Central issue:
Problems and solutions in industrial field testing
solution:
Electrical isolation solves the problem of ground loop
Use enhanced isolation signal conditioning
Effective noise suppression measures in signal conditioning systems
In industrial field testing, we often encounter the following five problems:
Ground loop
The “ground loop†is a very difficult problem for engineers and technicians, and the resulting fault is difficult to find and is a waste of time. Possible failures are:
Although the sensor has not changed, the meter reading is slowly drifting;
When the other equipment is connected, the reading of the meter drifts;
The verification device is connected to the end of the instrument cable and directly connected to the input end of the instrument, and the two measurement results are different;
A 50Hz/60Hz sine wave is superimposed on the DC input signal;
There are some faulty measurement equipment problems that are difficult to explain.
All of these problems may be caused by "ground loops." Some "ground", "common point" and "reference point" that appear to be equipotential on the surface actually have potential differences. When they form a path, there is a current flowing through the path due to the potential difference, which is the "ground loop." All of the above problems can be solved by "isolation."
Sometimes the two devices are individually grounded to cause a potential difference at the ground point, causing current to flow through the signal line. Why is this problem caused by both devices being grounded? This is because the earth and the metal casing are actually poor conductors of electricity compared to the copper wires that transmit electrical energy and signals. The internal impedance characteristics that block the flow of current vary with seasons and weather, and allow current to flow through the connections. Any wire for these two devices. Many factories and workshops have potentials of tens or even hundreds of volts. With proper signal conditioning – “electrical isolationâ€, ground loops can be eliminated, effectively preventing potential problems before accessing sensitive test systems. The pressure is damaged, thus protecting the device. The isolator provides a complete floating input and output port with no electrical path from the field input to the output and from the field input to the power supply, so there is no current path and there is no ground loop.
How can a path be provided to pass signals from the input to the output without forming a current path? The method of "magnetic isolation" can be adopted, and the signal generated by the transformer, the input and output signals are no longer connected by a circuit but by a magnetic circuit, so that the electrical path can be eliminated. The transformer is used to accurately and reliably isolate the low-level signal, and the modulator and demodulator are used to transmit the signals at both ends of the transformer isolation medium, and the isolation voltage RMS is up to 4000Vrms.
Sometimes we measure small signals from thermocouples or other sensors at ground potentials up to several hundred volts. This ground potential is called "common mode voltage." A high-quality signal conditioner can suppress the error caused by the "common mode voltage" while still accurately amplifying the signal. This ability is called "common mode rejection (CMR)". The 5B signal conditioning module has sufficient common mode rejection to reduce the effects of common mode voltage.
2. Wrong wiring and overvoltage
Think about it, what happens when you connect a cable from a sensitive data acquisition card to another cabinet or other part of the plant at an industrial site? Input and output terminals are mixed in hundreds of terminals that connect signals of various sizes and properties: DC signals, AC signals, millivolt signals, thermocouples, DC power supplies, AC power supplies, proximity switches, Relay circuit, etc. It is not difficult to imagine that even a well-trained technician or electrician may be misplaced. When the system needs to be modified, the wiring diagram needs to be modified in time; sometimes the power supply fails, and the excessive voltage is unintentionally added to the system. How can we protect the measurement system?
The answer is: Enhanced isolation signal conditioning is used on the pins of each analog signal. This inexpensive insurance measure prevents problems with each input and output signal line due to misconnection or overvoltage. For example, when the input circuit is used to measure millivolt-level thermocouple signals, the SCM5B series of signal conditioners can provide 240V AC protection. In other words, you can connect 240V AC voltage across the input line of the thermocouple without any damage to the device. Signal conditioning and field I/O connections are used on the system side to protect all measurement and data acquisition equipment in the system.
3. Reduced resolution
Resolution is the smallest change that an analog-to-digital conversion (ADC) system can detect and respond to. There are two ways to increase the resolution to measure even smaller changes: "use a higher resolution ADC" or "reduce the measurement range", for example, if you know that most of the time the measured temperature is around 100 °C, then you A thermocouple signal conditioner with a more accurate measurement range can be customized. A signal conditioner with a temperature measurement range of 50~150°C will greatly improve the resolution when the measurement range is 0~1200°C.
4. Multiple signals have different characteristics
In the traditional measurement method, 4, 8, or 16 input signals need to be connected to the same type of signal interface. For example, if you need to measure 2 J-type thermocouples, 1 0~10V signal, 4 4~20mA signals, and 2 platinum thermal resistance (RTD), you need to buy a transmitter for each channel, and then They are connected to a 4-20 mA public input board. It is now possible to use the 5B signal conditioning scheme, that is, to configure corresponding signal conditioning modules for each channel, and these modules are collectively placed on one carrier board. These carriers provide all the terminals for connection to the input, output and field devices. The following outputs are available: 0~5V, 0~10V, 4~20mA, RS-232/485, etc. Various types of conditioning modules can be mixed and installed on the same carrier board, which is convenient to use and can be hot swapped.
5. Electromagnetic interference
There are a variety of sources of interference in modern factories and workshops: engines, electric motors, fluorescent lights, radios, generators, etc. Each source of interference radiates electromagnetic noise that can be accepted by the line, board, and measurement module. Even with the best shielding and grounding measures, these disturbances are manifested in the form of noise in the signal measurement. How to eliminate these interferences? These interferences can be eliminated by using effective noise suppression measures in the signal conditioning system.
Low-frequency noise can be filtered out by using a signal conditioning system with common mode rejection and constant mode rejection. When measuring positive or negative input signals on the common side, both the positive input and the negative input have common mode noise. The difference between the positive input noise of the positive input and the negative input is the normal mode noise. In the signal conditioning subsystem, the typical common mode rejection index is 160 dB. This logarithmic proportional relationship means that the effect of common mode voltage noise on the measurement is attenuated by 108:1 relative to the signal.
Ultra-high frequency noise in the RF band causes DC offset due to rectification, which requires other methods to eliminate noise, including the use of special wiring designs and the use of RFI filters (such as ferrite). Performance testing is required for electromagnetic sensitivity EN certification requirements published in accordance with the requirements of the European Community CE Marking. A typical application that is important is the use of transceiver radios within a few feet of the input line and signal conditioning subsystem. The signal conditioning must be capable of suppressing measurement errors during wireless signal transmission. Proper circuit board routing and signal conditioning measures ensure maximum accuracy in noisy environments.
Special Note:
1. Avoid installing sensitive measuring instruments or arranging lines that carry low-level signals near electromagnetic noise sources such as circuit breakers, transformers, motors, thyristor drivers, welding machines, fluorescent controllers or relays.
2. Use 10~12 twist/ft twisted pair to reduce magnetic noise interference.
3. Use a shielded cable and connect its shield to the circuit common on the input.
4. Do not route the signal-carrying line with the power line, relay leads, and other high-voltage or high-current cables in the same jacket.
5. In an environment where interference is severe, place the signal conditioning circuit and measurement equipment in a grounded and enclosed shielded room.
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