Signal conditioners ensure signal integrity

Like a fuel injector in a high-performance engine, signal conditioners perform a critical, yet often under-recognized function. Signal conditioners isolate, transmit and convert a wide range of process signals into a format that is compatible with the receiving device such as a programmable logic controller.

04/01/2010


 

 

Like a fuel injector in a high-performance engine, signal conditioners perform a critical, yet often under-recognized function. Signal conditioners isolate, transmit and convert a wide range of process signals into a format that is compatible with the receiving device such as a programmable logic controller.

 

As the number and variety of interconnected sensors and control devices continue to grow, plant floor engineers face increasing demands to guard against the many electrical interfacing problems that can occur. Signal conditioners can help resolve these issues.

 

Employed wherever temperature, pressure, flow, speed or other process values are measured and controlled, signal conditioners help protect these measurements from being corrupted as they transmit the measurement data from the field where the sensors collect data to the plant control room.

 

The latest advances in signal conditioner technology include more rugged, robust devices. These conditioners provide more functionality in smaller packages, giving users greater design and integration flexibility while helping reduce cabling and maintenance costs.

 

Practical applications

A signal conditioner’s primary function is to isolate measurement and control signals. Such isolation prevents interference that can occur due to ground loops or common mode noise. A ground loop occurs when more than one ground connection is made to a single control signal.

 

In such a case, a difference in potential of the Earth ground at each location can create an additional and unpredictable amount of current in the loop, which distorts the measurement. Signal conditioners break the ground loop current path and maintain the integrity of the measurement.

 

Often the signals from the sensors and transducers are small voltages. As such, they are susceptible to noise pickup from motors, variable speed drives, welding equipment and general switching spikes carried by a plant’s electrical system.

 

If there is no isolation between input and output, this common mode noise will produce unacceptable changes in the output current or voltage. Applying signal isolation helps prevent the noise from passing to the output, even if it is relatively high in magnitude.

 

Many industrial applications such as oil and gas or chemical production often require process measurement signals to be accurately transmitted over long distances. Instead of running expensive and noise-sensitive thermocouple or RTD cable to the control system, signal conditioners can isolate and convert to a higher-level 4-20 mA current signal that can be transmitted over standard copper conductors.

 

Standard copper wiring (typically shielded, twisted pair) is less expensive than specialized wiring such as thermocouple lead wire. Also, the 4-20 mA signal is more noise-tolerant than thermocouple signals, which are millivolt levels.

 

Signal conditioners also allow for the low-cost addition of an analog signal in an established process that has limited analog I/O expansion capability.

 

Rather than buying another I/O card, plant floor managers can use external signal conditioning modules to convert an analog signal into a format that can be used on an open channel in an existing I/O module in their control system.

 

Also, by sticking to one multi-channel input card type, users may be able to extend the performance of their legacy control systems and cut the cost of stocking spares for each module type.

 

Additionally, many small PLCs offer only a 0-10 Vdc analog input on the base unit. Signal conditioners can be used to convert the more common 4-20 mA analog signals into a 0-10 Vdc signal, allowing for cost-effective expansions to a system that uses small PLCs.

 

Another core function of signal conditioners is signal splitting, in which the conditioner takes one signal input and provides two identical outputs. All inputs, outputs and power are isolated from one another.

 

The splitter approach allows managers to disconnect either control system for maintenance without affecting the signal going to the other system.

 

Signal splitting is a common tactic in applications where the user needs to monitor the input from a field transmitter at two locations simultaneously.

 

Here, a dual-output signal conditioner takes the input from one transmitter and sends two identical isolated outputs to two receiving devices for monitoring, recording and control. Users also employ this technique for isolating validated systems from non-validated systems in the bio-pharmaceutical market.

 

Typical symptoms that indicate the need for signal conditioning include:

 

  • A transmitter or some other piece of equipment works perfectly well on the test bench but will not work when installed in the plant

  • The readings received from a transmitter in the field change noticeably when another seemingly unrelated piece of equipment is connected to the system

  • Two pieces of equipment have different grounding requirements and cannot be interconnected directly

  • The measurement into a receiving system varies unexpectedly when nearby electrical devices are operated.

    • Selection criteria

      Even with the wide variety of analog I/O available in today’s modern process control systems, signal conditioners continue to deliver valuable problem-solving capabilities. Modules are available for most types of analog signals, but selection criteria beyond signal types must first be considered. Users must be careful to match the response time to the application, and make sure the signal conditioner can handle the sensor load (impedance).

      Signal conditioners with two-way isolation separate the input and output signals from each other electrically and decouple the measuring circuits. This eliminates differences in potential caused by long-line lengths and common reference points. The electrical separation also protects against irreparable damage caused by overvoltages.

      Signal conditioners with three-way isolation also separate the supply voltage from the input and output circuits and enable the analog circuit to operate with one operating voltage.

      Signal conditioners with passive isolation such as input-loop-powered or output-loop-powered devices offer an additional advantage: They don’t require an additional power supply. The input or output circuit can provide the power supply to the signal conditioner.

      Accurate signals are integral for keeping plant operations running efficiently and without incident. Signal conditioners are cost-effective tools for safeguarding signals and maintaining uptime. A variety of signal conditioner models and capabilities can seamlessly fit the particular needs of an existing control system.

      Though often invisible, signal conditioners provide a critical link for analog signal transmission in new and existing installations.

       





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