Page 45 - The Mechatronics Handbook

data along with a 24-hour clock and the user requested temperatures to produce a digital control signal.
This signal directs the actuator, usually a simple electrical switch in this example. The switch, in turn,
controls a motor to turn the heating or cooling unit on or off. New measurements are then taken and
the cycle is repeated. While not a mechatronic product on the order of a camcorder, it is a mechatronic
system because of its combination of mechanical, electrical, and computer components. This system may
also incorporate some additional features. If the temperature being sensed is quite high, say 80°C, it is
possible that a fire exists. It is then not a good idea to turn on the blower fan and feed the fire more
oxygen. Instead the system should set off an alarm or use a data communication device to alert the fire
department. Because of this type of computer control, the system is “smart,” at least relative to the older
mercury-switch controlled systems.

An Automotive Example

A second example is the Antilock Braking System (ABS) found in many vehicles. The entire purpose of
this type of system is to prevent a wheel from locking up and thus having the driver loose directional
control of the vehicle due to skidding. In this case, sensors attached to each wheel determine the rotational
speed of the wheels. These data, probably in a waveform or time-varied electrical voltage, is sent to the
microcontroller along with the data from sensors reporting inputs such as brake pedal position, vehicle
speed, and yaw. After conversion by the ADC or input capture routine into a digital value, the program
in the microprocessor then determines the necessary action. This is where the aspect of human computer
interface (HCI) or human machine interface (HMI) comes into play by taking account of the “feel” of
the system to the user. System calibration can adjust the response to the driver while, of course, stopping
the vehicle by controlling the brakes with the actuators. There are two important things to note in this
example. The first is that, in the end, the vehicle is being stopped because of hydraulic forces pressing
the brake pad against a drum or rotor—a purely mechanical function. The other is that the ABS, while
an “intelligent product,” is not a stand-alone device. It is part of a larger system, the vehicle, with multiple
microcontrollers working together through the data network of the vehicle.

3.2 Input Signals of a Mechatronic System

Transducer/Sensor Input

All inputs to mechatronic systems come from either some form of sensory apparatus or communications
from other systems. Sensors were first introduced in the previous section and will be discussed in much
more depth in Chapter 19. Transducers, devices that convert energy from one form to another, are often
used synonymously with sensors. Transducers and their properties will be explained fully in Chapter 45.
Sensors can be divided into two general classifications, active or passive. Active sensors emit a signal in
order to estimate an attribute of the environment or device being measured. Passive sensors do not. A
military example of this difference would be a strike aircraft “painting” a target using either active laser
radar (LADAR) or a passive forward looking infrared (FLIR) sensor.
As stated in the Introduction section, the output of a sensor is usually an analog signal. The simplest
type of analog signal is a voltage level with a direct (though not necessarily linear) correlation to the
input condition. A second type is a pulse width modulated (PWM) signal, which will be explained further
in a later section of this chapter when discussing microcontroller outputs. A third type is a waveform,
as shown in Fig. 3.3. This type of signal is modulated either in its amplitude (Fig. 3.4) or its frequency
(Fig. 3.5) or, in some cases, both. These changes reflect the changes in the condition being monitored.
There are sensors that do not produce an analog signal. Some of these sensors produce a square wave
as in Fig. 3.6 that is input to the microcontroller using the EIA 232 communications standard. The square
wave represents the binary values of 0 and 1. In this case the ADC is probably on-board the sensor itself,
adding to the cost of the sensor. Some sensors/recorders can even create mail or TCP/IP packets as output.
An example of this type of unit is the MV100 MobileCorder from Yokogawa Corporation of America.

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