Pneumatic controllers, such as thermostats, are connected between a sensing element located in a regulated space and a controlled device operable to change the conditions of the space (e.g. HVAC systems and components). The sensor is sensitive to changes in the controlled space and able to consistently provide an output signal proportional to the level of a controlled variable, e.g. temperature or humidity. In response to the output signal, the controller actuates the controlled device to prevent excessive deviation of the variable from the desired setpoint. Generally, there are two main elements in a controller, a sensing element and a relay which produces an output in response to the sensing element to actuate the controlled device. The controlled device may take a variety of forms including, for instance, a valve or damper for admitting warm air into a room.
Often with heating, ventilating, and air conditioning systems, pneumatic controllers are used to provide a controlled pressure output which regulates the position of the controlled device. For example, a typical pneumatic system includes a compressor for supplying a pressurized source of air, typically at 15 or 20 PSIG. This supply air is delivered through conduits to a pneumatic relay having a valve for selectively permitting passage of the supply air through the relay to the controlled device. The valve is typically a diaphragm type valve whose position is controlled by the pressure level in a pilot circuit. The pilot circuit is a passage which allows a portion of the supply air to pass along one side of the diaphragm before exiting through a control port. The amount of resistance to air passing from the control port controls the pressure build-up within the pilot circuit and thus the position of the control valve.
A sensor, such as a bi-metal element, is disposed in proximity to the control port to change the amount of resistance relative to the level of the controlled variable. For instance, when the controlled variable, e.g. temperature, is at an equilibrium level with the setpoint, the bi-metal element will be at an equilibrium position. However, if the temperature deviates from the setpoint in one direction, the bi-metal element will bend away from the control port allowing free passage of supply air. On the other hand, as the temperature moves beyond the setpoint in the opposite direction, the bi-metal element will bend towards the control port tending to further restrict passage of air from the control port as the temperature change continues. This will of course change the pressure in the pilot chamber which, in turn, changes the position of the control valve to thereby increase or decrease the airflow and pressure to the controlled device as required in a particular application.
Although pneumatic control systems like the one described above work well in certain applications, other applications present greater difficulties. One difficulty is that use of a bi-metal element to sense a given variable and then control the pneumatic relay is often less precise than desired. For example, the bi-metal element may not react consistently to temperature change or it may react too slowly. This type of system makes it more difficult to maintain the temperature, for instance, of a room within a close range from the setpoint.
In other systems, digital controllers have been used to control the flow of air to the controlled device. For example, in Lilja et al., U.S. Pat. No. 5,114,070, a pneumatic thermostat is disclosed which uses an electronic to pneumatic transducer controlled by a microprocessor. This, of course, presents further obstacles because either electric wires must be run to the microprocessor to provide power or a remote power supply must be used. In Lilja, the use of a battery, solar cell, or air turbine is suggested to supply or supplement electric power to the microprocessor. One problem with this system is that when the battery goes dead the control system is no longer powered. Similarly, if there is an insufficient light or air supply to run the solar cell or air turbine, the pneumatic thermostat will not function.
The electronic to pneumatic transducer disclosed in Lilja uses a pair of solenoids to allow passage of air between the inlet pipe and the outlet pipe of the transducer or between the outlet pipe and atmosphere respectively. The solenoid valves are controlled by the microprocessor. This system requires solenoids capable of regulating the substantial air volume and air pressure in the main air supply conduit. Manipulating such solenoid valves requires a relatively large amount of power from the power source which increases the rate at which the power source is depleted. It would be advantageous to have a digitally controlled system that did not operate directly in the main air supply conduits and thus did not require such a substantial amount of energy.