Building Services Controls Mechanical Services (UBLMPB-30-3) Reference: CIBSE (2009) Guide H: Building Control Systems, 2nd Ed.

Components Notes & terms: Input, i - the error between set point and value of controlled variable Output, o - e.g. a movement of the actuator motor there may be a distinction between the set point and the desired value of the controlled variable the graphic shows a closed loop control system – has a feedback pathway i feedback o

Control Modes Two Position Modulating on/off could be extended have more steps e.g. high/low/off control of burners can be floating control, with slow moving actuator Modulating Proportional ... Integral ........... Derivative ....... or combinations of above, e.g. PI or PID o  i o  ∫i or do/dt  i o  di/dt

Two-postion control

Two-postion control Set point On Off Dead (or neutral) zone Controlled condition (e.g. Temperature) On Off Set point Dead (or neutral) zone

Floating control

Proportional control

- response to step transient load change Proportional control - response to step transient load change Note! There is a discrepancy in this graphic

System response T DV = distance-velocity lag T = transfer lag

Damping Underdamped Critically damped Over-damped The response characteristic of a control system is defined by its transfer function

PID control modes Proportional (P) (P + I) (P + I + D) Refer to lab demo ...

AC controls

Mixing box – enthalpy control Θ h Mixing box – enthalpy control

Comfort cooling controls + fan − C T 100% 0% set point Heating pb Cooling pb Mixing pb Enthalpy control