Systems Control Design considerations for Control systems and

Systems Control Design considerations for: Control systems and building energy management systems

Control? • Control can be defined as: “the modification of the behaviour of a system so that it acts in a pre determined manner”

Example

Example • Tossing a caber requires strength but balancing a caber requires control! • For caber to remain upright continual adjustments to the position of the caber tosser’s body are required • e. g. if the caber falls to the left then the caber tosser moves to the left to regain control • The upright position is known as the “reference” • The difference between the reference and the actual angle of the caber is known as the “error” reference error

Example • The body (legs) is commanded by the brain • Command based on the error that is seen by the eyes Ø The brain is therefore the “controller” Ø The body is the “actuator” Ø The eyes are the “sensor” Ø The caber is the “controlled system” • These are the basic components of ALL control systems! caber reference eyes error body brain

Block Diagram • We can represent the “caber+tosser” control system using a “block diagram” of the bio-mechanical control system Desired angle Actual angle Error Actual angle Observed angle

Example We can see the same basic control elements in the function of a heating or air conditioning system, where the aim is to maintain comfortable conditions within a space for a specified period of time. This is achieved through the use of a electromechanical control system.

A basic room temperature control system heating coil Qo room flow control valve T s valve position controller +/ sp error (e) set point

A basic room temperature control system heating coil Qo flow control valve T s valve posit ion control ler roo m error (e) + / set poi nt This type of control mechanism is commonly termed feedback control, where the controlled variable (temperature) is fed back to the control system sp A sensor (thermostat) sends a temperature (usually a mix of air and radiant temperature) back to a controller. This is usually passed in the form of an electrical voltage, where the voltage magnitude is proportional to the temperature. The controller compares this temperature to a set-point (desired) temperature and generates an error value. The error is the difference between the set-point temperature and the sensed temperature. Depending on the magnitude of the error, the controller will adjust the output of the heating system up or down. In practice, this will involve operating a mechanical component such as a valve to increase or decrease the flow of hot water through a heating coil or radiator. The controller employs an algorithm to determine the heat output as a function of the error.

Components of control • We can represent the “HVAC+room” control system using a “block diagram” of the electro-mechanical control system desired temperature actual Error CPU measured temperature valve + heating coil temp

ON/OFF control This is the simplest type of control used in buildings: • If the sensed temperature is below the set-point then the heating system is fully ON. • If the sensed temperature rises above the set-point then the heating system is OFF. o C heat output set point temp. room air temperature Q max This shows the temperature and heat output in a room controlled by an ON/OFF controller. In practice the use of ON/OFF control can cause problems. As can be seen, the heating system rapidly switches ON an OFF leading to inefficient system operation and increased mechanical wear.

ON/OFF control with deadband To address this deficiency a 'dead band' may be introduced, i. e an upper and lower set-point. • if the sensed temperature is below the lower set-point then the heating system is ON; • if the sensed temperature rises above the lower set-point but is still below the upper set-point then the heating system is ON; • if the sensed temperature is above the upper set-point then the heating system is OFF; and • if the sensed temperature falls below the upper set-point but is still above the lower set-point then the heating system is OFF. o C h eat u pper temp. lower temp. output r oom air temperature The addition of the upper and lower set-points acts to reduce the frequency of the plant switching at the expense of poorer control of the controlled variable (temperature). Q max

Proportional control This is a more advanced control algorithm, where the control action is proportional to the size of the error: where K is known as the gain of the controller. o C heat output room air temperature upper temp. Qmax lower temp. offset error Taking the example of room temperature control: • if the temperature is below the set-point then the heating is ON and the output is proportional to the difference between the sensed temperature and the desired temperature; and • if the temperature is above the set-point then the heating is OFF. As the sensed temperature gets closer to the set-point temperature, so the output of the heating system is reduced.

Proportional control (continued) In practice the operation of a proportional controller is often limited as the output of the heating system is limited (i. e. it has a maximum capacity). This is achieved by introducing a 'proportional band' or 'throttling range'—this is similar to a dead band in that a single set-point is replaced by an upper and lower limit. The control action now follows the following rules: • if the temperature lies above throttling range then the heating system is OFF; • if the temperature lies below the throttling range then the heating system is ON at full power; and • if the sensed temperature is within the throttling range then the output is a function of the error. u & set point o. C heat output throttling range l 0 0 Qm ax

Proportional control (continued) Within the throttling range, the output of the heating system is: where, in this case, u = sp and so u - s = e As with ON/OFF control, the throttling range affects the operation of the system: a narrow throttling range gives close control (a small error) at the expense of the system switching ON/OFF frequently; and a wide throttling range reduces the ON/OFF switching off the system (cycling) at the expense of poorer control.

