2263Addressable Smart Devices and Wiring

Also called 'smart' devices, these devices can be controlled directly by a digital control system, by giving the device a unique identification code or "address" to differentiate it from all other similar devices. Usually each addressable device has a means of setting up its own unique address or name, to permit uniqueness in communication. Addressable devices relay their information or accept their commands digitally, and do not rely on transmitters or the like. Addressable devices have the distinct advantage of reduced wiring, since a single loop of communication trunk wiring can be shared by multiple devices; on systems with large number of points, the difference can be remarkable. However, addressable devices cost more than conventional (non-addressable) devices, since they include on-board communication hardware, as well as A/D and D/A converters for addressable analog devices. Since they cost more, their application requires balancing the cost vs. the benefits. Of the devices listed below, the ones currently most popular are the simple contact closure input/output devices used in large quantities for security, fire alarm, and lighting control systems, where the economies of scale show an advantage. The single loop communication can present new failure modes compared to conventional home run wiring methods; i.e. if a single cable is cut how many devices are affected?

Common Addressable Devices:

• Fire alarm devices—input and output

• Lighting control devices—input and output

• Security devices—input and output

Other Addressable Devices:

Lighting ballast

• Transmitters and sensors

• Thermostats

22.6.4 Linearization

"For simplicity of design, a linear relationship between input and output is highly desirable." [3: pp 28]

Linearity is a high priority in instrumentation, for both inputs and outputs. This is because the controller's algorithms or ratios are arithmetic in nature, comparing the input to a standard and producing a derived output. Linearization makes the effect of the controller's decisions predictable and manageable. Understanding how linearization can affect the control system is important to assure project success.

Input/Output instruments are considered linear when an incremental change of input value produces an equal increment of output value regardless of the value of the input or location of the device's range. For example, a change of 1 degree F might produce a change of 0.16mA in transmitter output. If truly linear, the device would produce this 0.16mA change in current if reading 0 to 1 degF, or if reading 100 to 101 degF. To the extent that non-linearities exist in the control loop, errors and unpredictability will also exist and so should be identified and minimized.

Many natural phenomena are linear, and many are not. Linear Examples include metal resistance with respect to temperature changes, static pressure with respect to depth of liquid, and volume of a vertical cylinder with respect to level. Non-Linear Examples include the volume of a conical container or a horizontal cylinder with respect to level, the change in flow with respect to butterfly valve or single blade damper position, and a heat exchanger heat transfer rate with respect to flow rate. Some natural phenomena behave in predictable, but non-linear fashion. These include thermocouples and differential pressure flow meters (head loss devices). Thermocouples are linearized by a look-up table or mathematical expression that defines the non-linearity, while head-loss meters are linearized with a square root extractor. Most common input measurements have already been linearized by the instrument manufacturers. Instead of hardware characterization, it is possible to linearize inputs and outputs through software. This "software linearization" is sometimes used with industrial controls, but seldom with commercial controls.

A common control issue with linearization is control valves and dampers. These are notorious for having nonlinear response with respect to travel position. Control valves are generally characterized as either linear, quick opening or equal percentage. For valves, the flow "character" is achieved by specially contouring the valve plug, to influence the flow rates at different valve stem positions. Characterized ball valves are also available, to greatly improve the inherent quick-opening flow pattern of these valve trim shapes. Control dampers and butterfly valves are either flat blade or flat disk shapes and do not have selectable characterized flow patterns like control valves do. In the case of control dampers, about all that can be done to reduce non-linear air flow through the damper is to down-size it to create a high pressure drop, which may create other complications or costs, especially in outside air ducts and large ducts without room for transitions. Where linear control of an air stream is important, air valves can be used which are available with characterized flow patterns.

Modulation of a heat exchange process is a common automatic control application. The following example applies for most types of heat exchanges including shell and tube, tube and fin, etc., and includes all HVAC air coils. The heat transfer characteristics of a heat exchanger can be likened to a 'quick opening valve' since the incremental change in heat transfer for the first fraction of fluid input is much higher than the last fraction. Controlling flow linearly through a heat exchanger will yield a non-linear output with associated control problems, especially tuning issues. In this case, the non-linear flow characteristic of a control valve is deliberately used to improve process control. The standard approach to correct this is to use a control valve with a flow characteristic that is a mirror image of the inherent heat exchanger performance (equal percentage type), canceling this inherent phenomenon so the overall control effect is nearly linear. This example not only illustrates how linearity is important to a control system, but also points out that for heat exchanger applications, the control valve selected should almost always be the equal percentage type.

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