The Reliability of Radio for Monitoring and Control in
Industrial Environments
The Wireless radio technology has long been used in the
form of fixed frequency radio in our homes and cars and to transmit data
in industrial applications. Operation requires a government license t
hat
theoretically prevents other broadcast signals inside the "bandwidth"
and territory covered by that license. This high power output allows
transmission across great distances and "blasting" through obstacles.
The downside is an almost immediate drop off in performance if
interference (man-made or environmental) moves into the allocated
bandwidth. Limited available frequencies also means that users,
particularly in urban areas, must often wait years for a license.
To allow greater access and utilize new radio technologies
dealing with interference, in 1987 the FCC allocated ISM (Industrial,
Scientific, Medical) spread spectrum bands.
Radio technology has been used in the telemetry world for
years instead of costly long run cable.
Licensed radios and even spread spectrum radios are
commonplace in the wide-open spaces of the oil fields and outlying
municipal water systems around North America. Here, reliability depends
on the FCC maintaining the end-user’s exclusive rights to that portion
of the bandwidth. Reliability is purchased or, in the case of spread
spectrum radios, often maintained simply because there aren’t other
radios competing for the bandwidth in the same area.
New technology is enabling greater use of spread spectrum
radio for monitoring and control in industrial environments. Today, we
can securely move small amounts of sensor and control information
including transmission of mission critical data through heavy
interference. Distances between the transmitter and receiver of 300 feet
to 15 + miles are achieved while maintaining reliability and information
integrity.
A typical dilemma faced by tank farms illustrates the
technology. Scattered I/O from multiple tank levels on one side of a
highway must be relayed to a DCS or similar system across the road.
Digging trenches, laying conduit, and pulling cable makes acquiring
these signals costly - not to mention the costs of engineering and
inspections and the time needed to acquire right of way prior to
implementing the solution. Wireless I/O interfaces are less expensive -
in some cases costing tens of thousands of dollars less.
An industrial wireless I/O interface can send analog and
discrete signals from a sensor to a PLC or from a PLC to a pump. In this
case, reliably reporting levels, pressure, flow and alarms or to control
pumps, valves and switches by updating data far more often than
required.
The Key to a reliable Industrial wireless I/O interface
Reliability is maximized through frequent sampling of
small data packets. Small information packets are a critical component
to designing a reliable industrial wireless interface. Whereas
traditional telemetry SCADA requires lots of information to be sent
through the air, cable replacements for industrial I/O require only
bytes of information to be moved. Since errors occur when bits are
received incorrectly, the smaller the packet the less chance for error.
Applications such as alarms are essentially one bit of
information that is either ON or OFF (4-20mA current loop output is
usually transmitted in one or two bytes). Data checks values to detect
changes.
Each packet in an independent update, eliminating the need
to include networking information. Sampling more often than needed
provides real time data and allows data to be lost if the radio
environment is cluttered by heavy interference.
Small packet size can also yield more Power Per Bit. Given
a pristine radio environment, there is a clear relationship between
speed and distance (here, speed equates to baud rate). In a setting with
no interference, if one watt of transmit power is applied to a
transmitter sending out information at a slow speed, that radio will
fire its signal farther than a radio sending out information at a high
speed. The more Power Per Bit, the better able you are to penetrate
walls, bounce around tanks and propagate through maze-like metallic
structures. The flip side is that more bits per second in transmission
reduces Power Per Bit. Therefore, in applications where I/O is moved 300
feet to 15 miles, a small number of bits stand a better chance of making
it to the receiver than a large number.
The Types of Spread Spectrum:
FCC allows two methods for building a license-free spread
spectrum radio: Direct Sequence Spread Spectrum (DSSS) or Frequency
Hopping Spread Spectrum (FHSS). Differing physical mechanisms for
dealing with and rejecting interference means DSSS and FHSS behave
differently in industrial settings.
Interference and how DFSS and FHSS address it are vital to
understand. Wireless radios encounter interference through EMI or RFI
from industrial equipment; from other licensed users (even in ISM
bands), or from unlicensed radios (especially in ISM).
In DSSS radios, a data packet starts out as "narrow data."
It then generates a random code word for every bit in that packet. These
code words spread the narrow data being sent and "widen" it across a
much wider bandwidth.
