2-Wire and 4-Wire Transmitters and Control Loops

Digital Meters

Industrial process control transmitters commonly provide analog signals, such as 4-20 milliamps (mA), 1-5 volts DC, and 10-50 milliamps as outputs which can be scaled to the control range of the process variable they are sensing. Industrial transmitters are either “4-wire” or “2-wire” which defines how the transmitter gets its supply or “excitation” voltage.

4-wire transmitters have four wires exiting the device. Two wires are for the supply power of the device, typically 120 volts AC, 240 volts AC or and external 24 volts DC supply. The other two wires provide the analog output signal provided by the transmitter circuitry.

2-wire, loop power example

2-wire transmitters have only two wires exiting the device and rely upon the control “loop” for the excitation voltage - typically 24 volts DC which normally comes from the loop controller, PLC or DCS.

4-wire devices are also classified as “active” (supplying power) devices, while 2-wire devices are classified as “passive” (loop powered) devices.

For example, a digital meter (active) may provide loop power to a pressure transmitter. The pressure transmitter regulates the current on the loop to send the signal back to the digital meter, but since the transmitter does not provide power to the control loop, it is deemed passive. In another example, a passive (loop powered) digital meter and a passive pressure transmitter may be used in the same loop, but uses a 24 V battery as the active device to power the loop.

Industrial Electric Immersion Heaters

Screw plug and flanged
immersion heaters
(courtesy of Durex)

Electric immersion heaters are used in a myriad of industrial applications. From drying industrial gasses, to freeze protecting cooling tower sumps, to heating acids in plating applications, the versatility of electric heating element can save time, energy and space.

Industrial immersion heaters are used to directly heat a standing or moving fluid by using electric heating elements. There are three primary types of industrial electric immersion heaters; screw-plug heaters, flanged immersion heater, and over-the-side heaters.

At the heart of industrial immersion heaters are the individual heating elements, normally constructed from a stainless steel or Inconel tube containing a magnesium oxide filler and a nichrome resistance wire. Current is applied to the wire which produces the heat, while the compacted magnesium oxide powder provides the electrical insulation, and the metallic tube provides the physical protection.

Custom Electric Heaters for Unique Thermal Systems Require the Right Thermal System Partner

OEMs in the analytical, semiconductor, biomedical, life-science, food service and environmental industries continually design new pieces of equipment offering their customers greater efficiencies, smaller foot prints and greater production rates. When the piece of OEM equipment requires precise heating, consultation with an experienced thermal systems engineer will provide significant time savings and budget control.

More specifically, working with an experienced thermal system consultant provides these important benefits: front-end, practical design review to optimize manufacturability; timely prototype development; partnerships and alliances with platers, brazers, casters and heating element manufacturers; single source responsibility; testing and calibration; inventory management; value-added assembly; and cleaning and packaging.

Aluminum Nitride Ceramic Heaters Open Doors to Better Machine Design

Aluminum Ceramic Heaters
(Courtesy of Durex)

Aluminum Nitride (AlN) Ceramic heaters are a relatively new entry in the very high watt density heater market and are an attractive alternative to traditional metal sheathed heaters. Capable of achieving up to 2000 watts per square inch, and operating temperatures of up to 1000 deg. C, these heaters show great promise for semiconductor processing applications such as crucible heating, fluid and gas handling and chemical vapor deposition. 

The heaters are made by "tracing" a resistance material (Tungsten) on a the ceramic base at various thicknesses, corresponding to the performance requirements of the heater. The Tungsten and AIN expand and contract at very similar rates, which greatly reduces the mechanical concerns of delamination. Binders and trace additives are added to the ceramic and Tungsten for additional strength. The resulting construction allows for some pretty impressive thermal cycling - one example is an application with a 200 deg. C temperature swing every 30 seconds.

AIN ceramic heaters offer significant advantages over metal sheathed heaters and their inherent performance limitations. Material compatibility, fatigue, outgassing and thermal lag must be considered when applying metal sheath heaters. Ceramic heaters combine excellent thermal conductivity with outstanding chemical resistance, strength, inertness and design flexibility. Additionally, RTD sensors can be deposited right on the ceramic heater itself for optimum control. 

Capable of forming virtually any shape, along with their excellent mechanical, thermal, dielectric, chemical resistant and embedded sensors, Aluminum Nitride Ceramic heaters open the doors for engineers to design equipment to new levels of performance.

Mass Flow Controller Calibration, Accuracy Check and Maintenance Solution

An innovative solution to solving the high cost and downtime caused when servicing mass flow controllers (MFC's).

Introduction to Programmable Logic Controllers (PLC)

Typical Modular PLC

Programmable logic controls, or PLC's, are used for plant automation and control. The PLC is a specialized, industrial computer which includes onboard random access memory (RAM) and read only memory (ROM). As with any other computer, the PLC has a central processor unit (CPU) for data processing. A single PLC has the switching and logic capability to replace thousands of control relays. PLC's are ubiquitous and are used in many different applications in all industries including semiconductor manufacturing, pharmaceutical production, chemical processing, food production, primary metals, and HVAC. Because of their wide industry use, they are manufactured in many shapes and sizes.

Basic Concepts of PID Control

PID control loop diagram
(courtesy of Wikipedia)

PID is short for "proportional plus integral and derivative control", the three actions used in managing a control loop. Process loop controllers use one, two or all three of these to optimally control the process system. PID control is used in a wide variety of applications in industrial control and process system management.

Many types of PID controllers exist on the market and are used for controlling temperature, pressure, flow, and just about every other process variable. Here is a brief explanation of the three actions that make up PID.

Proportional Control Action (P):  The controller output responds in proportion to error signal. The characteristic equation for this action is:

• Where, Kp is called proportional gain, e is the error magnitude and B is the output from controller when there is no error. It is also called bias. 
• In a proportional controller, the value of gain is set as required by the process and can be varied from 0 to ∞. 

Integral Control Action (I): The control system will respond if the error is present over a period of time. This type of control action is called Integral Control Action. The integral action is defined mathematically as:

• Where, e= error, Ti= Time interval of integral action.
• Purpose of integral action is to provide adequate control action on varying demands of process. In this type of action, output varies as per the time integral of error. This action does not exist independently and always associated with proportional control. 

Derivative Control Action (D): To achieve a stable process, wide proportional band and low integral action are set. Due to these settings, the control system can be too slow. If large system disturbances occur over a wide interval, PI controllers are inadequate. These large system disturbances can be managed if the controller output responds not only to the magnitude of deviation, but also to the rate of change of deviation. Derivative control action is that control action. 

Today's loop controllers are much easier to set the PID, thanks to auto-tuning algorithms. What used to be a very time consuming and tedious job can now be done with the push of a button and allowing the controller to "learn" the process dynamics. PID controllers minimize error and optimize the accuracy of any process.