U.S. patent application number 14/504637 was filed with the patent office on 2015-04-16 for hart sampling.
The applicant listed for this patent is Rockwell Automation Limited. Invention is credited to Philip J. Agar, Alison Cooper.
Application Number | 20150103957 14/504637 |
Document ID | / |
Family ID | 49328404 |
Filed Date | 2015-04-16 |
United States Patent
Application |
20150103957 |
Kind Code |
A1 |
Agar; Philip J. ; et
al. |
April 16, 2015 |
HART Sampling
Abstract
A method for processing an electrical signal comprises receiving
an electrical signal comprising a frequency modulated signal
encoding digital data, sampling a first portion of the electrical
signal to provide a first sample set, and determining an
intermediate value from the first sample set. A first output value
is selected from a plurality of stored output values based on the
intermediate value, the output value indicating a frequency
modulation encoded value within the first portion of the electrical
signal. An indication of the obtained output value is output.
Inventors: |
Agar; Philip J.; (Danbury,
GB) ; Cooper; Alison; (Danbury Chelmsford,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rockwell Automation Limited |
Milton Keynes |
|
GB |
|
|
Family ID: |
49328404 |
Appl. No.: |
14/504637 |
Filed: |
October 2, 2014 |
Current U.S.
Class: |
375/334 |
Current CPC
Class: |
H04L 27/106 20130101;
H04L 27/14 20130101; H04L 27/1563 20130101; H04L 27/144
20130101 |
Class at
Publication: |
375/334 |
International
Class: |
H04L 27/14 20060101
H04L027/14; H04L 27/10 20060101 H04L027/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 10, 2013 |
EP |
13188181.5 |
Claims
1. A method for processing an electrical signal comprising:
receiving an electrical signal comprising a frequency modulated
signal encoding digital data; sampling a first portion of the
electrical signal to provide a first sample set; determining an
intermediate value from the first sample set; obtaining a first
output value from a plurality of stored output values based on the
determined intermediate value, the first output value indicating a
frequency modulation encoded value within the first portion of the
electrical signal; and outputting an indication of the first output
value.
2. The method of claim 1, wherein the first output value is a HART
data bit.
3. The method of claim 1, wherein determining the intermediate
value comprises determining a binary value associated with each of
a plurality of samples in said first sample set, the intermediate
value comprising said binary values.
4. The method of claim 1, wherein sampling the first portion of the
electrical signal comprises sampling the first portion of the
electrical signal at a sample rate of 83.3 .mu.s.
5. The method of claim 1, wherein sampling the first portion of the
electrical signal comprises obtaining ten samples from the first
portion of the electrical signal such that the first sample set
comprises ten samples.
6. The method of claim 1, wherein said first portion is one of a
plurality of sampled portions of the electric signal, each sampled
portion providing a respective sample set.
7. The method of claim 6, wherein the plurality of sampled portions
are contiguous.
8. The method of claim 7, wherein each of the plurality of sample
sets differs from an immediately preceding one of the plurality of
sample sets by a single sample such that the plurality of sample
sets define a sliding window over the received electrical
signal.
9. The method of claim 6, wherein each, of said sample sets is
associated with a respective count and the method further comprises
selecting the first sample set from the plurality of sample sets
based on a count associated with the first sample set.
10. The method of claim 9, wherein the count is in a phase locked
loop with respect to data bits encoded within the frequency
modulated signal such that the count is synchronized with the data
bits encoded within the frequency modulated signal.
11. The method of claim 10, wherein the count associated with said
first sample set indicates that the first sample set is associated
with a substantially middle portion of a data bit encoded within
the frequency modulated signal.
12. The method of claim 1, further comprising: populating said
plurality of stored output values, said populating comprising:
determining a plurality of possible intermediate values; and for
each of said possible intermediate values selecting a respective
data bit for association with that intermediate value.
13. The method of claim 12, wherein selecting the respective data
bit for association with the intermediate value comprises applying
a Fourier transform to the intermediate value and selecting a data
bit in dependence upon the resulting frequency distribution.
14. The method of claim 13, further comprising selecting a data bit
value of `1` when the resulting frequency distribution is primarily
centered around 1200 Hz and selecting a data bit value of `0` when
the resulting frequency distribution is centered around 2200
Hz.
15. An apparatus for processing an electrical signal comprising: a
receiver arranged to receive an electrical signal comprising a
frequency modulated signal encoding digital data; a sampler
arranged to sample a first portion of the electrical signal to
provide a first sample set; a memory storing a plurality of output
values, each output value indicating a frequency modulation encoded
value; and a processing module arranged to process the first sample
set to determine an intermediate value, to obtain a first output
value associated with the first sample set from a plurality of
stored output values based on the determined intermediate value,
and to output an indication of the obtained output value.
