage isolation converter product model is FZ1521, FZ1522.

1. No matter the PLC sends a signal to the external equipment instrument and external equipment instrument sends a signal to the other devices often encountered in the case of a:

Require a signal can be sending signals to display instrument, and transmitted to the equipment such as frequency converter.

This may cause interference between the two devices, to thoroughly solve the problem of interference, it is recommended to use isolation type signal distributor, it is between two output isolation.

It can realize the input signal and the external device isolation, isolation between received signal equipment at the same time.

As shown in figure 2.

2. The field instruments in supporting that may coordinate adverse consisted of received signal devices (such as receiving 4-20 ma) interface for two wire connection way (that is, the receiver mouth for a 24 v power supply in series with a 250 Ω), the two wires of the interface for 24 v battery anode, a 250 Ω end, suitable for connecting the scene two wire transducer.

But the scene equipment for four wire transducer, 4-20 ma output, this direct connection will cause the power conflict. Solution is to adopt the isolator to the scene to 4-20 ma receives and isolation, the output part of the isolator installed a standard second-line transmitter, in response to receiving device interface. This product model for FZ9030.

Isolator to ensure the two parts of the input/output isolation, work with 24 v power supply for input, output, part of the power supply at the same time, must ensure that with two parts on electrical isolation.

The input/output/plus power between all isolated device called three isolated or full device. In theory, the way of power supply and no matter how much number of isolator are available a 24 v power supply, will not produce interference.

3. Commonly used for 4-20 ma to 4-20 ma current signal isolation, here recommend a kind of don't need to add other power isolator WFZ1562.

As shown in figure 3.

Clearly without external power supply connection is more simple, and low power consumption, low heat, high reliability.

FZ1562 is the biggest characteristic of don't need an external power source, it brings the advantages of simple and reliable at the same time also brought the limitations of use. WS1562 isolation for 4-20 ma signal transmission, in a sense is transmitted power, internal must have a power loss. Loss on the input and output current/voltage differential value of product. Load resistance RL = 250 Ω, for example, when the output is 20 ma output voltage of 250 Ω, and ends at the input voltage for testing. Simple calculation shows that the internal loss is equal to 20 ma x () = 76 (mW), which is the internal loss for 76 milliwatts. From the Angle of users, if the output load resistor RL is equal to 250 Ω, then from the input into the equivalent resistance of a maximum of (Ω). In this case the input 4-20 ma current source must have driven 440 Ω load ability, ability makes WS1562 passive isolator on the output side load resistance RL = 250 Ω under the condition of normal work.

However, from experience to see most of the field instrument can meet these conditions.

4. Talk about the commonly used two wire transducer (including pressure, temperature, flow rate...). , from the perspective of isolation can be divided into isolation type and the isolation type.

Use isolation type two wire transducer's main purpose is to improve the anti-interference ability.

Two wire transmitter isolation has two ways: one way is sensor and transmitter and designated place must be placed on the site, in this case the general settled isolator in the central control room in the cabinet. To this kind of scene two wire transducer power distribution form, there are two kinds of interface according to the specific site conditions.

Figure 4 shows the for PLC and two wire transducer two interfaces of a chart.

Another way is divided into two parts, sensor and transmitter specified location sensors placed on the site, the transmitter is designed for isolation type is placed in the control room. In the face of PLC two interface mode, figure 5 shows the Pt100 as sensor isolation transmitter using a chart. Product model for FZ9050 and FZ2050.

With said that such processing Pt100 temperature transmitter is considered Pt100 long-term compensation and linearization. Similar and isolation transmitter for a variety of thermocouple sensor, it is the same as the above basic. The difference is increased the cold junction compensation. Product model for FZ9060 and FZ2060. These FZ2050 and FZ2060 belong to two wire transducer. FZ9050 and FZ9060 belong to the whole isolation transmitter.

If the input signal is 5 a alternating current or hundreds of volts ac voltage can choose the product model is FZM -i, FZM -u.

In short, as long as the external equipment and determines the receiving equipment way, choose is simple.

Isolator appearance using guide installation, wiring USES the connection type, this way also called isolation terminal, apply is installed in the cabinet.

Installed in isolated front terminal circuit stmicroelectronics (ST) ASIC chip (ASIC) circuit, realize the temperature segregation transmitting, although the parts packaged (such as with cheap OP amp) cost is high, but in terms of long-term stability, reliability, performance is finishing. Introducing ASIC chip quality is the basic guarantee for the stable and reliable isolation terminal, at the same time dedicated SIC chips on the function, such as long-term compensation, constant current drive, linear performance.

Increasingly miniaturized isolation terminal design, so the purpose of miniaturization is less occupy a space. Of course should be installed allows the user to intensive, intensive installed on heat dissipation problems. In other words, it is necessary to reduce the power consumption of the interior.

Now there appear a lot of CPU as the core of market segregation terminal, with field programmable functions and communication functions, has high flexibility, for customers to reduce the amount of inventory, reduce the backlog of funds. CPU as the core of the isolation terminal will certainly become a mainstream in the field.