Lag; General; Measurement Lag - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

FLOW (MANIPULATED
VARIABLE)
VALVE
STEAM
(CONTROL AGENT)
CONVERTER
Fig. 42. Steam-to-Water Converter.
A setpoint change can be disruptive because it is a sudden
change in the system and causes a disturbance to the hot water
supply. The resulting change passes through the entire process
before being measured and corrected.
Ambient (environmental) variables are the conditions
surrounding a process, such as temperature, pressure, and
humidity. As these conditions change, they appear to the control
system as changes in load.

LAG

General

Time delays, or lag, can prevent a control system from
providing an immediate and complete response to a change in
the controlled variable. Process lag is the time delay between
the introduction of a disturbance and the point at which the
controlled variable begins to respond. Capacitance, resistance,
and/or dead time of the process contribute to process lag and
are discussed later in this section.
One reason for lag in a temperature control system is that a
change in the controlled variable (e.g., space temperature) does
not transfer instantly. Figure 43 shows a thermostat controlling
the temperature of a space. As the air in the space loses heat,
the space temperature drops. The thermostat sensing element
cannot measure the temperature drop immediately because there
is a lag before the air around the thermostat loses heat. The
sensing element also requires a measurable time to cool. The
result is a lag between the time the space begins to lose heat
and the time corrective action is initiated.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROLLER
WATER
TEMPERATURE
(CONTROLLED
VARIABLE)
HOT WATER SUPPLY
(CONTROLLED
MEDIUM)
LOAD
HOT WATER
RETURN
CONDENSATE RETURN
STEAM TRAP
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Fig. 43. Heat Loss in a Space Controlled by a Thermostat.
Lag also occurs between the release of heat into the space,
the space warming, and the thermostat sensing the increased
temperature. In addition, the final control element requires time
to react, the heat needs time to transfer to the controlled medium,
and the added energy needs time to move into the space. Total
process lag is the sum of the individual lags encountered in the
control process.

Measurement Lag

Dynamic error, static error, reproducibility, and dead zone
all contribute to measurement lag. Because a sensing element
cannot measure changes in the controlled variable instantly,
dynamic error occurs and is an important factor in control.
Dynamic error is the difference between the true and the
measured value of a variable and is always present when the
controlled variable changes. The variable usually fluctuates
around the control point because system operating conditions
are rarely static. The difference is caused by the mass of the
sensing element and is most pronounced in temperature and
humidity control systems. The greater the mass, the greater the
difference when conditions are changing. Pressure sensing
involves little dynamic error.
Static error is the deviation between a measured value and
the true value of the static variable. Static error can be caused
by sensor calibration error. Static error is undesirable but not
always detrimental to control.
Repeatability is the ability of a sensor or controller to output
the same signal when it measures the same value of a variable
or load at different times. Precise control requires a high degree
of reproducibility.
27
CONTROL FUNDAMENTALS
COLD
AIR
HEAT LOSS
SPACE
THERMOSTAT
VALVE
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