The PRS1 Speed-Switch normally operates from a variable frequency signal
supplied by a machine mounted sensor. The sensor signal varies in frequency in
direct proportion to machine speed, and may be a sinusoidal, triangular, square,
or pulse-type waveform. The sensor arrangement can take a variety of forms
such as a Magnetic Pickup or Proximity Sensor detecting passing teeth on a
sprocket or gear, a Photo-Electric Scanner viewing passing pulley spokes, a
Rotary Pulse Generator coupled to a machine shaft, or a Length Sensor driven
by a web or ribbon of material passing through the machine. (See Sensor
Section of the catalog for more information on sensors.)
Since the PRS1 operates from the frequency content of the incoming signal,
the response time of both devices is also related to the signal frequency. This
gives rise to the cardinal rule of selecting a sensor arrangement:
WHEN RESPONSE TIME IS IMPORTANT, SELECT A SENSOR
ARRANGEMENT & LOCATION THAT WILL PROVIDE A HIGH
FREQUENCY OUTPUT AT OPERATING SPEED.
When a PRS1 application is first contemplated, it seems to be natural to think
in terms of applying the sensor to the low speed end of the power drive train. In
some cases this may be the only practical location for the sensor, and if fast
response is needed from the PRS1, a sensor arrangement capable of delivering
a high number of cycles or pulses/revolution (PPR) will be required. In a great
number of applications however, generating a higher frequency sensor signal is
simply a matter of locating the sensor on a intermediate or high speed shaft such
as directly on the drive motor shaft.
Another advantage of moving the sensor location up toward the high speed
end of the drive train is that the shaft rotary motion is usually much smoother
and more regular. Slow speed shafts will often rotate irregularly due to gear
backlash, “slop” in couplings, or slack in chain drives. This irregular motion
can have an adverse effect on the resulting output, especially when using the
PRS1 to perform a speed switching function near normal running speed.
SELECTING AN APPROPRIATE SENSOR ARRANGEMENT
There are no exact rules governing the selection of a sensor arrangement
since machine geometry and conditions can vary widely from one application
to the next. However, the following generalized criteria will prove useful as
guidelines toward selecting the best sensor arrangement. (See Sensor Section of
the catalog for more information.)
ULTRA-LOW SHAFT SPEEDS (10 RPM or less) - Proximity Sensors, Photo-
Electric Scanners, or Rotary Pulse Generators, are usually the best selections.
In most ultra-low speed applications, it is advisable to provide as many pulses
per revolution as possible (high PPR) to get acceptable response times.
LOW-SHAFT SPEEDS (10-100 RPM) - LMPC (Super-Sensitive Magnetic
Pickup), Proximity Sensors, Photo-Electric Scanners and RPG’s can usually
be applied in this speed range.
PRS1 SENSOR & FREQUENCY RANGE SELECTION
INTERMEDIATE SHAFT SPEEDS (10-1000 RPM) - Magnetic Pickups, the
LMPC, RPG’s and some Proximity Sensors are appropriate at these speeds.
HIGH-SHAFT SPEEDS (1000 RPM and up) - Magnetic Pickups, the LMPC
and RPG’s are usually the best choices.
FOR LINEAR SPEEDS ON PAPER WEBS, TEXTILE, RIBBON, STRIP
AND WIRE - The LSC Length Sensor may prove desirable.
CAUTION: When selecting a sensor for operation at any speed, make sure the
sensor is also capable of delivering an output for the entire speed range up
through maximum machine speed.
DETERMINING SENSOR FREQUENCY OUTPUT &
SELECTING THE PROPER FREQUENCY RANGE
Machine speeds are normally expressed in revolutions/minute (RPM) while
the PRS1 has adjustable frequency ranges in cycles/second or Hz. In addition,
sensor arrangements usually deliver a number of signal cycles or pulses for
each shaft revolution. The following formula provides a convenient way to
relate these variables:
FRQ (CPS or Hz) =
RPM
6
x
0
PPR
WHERE:
RPM is the speed of the shaft where the sensor is located in revolutions
per minute.
PPR is the number of pulses (or cycles) produced by the sensor for one
shaft revolution.
EXAMPLE 1
A machine is to be equipped with a PRS1 Speed Switch. A 42-tooth timing
belt pulley is available in the power drive train, and an LMPC is to be used to
sense passing teeth. The PRS1 set-point is to be adjusted to provide overspeed
output when the timing belt pulley reaches 730 RPM. What should the
frequency range of the PRS1 be?
FRQ @ set-point =
730 RPM
60
x 42 PPR
= 511 Hz
SELECT: PRS11021 (or PRS11022 for 230 VAC) which has an adjustable
range of 100-1000 Hz.
Red Lion Controls
20 Willow Springs Circle
York PA 17402
Tel +1 (717) 767-6511
Fax +1 (717) 764-0839
Red Lion Controls AP
31, Kaki Bukit Road 3,
#06-04/05 TechLink
Singapore 417818
Tel +65 6744-6613
Fax +65 6743-3360
Red Lion Controls BV
Basicweg 11b
NL - 3821 BR Amersfoort
Tel +31 (0) 334 723 225
Fax +31 (0) 334 893 793
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