2251 Introduction

Deciding which control mode to apply is important, regardless of the technology used. It is important to understand that these modes can be implemented using many of the available technology types. In many cases, simple on-off control is adequate, and very appropriate. In other cases, the desired effect can only be achieved with modulating controls. The following are basic control

TECHNOLOGY PROS CONS

• Easy to understand and troubleshoot

• Inexpensive

• Accommodates "and-or" logic with simple series-parallel circuits (relay logic).

• Floating control variation approaches analog control quality with reduced cost.

• Definite purpose controls are not flexible without hardware change.

• Limited capabilities for optimization.

Analog Electronic

• Lowest cost stand-alone modulating control option.

• Often standard equipment on packaged HVAC equipment.

• Long term drift unless properly maintained.

• Short-lived components.

• Definite purpose controls are not flexible without hardware change.

• Often not user-friendly.

• Limited capabilities for optimization.

• Input/Output hardware items, especially actuators, are more expensive than conventional on-off devices.

• End devices extremely durable

• Can be long lived if designed and maintained properly.

• Long term drift unless properly maintained.

• Temperature dependent drift.

• Definite purpose controls are not flexible without hardware change.

• Limited capabilities for optimization.

• Susceptible to system-wide failure if compressed air system is contaminated.

• Complicated adjustments for pneumatic controllers

• First cost includes infrastructure cost (tubing).

• Future operator work force may have a reduction in skill level for this technology.

Digital Control (DDC)

• Excellent flexibility, using software changes with existing hardware.

• Best option for optimization due to computing power.

• Opportunities for communication linkage to other, compatible equipment for large data gathering/workstation benefit.

• Convenient method of recording key measurements, baseline information, historical data, trends, run-times, etc.

• Convenient remote monitoring.

• First cost includes infrastructure cost (cabling).

• Often outdated before physically worn out, due to rapid technology advances.

• Tendency for proprietary equipment manufacturers to create captive customers and high life cycle costs for repair parts and software upgrades.

Figure 22.1 Pros and Cons of Different Control Technologies

Figure 22.1 Pros and Cons of Different Control Technologies modes. The accompanying diagrams will illustrate typical system performance.

The term "system capacitance" refers to the rate of response of a system to a stimulus. Systems with a large capacitance tend to resist change and the effects of control are felt more slowly than systems with smaller capacitance. Comparing the effect to a flywheel or relative mass is a good way to describe this concept. Another useful example to illustrate system capacitance is an instantaneous electric water heater (small volume of water) compared to a standard residential tank-type water heater—upon energizing the heater elements, the water temperature in the tank unit changes much more slowly because it has more mass, and we say it has greater system capacitance.

The term "gain " is a control term synonymous with sensitivity, and is usually an adjustable value used to tune the controls. If a quicker response is desired for a small input change, the gain is increased. This makes the input change more noticeable, and results in a stronger output reaction from the controller.

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