This document provides an overview of on-off and continuous control systems for mechanical, thermal, and chemical systems. It discusses how an on-off controller works by switching fully on when the error is positive and fully off when the error is zero or negative. As an example, it describes how an on-off controller could be used to control the temperature of an industrial oven by turning the heating elements fully on or fully off based on the difference between the setpoint and measured temperatures. The document also notes that systems have characteristics like dead time and capacitance that affect how quickly a controller can respond to disturbances.

1. CONTROL SYSTEMS
ENGINEERING
Course Code: MEB 4101
Course Credit 4
Dynamic response
Prepared by;
Masoud Kamoleka Mlela
BSc, Electromechanical Eng. (UDSM)
MSc, Renewable Energy (UDSM)
PhD, Mechanical Engineering (HEU, CN).

2. OVERVIEW ON ON/OFF AND CONTINUOUS
CONTROL OF MECHANICAL, THERMAL AND
CHEMICAL SYSTEMS
CONTROL SYSTEMS ENGINEERING

3. • In an industrial plant, a closed-loop control system has
the role of keeping a measured physical signal to a
predefined value (set point). The physical signal, also
called controlled variable, can be of any kind,
electrical (voltage, current, power), mechanical
(position, speed, force, torque), hydraulic (pressure,
flow) or thermal (temperature). The difference between
the controlled variable (measured) and the predefined
value is called error.
• The input of the controller is the error and the controller
output is an actuation signal which is send to an
actuator. The controlled variable is further measured with
a sensor and the information is feed back to the
controller.
On-off control system

4. The difference between the setpoint and the plant output
(measured) occurs because of the disturbances which affect the
plant (process). The role of the controller is to reject these
disturbances and keep the plant output (controlled variable) to
the predefined value (setpoint).
On-off control system

5. • In industrial applications there are several control laws used, most of
them being on-off control, PID control or other more advanced laws
(fuzzy, neuro-fuzzy, optimal, etc.).
• The on-off control is the simplest form of a controller, which switches
ON when the error is positive and switches OFF when the error is zero
or negative. An on-off controller doesn’t have intermediate states but
only fully ON or fully OFF states. Due to the switching logic, an on-off
controller is often called a bang-bang controller or a two-step
controller
On-off control system

6. Regardless of the size of the error, the
output of the on-off controller can only
be fully ON or fully OFF, it is not
proportional with the error.
Let’s take as example the temperature
control of an industrial oven. The
temperature inside the oven is
measured with a sensor and feed
back to the controller. Based on the
error (difference between setpoint
temperature and measured
temperature), the heating elements
are turned ON or OFF by the
controller. There are no intermediate
values of the heating element, they
are fully ON or fully OFF.
On-off control system

7. • The industrial oven has two important characteristics which
need to be explained, because they affect the response of the
controller:
– dead time
– capacitance (inertia)
• In most of the control systems with feedback loop, the system
can not respond instantly to any disturbance and it takes time
(delay) until the controller output has any effect on the
measured (plant) output. This time delay is know as dead time.
In the case of the industrial oven, if the access door is opened, it
takes time until the temperature drops, the controller senses
the difference, turns the heaters on and the temperature is
brought back to setpoint. Dead time has the effect of hiding the
disturbance from the controller and limits its ability to react
quickly.
On-off control system

8. The capacitance of a system is seen as the resistance to
changing inputs. The higher the capacitance of a system, the
longer the time it takes to react to changes. With the oven cold,
even if turning the heaters on, takes time for the temperature
to start increasing and reach the nominal value. The advantage
is that capacitance has the tendency to filter (dampen) out the
effect of disturbances on a system.
On-off control system