PID control It is impossible to completely eliminate the error between the desired temperature and the sensed temperature using only proportional control. There is always an offset error, where the controlled temperature never quite reaches the desired temperature. o C heat output set point temp. room air temperature Qmax A PID controller incorporates a mix of proportional, integral and derivative control action. In this case the control output is a function of the size of the error, the rate of change of the error with time, and the integral of the error over time. where Td is the derivative action time (s), Ti the integral action time (s) and K the gain

Building Energy Management Systems We’ve looked at a single controller – in reality a large building will have hundreds of different controllers operating – BEMS is means of monitoring and coordinating their operation ventilation cooling heating BEMS fire humidity security lighting

Building Energy Management Systems - facilities Monitoring They allow plant status, environmental conditions and energy to be monitored, providing the building operator with a real-time understanding of how the building is operating. This can often lead to problems being identified which may have gone unnoticed, e. g. high energy usage. Energy meters can easily be connected to a BEMS, providing real-time consumption patterns and ultimately a historical record of the building's energy performance. BEMS can therefore improve management information by trend logging performance, benefiting forward planning and costing. This can also encourage greater awareness of energy efficiency amongst occupants and management. Alarms They allow alarms to indicate that plant has shut down or requires maintenance, or that environmental conditions are outside limits. This is particularly important when applied to geographically remote buildings. Integrated Control They allow close integrated control of building services equipment, saving energy and improving comfort levels for the occupants.

BEMS – layout of centralised system Centralized systems control all connected site services from a single computer unit, and are most appropriate for large commercial buildings, such as hospitals with over 500 beds, motor factories and airport terminals.

BEMS – layout of distributed system Distributed systems comprise a number of local "intelligent" outstations which each control a small building, part of a large building, or a particular service. The outstations feed data back to a central unit for data collation and perhaps supervisory control (where, for example, set points can be changed by the central station). This type of system is generally used for a group of small to medium-sized buildings under common ownership (e. g. schools, hotels).

BEMS – communications

BEMS – typical control functions - optimiser An optimiser (or optimum start controller) is basically a device incorporating a time switch, which switches the plant on at such a time that the room temperature reaches the required value at or just before the predetermined occupancy start time. Most optimisers calculate the start-up time from a combination of space and ambient temperature sensor measurements. Similar functionality is used for optimum stop, so that heating systems can be switched off before the end of the working day.

BEMS – typical control functions - compensator For buildings other than dwellings, the Building Regulations require that the temperature of the heating system is regulated according to outside temperature. This can be achieved by using a compensator which adjusts the flow temperature in the heating circuit as the outside temperature rises or falls. Adaptive compensators are self-learning and can adjust the flow temperature based on the flow temperature/outside temperature relationship of previous days. Similarly, some optimisers are self-adapting.

BEMS – typical control functions - compensator The 3 -way valve reduces the flow water temperature, according to the heating load, by reducing the water flow from the boiler while increasing the return water flow. The flow water temperature is set in proportion to the outside temperature measured on an external north-facing wall. Control of the valve is with a PI control loop.

BEMS – typical control functions - others Sequence control: boiler sequence control enables only the number of boilers that are actually required to meet system demand, and avoids the low efficiencies that can result from boilers being used at low part-loads. Frost and fabric protection by switching on heating at night to prevent condensation and freezing Lighting control based on measured workplane illuminance levels Supply and return fan control in air-conditioning systems Duty cycling: regular on-off switching to achieve required temperatures Mixing of re-circulated and ventilation air: e. g. to use outside air to reduce cooling requirements.

BEMS - disadvantages Capital cost This is high - a large building BEMS could be £ 50 -100 k, although smaller systems with intelligent outstations can be effective. Paybacks in energy terms can be 510 years, but this excludes monitoring and alarm benefits. Training An important consideration - poor operation of BEMS can negate any energy benefits. Over-sophistication There is a danger in installing a high-tech control system that is difficult to understand.

“Intelligent” buildings This term is often used to describe buildings with a high level of computerised control, with integration of different control systems. For example, energy management, security, office automation systems and external communication systems may be linked. Fully integrated systems are estimated to cost up to 50% more than for a conventional building, but potentially offer energy savings, efficiency and adaptability.

Conclusions Most of the examples have involved control applied to room temperatures. Many other control targets may be considered within a building design context. For example: • control of blinds located on the façade; • control of fan speed; • control of damper/valve positions; • control of hot water temperature; and • control of re-circulated air Good control is vital to acceptable performance in terms of comfort and energy efficiency. The control algorithms such as ON/OFF, proportional and PID must be configured for optimum performance: • selection of set-points; • selection of proportional/dead band values; and • selection of integral/derivative action times.

Conclusions Inappropriate control parameters lead to a poorly configured control system, which in turn may give rise to uncomfortable conditions, energy waste and a reduction in the lifetime of system components. It is important that a building's control system is well designed, commissioned and maintained.