Specifications

Position Sensing
In addition to their use to complete tachometer feedback
loops, used in position transducer circuits, the LM2907,
LM2917 devices can also be used as position transducers.
For example, the timing resistor can be removed from pin 3
so that the output current produces a staircase instead of a
fixed dc level. If the magnetic pickup senses passing notch-
es or items, a staircase signal is generated which can then
be compared with a reference to initiate a switching action
when a specified count is reached. For example,
Figure 19
shows a circuit which will count up a hundred input pulses
and then switch on the output stage. Examples of this appli-
cation can be found in automated packaging operations or
in line printers.
The output of the tachometer is proportional to the product
of supply voltage, input frequency, a capacitor and a resis-
tor. Any one of these may be used as the input variable or
they may be used in combination to produce multiplication.
An example of a capacitive transducer is illustrated in
Figure
20,
where a fixed input frequency is employed either from
the 60 Hz line as a convenient source or from a stable oscil-
lator. The capacitor is a variable element mechanically cou-
pled to the system whose position is to be sensed. The
output is proportional to the capacitance value, which can
be arranged to have any desired relationship to the me-
chanical input by suitable shaping of the capacitor elec-
trodes.
TL/H/7451–35
FIGURE 18. Motor Speed Control
TL/H/7451–36
TL/H/7451–37
FIGURE 19. Staircase Counter
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