Page 843 - Mechanical Engineers' Handbook (Volume 2)

834 Mechatronics

1.4 Sources and Meters
Some practical considerations are important when building mechatronic systems. Sources
and meters (and all elements) do not behave as the ideal representation described above.
Some of the limitations are described here.
1. Voltage Source. An ideal voltage source as described above does not have any output
resistance (no resistor shown in the above representation) and can supply infinite current.
Obviously, a real voltage source does have an output resistance. The output resistance (usu-
ally 1 ) can be represented as the model given below. Now, as current is output from the
source, the output voltage (V ) is no longer the same voltage as the source voltage (V ).
s
out
The magnitude of this output resistance (and the corresponding voltage drop) changes with
many factors. For example, the output resistance of a rechargeable battery (such as a car
battery) increases as the battery ages. The most significant difference between an ideal volt-
age source and a real source is that the real source can provide only a limited current. This
is a limitation on both the instantaneous current that can be produced as well as the total
amount (time integral) of current that can be produced. For example, the power supply
limitation on instantaneous current is often a constraint for battery-operated devices such as
cell phones or robots. See Fig. 14.
2. Current Source. An ideal current source as described above does not have any output
resistance (no resistor shown in the above representation) and can supply infinite voltage.
Obviously, a real current source does have an output resistance. The output resistance (usu-
ally 1M ) can be represented as the model given below. Now, as current is output from
the source, the output current (I ) is no longer the same current as the source current (I ).
out
s
The magnitude of this output resistance (and the corresponding voltage drop) changes with
many factors. The most significant difference between an ideal current source and a real
source is that the real source can provide only a limited voltage. This is a limitation on both
the instantaneous voltage that can be produced as well as the total amount (time integral) of
voltage that can be produced. See Fig. 15.
3. Voltmeter. An ideal voltmeter has infinite input resistance and draws no current from
the voltage being measured. A real voltmeter has a finite input resistance as modeled below
and does draw some current from the source which can change the voltage being measured.
However, the input resistance of most real voltmeters is very large (usually several mega
ohms) and this makes the voltmeter a very safe device as it does not draw ‘‘significant’’
current. See Fig. 16.
4. Ammeter. An ideal ammeter has zero input resistance and does not produce a voltage
drop. A real ammeter has a finite input resistance as modeled below and does have a small
voltage drop across the leads of the meter. However, the input resistance of most real am-
meters is very small (a few ohms), and this makes the ammeter a device that requires careful
consideration before use. If the ammeter leads are placed between two points with a potential
difference, a short circuit will occur and very large current will be produced. Therefore, most

+
R
+ out
V V
S out
–
–
Figure 14 Real voltage source.