Specifications

CONSTRUCTION
PC BASED SPEED
MONITORING SYSTEM
T
his project describes the software
and hardware necessary to moni-
tor and capture in real time the
speed of any rotating object. The speed
may be defined/stored/displayed in any of
the three units: RPM (rev./minute), RPS
(rev./second), or RPH (rev./hour). The sys-
tem uses a sampling time of two seconds
and can store up to 16 minutes of data per
file. The x and y axes can be scaled to read
any speed and the x-axis can be ‘stretched’
to observe clustered points.
The hardware mainly comprises a
proximity switch whose output is con-
nected to the printer
(LPT1) port of the com-
puter through an opto-
coupler. The proximity
switch is used as a
speed-sensor. The pro-
gram is written in C++
and has effective error
handling capability and
a help facility. This sys-
tem can be used to
monitor the speed of ro-
tating parts in the in-
dustry or to read and
record wind speeds.
The hardware interface
The hardware interface circuit is given
in Fig. 1. A 230V AC primary to 0-9V,
250mA secondary transformer followed
by IC 7805 is used for catering to the
power supply requirement for proximity
switch and the opto-coupler. The proxim-
ity switch, as shown in Fig. 2, is a 3-wire
switch (e.g. PG Electronics’ EDP101)
which operates at 6V to 24V DC.
The inductive type proximity switch
senses any metal surface from a distance
of about 5 mm to 8 mm. Thus, a gear or
fan blade is ideal for counting the number
of revolutions. The number of teeth that
trigger (switch-on) the proximity switch
during every revolution are to be known
for the software to calculate the speed of
G.S. SAGOO
SANTHOSH JAYARAJAN
the machinery. The output of the circuit,
available across resistor R2, is fed to the
PC via 25-pin ‘D’ connector of parallel port
LPT1. Pin 11 pertains to data bit D7 of the
input port 379(hex) of the LPT1 port hav-
ing base address 378(hex), and pin 25 is
connected to PC ground. (In fact, pins 18
through 25 of the parallel port are strapped
together and connected to ground.)
The proximity switch is mounted on
a stationary part, such as a bolt or stud,
in such a way that it senses each tooth of
the rotating part as shown in Fig. 3. Two
fixing nuts are provided on the threaded
body of the proximity
switch for securing it
firmly onto a fixed
part of the machinery.
The software
prompts the operator
to enter the number of
teeth (being sensed
during every revolu-
tion), which is used by
the program for calculation
of RPM, RPS, or RPH, as
the case may be. In any
specific application, where
non-metallic rotating parts
are present and inductive
proximity switch cannot be
used, one may use photo-
electric switch to do the
counting for 2-second sam-
pling period.
As interface circuit can easily be wired
on any general-purpose PCB, no PCB lay-
out is included for it. The two wires to be
extended to 25-pin parallel port may be
connected using a 25-pin male ‘D’ connec-
tor.
Lab Note: Magnetic proximity
switches, from various manufacturers,
are available in the market. The impor-
tant specifications include operating DC
voltage range, operating current and its
sensitivity, i.e. the maximum distance
from a metallic object such that the
switch operates. These specifications are
normally mentioned on the proximity
switch itself or in the accompanying lit-
erature.
The software
The structural block diagram of the soft-
ware is shown in Fig. 4. The software has
the following four main modules, which
are activated from the main menu using
four of the function keys, F1 through F4.
Fig. 1: Interface circuit for PC based speed monitoring system
Fig. 2: Proximity switch
Fig. 3: Mounting of proximity switch
25