Page 48 - The Mechatronics Handbook

T = Time
= Period
Amplitude

t = time
50% Duty Cycle 20% Duty Cycle

FIGURE 3.8 Pulse width modulation.

Actuator Output

Like sensors, actuators were first introduced in a previous section and will be described in detail in a
later chapter of this handbook. The three common actuators that this section will review are switches,
solenoids, and motors. Switches are simple state devices that control some activity, like turning on and
off the furnace in a house. Types of switches include relays and solid-state devices. Solid-state devices
include diodes, thyristors, bipolar transistors, field-effect transistors (FETs), and metal-oxide field-effect
transistors (MOSFETs). A switch can also be used with a sensor, thus turning on or off the entire sensor,
or a particular feature of a sensor.
Solenoids are devices containing a movable iron core that is activated by a current flow. The movement
of this core can then control some form of hydraulic or pneumatic flow. Applications are many, including
braking systems and industrial production of fluids. More information on solenoid actuators can be
found in a later chapter. Motors are the last type of actuator that will be summarized here. There are
three main types: direct current (DC), alternating current (AC), and stepper motors. DC motors may
be controlled by a fixed DC voltage or by pulse width modulation (PWM). In a PWM signal, such as
shown in Fig. 3.8, a voltage is alternately turned on and off while changing (modulating) the width of
the on-time signal, or duty cycle. AC motors are generally cheaper than DC motors, but require variable
frequency drive to control the rotational speed. Stepper motors move by rotating a certain number of
degrees in response to an input pulse.

3.4 Signal Conditioning

Signal conditioning is the modification of a signal to make it more useful to a system. Two important
types of signal conditioning are, of course, the conversion between analog and digital, as described in
the previous two sections. Other types of signal conditioning are briefly covered below, with a full coverage
reserved for Chapters 46 and 47.

Sampling Rate
The rate at which data samples are taken obviously affects the speed at which the mechatronic system can
detect a change in situation. There are several things to consider, however. For example, the response of
a sensor may be limited in time or range. There is also the time required to convert the signal into a form
usable by the microprocessor, the A to D conversion time. A third is the frequency of the signal being
sampled. For voice digitalization, there is a very well-known sampling rate of 8000 samples per second.
This is a result of the Nyquist theorem, which states that the sampling rate, to be accurate, must be at least
twice the maximum frequency being measured. The 8000 samples per second rate thus works well for
converting human voice over an analog telephone system where the highest frequency is approximately
3400 Hz. Lastly, the clock speed of the microprocessor must also be considered. If the ADC and DAC are

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