Wednesday, October 7, 2009

HVAC/R Electronics #4

by Roger Desrosiers

In Electronics # 3 we looked at some working circuits of op amps, diac/triacs and transistors. Now as we talk about electronic controls, also known as conventional or single loop controls that control only one loop in an entire control system, let’s start to discuss the components as a working system that integrates many electronic components to a whole series of accurate control systems. Let’s begin by talking about sensors and output devices.



The sensors and output devices (e.g., actuators, relays) used for electronic control systems are usually the same ones used on microprocessor-based systems, which we will discuss in Electronic # 5. The distinction between electronic control systems and microprocessor-based systems is in the handling of the input signals. In an electronic control system, the analog sensor signal is amplified, and then compared to a set-point or override signal through voltage or current comparison and control circuits.

In a microprocessor-based system, the sensor input is converted to a digital form, where discrete instructions (algorithms) perform the process of comparison and control. Direct Digital Control Systems (DDC) can control many control sequences simultaneously.


Analog Electronic Controllers perform control functions based on inputs that can handle only one control loop at a time.

Convential Electronic control systems usually have the following characteristics:

Controller: Low voltage, solid state.
Inputs: 0 to 1V dc, 0 to 10V dc, 4 to 20 mA, resistance element,
thermister,thermocouple.
Outputs: 2 to 10V dc or 4 to 20 mA device.
Control Mode: Two-position, proportional, proportional plus integral (PI), step.

Figure 1 shows a simple electronic control system with a controller that regulates supply water temperature by mixing return water with water from the boiler. The main temperature sensor is located in the hot water supply from the valve. To increase efficiency and energy savings, the controller resets the supply water temperature setpoint as a function of the outdoor air temperature. The controller analyzes the sensor data and sends a signal to the valve actuator to regulate the mixture of hot water to the unit heaters.


Figure 1

These controllers measure signals from sensors, perform control routines in software programs, and take corrective action in the form of output signals to actuators. Since the programs are in digital form, the controllers perform what is known as direct digital control (DDC). Microprocessor-based controllers can be used as stand-alone controllers or they can be used as controllers incorporated into a building management system utilizing a personal computer (PC) as a host to provide additional functions.

Some electronic sensors use an inherent attribute of their material (e.g., wire resistance) to provide a signal and can be directly connected to the electronic controller. Other sensors require conversion of the sensor signal to a type or level that can be used by the electronic controller. For example, a sensor that detects pressure requires a transducer or transmitter to convert the pressure signal to a voltage that can be used by the electronic controller.

Electronic control, temperature sensors are classified as follows: Thermister, such as Resistance Temperature Devices (RTDs), change resistance with varying temperature. RTDs have a positive temperature coefficient (resistance increases with temperature).
Thermistors are solid-state resistance-temperature sensors with a negative temperature coefficient.

Controller:
The electronic controller receives a sensor signal, amplifies and/or conditions it, compares it with the set-point, and derives a correction if necessary. The output signal typically positions an actuator. Electronic controller circuits allow a wide variety of control functions and sequences from very simple to multiple input circuits with several sequential outputs. Controller circuits use solid-state components such as transistors, diodes, and integrated circuits and include the power supply and all the adjustments required for proper control.

Universal Controllers:
The input circuits of universal controllers can accept one or more of the standard transmitter/transducer signals. The most common input ranges are 0 to 10V dc and 4 to 20 mA. Other input variations in this category include a 2 to 10V dc and a 0 to 20 mA signal. Because these inputs can represent a variety of sensed variables such as a current of 0 to 15 amperes or pressure of 0 to 3000 psi, the settings and scales are often expressed in percent of full scale only.

Output Control:
Electronic controllers provide outputs to a relay or actuator for the final control element. The output is not dependent on the input types or control method. The simplest form of output is two-position, where the final control element can be in one of two states. For example, an exhaust fan in a mechanical room can be turned either on or off. The most common output form, however, provides a modulating output signal which can adjust the final control device (actuator) between 0 and 100 percent such as in the control of a chilled water valve.

Output Devices:
Actuator, relay, and transducer are output devices which use the controller output signal (voltage, current, or relay contact) to perform a physical function on the final control element, such as starting a fan or modulating a valve. Actuators can be divided into devices that provide two-position action and those that provide modulating action.

Two-Position:
Two-position devices such as relays, motor starters, and solenoid valves have only two discrete states. These devices interface between the controller and the final control element. For example, when a solenoid valve is energized, it allows steam to enter a coil which heats a room The solenoid valve provides the final action on the controlled media, steam. Damper actuators can also be designed to be two-position devices.

Modulating:
Modulating actuators use a varying control signal to adjust the final control element. For example, a modulating valve controls the amount of chilled water entering a coil so that cool supply air is just sufficient to match the load at a desired set-point (Fig. 17). The most common modulating actuators accept a varying voltage input of 0 to 10 or 2 to 10V dc or a current input of 4 to 20 mA. Another form of actuator requires a pulsating (intermittent) or duty cycling signal to perform modulating functions. One form of pulsating signal is a Pulse Width Modulation (PWM) signal.




In modulating control, when an actuator is energized, it moves the damper or valve a distance proportional to the sensed change in the controlled variable. For example, a Series 90 thermostat with a 10-degree throttling range moves the actuator 1/10 of the total travel for each degree change in temperature.

Series 90 controllers differ from controllers of other series in that the electrical mechanism is a variable potentiometer rather than an electric switch. The potentiometer has a wiper that moves across a 135-ohm coil of resistance wire. Typically the wiper is positioned by the temperature, pressure, or humidity sensing element of the controller.

The actuator has a low-voltage, reversible-drive motor which turns a drive shaft by means of a gear train. Limit switches limit drive shaft rotation to 90 or 160 degrees depending on the actuator model. The motor is started, stopped, and reversed by the electronic relay.

The feedback potentiometer is electrically identical to the one in the controller and consists of a resistance path and a movable wiper. The wiper is moved by the actuator drive shaft and can travel from one end of the resistance path to the other as the actuator drive shaft travels through its full stroke. For any given position of the actuator drive shaft, there is a corresponding position for the potentiometer wiper.

All Series 90 actuators have low-voltage motors. A line-voltage model has a built-in trnsformer to change the incoming line voltage to low voltage for the control circuit and the motor. Low-voltage models’ require an external transformer to supply the actuator (Fig. 19). Notice the Triacs in this circuit.



In reiterating the difference between Electronic Control and Direct Digital Control the electronic controls were quite good for the time but DDC is now the state of the art. These controllers measure signals from sensors, perform control routines in software programs, and take corrective action in the form of output signals to actuators. Since the programs are in digital form, the controllers perform what is known as direct digital control (DDC). Microprocessor-based controllers can be used as stand-alone controllers or they can be used as controllers incorporated into a building management system utilizing a personal computer (PC) as a host to provide additional functions. This is called distributed direct digital control. In part #5 we will talk about the various functions of DDC controls.

copyright(c)2009
Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.