15-01-2013, 12:00 PM
RPM Measurement Techniques
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Introduction
This document discusses field devices and methods for measuring
the rotational speed of a shaft in revolutions per minute (RPM).
RPM measurement is important when controlling or monitoring
the speed of motors, conveyors, turbines, etc.
Sensors for RPM Measurement
A sensor is necessary to sense shaft speed. Typical devices used for
this purpose are shaft encoders (rotary pulse generators), proximity
sensors, and photoelectric sensors.
Each of these devices sends speed data in the form of pulses. Two
factors affect the quality of this data:
• Number of pulses per revolution of the shaft (referred to as
PPR). Higher PPR values result in better resolution.
• Symmetry of pulses. The symmetry of one pulse to the next can
play a role in how consistent the RPM readings are. Symmetrical
pulses give more accurate data.
Encoders
Shaft encoders are the best solution for the sensing device. They
offer high resolution (typically 1 to 5000 PPR) and clearly defined,
symmetrical pulses. However, sometimes it is not feasible to mount
an encoder to the shaft being monitored.
Proximity Sensors
Proximity sensors provide medium- or low-resolution sensing,
depending on the number of pulses measured per revolution. The
best method of using a proximity sensor is to sense the teeth on a
gear. This type of sensing typically has options for 60, 120, or 240
PPR, and the pulses are relatively clearly defined and symmetrical.
If a gear is not available, a proximity sensor can be used to sense the
head of a bolt attached to the shaft. The drawback of this method is
the low PPR (low resolution). If more than one bolt head is used,
resolution improves, but pulses are often inconsistent and not
symmetrical.
Methods of Determining RPM
We’ll discuss two methods for determining RPM: the Frequency
measurement method and the Period measurement method.
Frequency measurement is better for fast-moving devices such as
motors and turbines that typically turn in thousands of revolutions
per minute.
Period measurement is better for devices that move more slowly,
such as shafts that turn in less than 10 RPM.
High PPR Solutions Using the Frequency
Measurement Method
For this discussion, high PPR is considered to be at least 60 PPR.
When using high PPR sensors, such as shaft encoders or proximity
sensors sensing gear teeth, the easiest way to determine RPM is to
monitor the pulse frequency from the sensor using a digital input
module and the Get Frequency command in PAC Control
Zero Speed Detection
Detecting when the shaft has stopped is important in many
applications, for example, to determine whether a conveyor has failed.
If you use the Frequency method to monitor RPM, you will know the
shaft has stopped when the frequency being sensed drops to zero.
If you use the Period method, the main issue is determining how
much time to allow between pulses before deciding that the shaft has
stopped. This time will vary from one system to the next. Essentially,
this method requires a little extra logic that will start (or restart) a
timer after each complete pulse and then monitor the timer while
checking to see if another pulse has been generated. If a pulse is not
received before the timer expires, then the shaft has stopped.
System Requirements
For either the Frequency or Period method of measuring RPM, the
following are required:
• PAC Control Professional, version 8.1 or higher
• One of these SNAP PAC I/O processors, which include high-speed
digital functions (frequency and period are measured in the processor,
not in the module):
- SNAP-PAC-R1
- SNAP-PAC-EB1
- SNAP-PAC-SB1
• For Frequency method, a SNAP digital input module configured as
a frequency input
• For Period method, a SNAP digital input module configured for
period measurement