Page 49 - The Mechatronics Handbook

Cutoff Frequency

Low Pass Band

Output

Frequency

FIGURE 3.9 Low-pass filter.

on the same board as the microprocessor, they will often share a common clock. The microprocessor
clock, however, may be too fast for the ADC and DAC. In this case, a prescaler is used to divide the clock
frequency to a level usable by the ADC and DAC.

Filtering

Filtering is the attenuation (lessening) of certain frequencies from a signal. This process can remove noise
from a signal and condition the line for better data transmission. Filters can be divided into analog and
digital types, the analog filters being further divided into passive and active types. Analog passive filters use
resistors, capacitors, and inductors. Analog active filters typically use operational amplifiers with resistors
and capacitors. Digital filters may be implemented with software and/or hardware. The software component
gives digital filters the feature of being easier to change. Digital filters are explained fully in Chapter 29.
Filters may also be differentiated by the type of frequencies they affect.
1. Low-pass filters allow lower set of frequencies to pass through, while high frequencies are atten-
uated. A simplistic example of this is shown in Fig. 3.9.
2. High-pass filters, the opposite of low-pass, filter a lower frequency band while allowing higher
frequencies to pass.
3. Band-pass filters allow a particular range of frequencies to pass; all others are attenuated.
4. Band-stop filters stop a particular range of frequencies while all others are allowed to pass.
There are many types and applications of filters. For example, William Ribbens in his book Under-
standing Automotive Electronics (Newnes 1998) described a software low-pass filter (sometimes also called
a lag filter) that averages the last 60 fuel tank level samples taken at 1 s intervals. The filtered data are
then displayed on the vehicle instrument cluster. This type of filtering reduces large and quick fluctuations
in the fuel gauge due to sloshing in the tank, and thus displays a more accurate value.

Data Acquisition Boards
There is a special type of board that plugs into a slot in a desktop personal computer that can be used
for many of the tasks above. It is called a data acquisition board, or DAQ board. This type of board can
generate analog input and multiplex multiple input signals onto a single bus for transmission to the PC.
It can also come with signal conditioning hardware/software and an ADC. Some units have direct memory
access (DMA), where the device writes the data directly into computer memory without using the
microprocessor. While desktop PCs are not usually considered as part of a mechatronic system, the DAQ
board can be very useful for instrumentation.

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