16. The apparatus of claim 15, wherein the sampler is arranged to
sample the first portion of the electrical signal at a sample rate
of 83.3 .mu.s.
17. The method of claim 15, wherein the sampler is arranged to
sample a plurality of portions, and wherein the first portion is
one of the plurality of sampled portions of the electric signal,
each sampled portion providing a respective sample set.
18. The apparatus of claim 17, wherein the processing module is
arranged to associate each of said sample sets with a respective
count and to select the first sample set horn the plurality of
sample sets based on a count associated with the first sample
set.
19. The apparatus of claim 18, wherein the count is in a phase
locked loop with respect to data bits encoded within the frequency
modulated signal such that the count is synchronized with the data
bits encoded within the frequency modulated signal.
20. The apparatus of claim 19, wherein the count associated with
said first sample set indicates that the first sample set is
associated with a substantially middle portion of a data bit
encoded within the frequency modulated signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to European patent
application number 13188181.5, filed Oct. 10, 2013, the entire
contents of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The subject matter disclosed herein relates generally to
methods and apparatus for processing electrical signals, and in
particular analogue electrical signals having digital signals
encoded therein and, more specifically, to processing analogue
electrical signals having HART signals encoded therein.
[0003] Analogue current loops, on which only one current level can
be present at any one time, are commonly used to remotely monitor
and/or control field devices. A 4-20 mA (four to twenty milliamps)
current loop is a well-known and widely adopted analogue electrical
transmission standard for industrial instrumentation and
communication. The 4-20 mA signal is provided via a current loop
where 4 mA represents zero signal and 20 mA represents maximum
signal. Use of a 4 mA signal to represent zero signal, rather than
0 mA, allows an open circuit condition to be detected.
[0004] The Highway Addressable Remote Transducer (HART)
Communications Protocol was developed to provide a digital
industrial automation protocol capable of being used over 4-20 mA
analogue current loops, sharing the pair of wires used by a 4-20 mA
system.
[0005] The HART protocol makes use of Bell 202 compatible Frequency
Shift Keying (FSK) signals to superimpose digital communication
signals at a low level on 4-20 mA analogue signals. Bell 202
compatible systems use 1200 Hz to represent a mark (`1`) and 2200
Hz to represent a space (`0`). HART enables two-way field
communication to take place and makes it possible for additional
information, beyond the process variables communicated by the 4-20
mA signal, to be sent to and from a HART enabled field
instrument.
[0006] While newer industrial process protocols, such as Fieldbus,
exist for providing real-time distributed control of field devices,
the installed base of 4-20 mA analogue current loops is large, and
as such HART remains in widespread usage. It is, therefore,
desirable to provide improved methods and systems for HART signal
processing.
BRIEF DESCRIPTION OF THE INVENTION
[0007] The subject matter disclosed herein describes an improved
method and system for processing analogue signals having digital
communication signals encoded using the HART protocol.
[0008] According to a first embodiment of the invention, there is
provided a method for processing an electrical signal. The method
comprises receiving an electrical signal comprising a frequency
modulated signal encoding digital data; sampling a first portion of
the electrical signal to provide a first sample set; determining an
intermediate value from the first sample set; obtaining a first
output value from a plurality of stored output values, the output
value indicating a frequency modulation encoded value within the
first portion of the electrical signal; and outputting an
indication of the first output value. The afore-described steps
provide a method for quickly and efficiently processing an
electrical signal to determine a frequency modulation encoded
value.
[0009] The electrical signal may further comprise analogue data.
For example, the electrical signal may comprise a 4-20 mA signal in
addition to the frequency modulated signal encoding digital
data.
[0010] The indication of the output value may be the first output
value itself. The first output value may be a binary value. For
example, the first output value may be HART data bit. HART data
bits can therefore be obtained from an electrical signal without
the need for an expensive HART decoder.
[0011] Determining an intermediate value may comprise determining a
binary value associated with each of a plurality of first samples
in said first sample set, the intermediate value comprising said
binary values.
[0012] According other aspects of the invention, the first portion
of the electrical signal may be sampled at a sample rate of 83.3
.mu.s. Ten samples from the first portion of the electrical signal
may be obtained such that the first sample set comprises ten
samples. The first portion may be one of a plurality of sampled
portions of the electric signal, each sampled portion providing a
respective sample set. The plurality of sampled portions may be
contiguous. Each of the plurality of sample sets may differ from an
immediately preceding one of the plurality of sample sets by a
single sample such that the plurality of sample sets define a
sliding window over the received electrical signal.
[0013] According to another aspect of the invention, each of the
sample sets may be associated with a respective count. The count
may be referred to as a transition reference count. The method may
further comprise selecting the first sample set from the plurality
of sample sets based on a count associated with the first sample
set. In this way, the sample set used to determine the intermediate
value, and thereby the first output value, may be selected so as to
improve an accuracy of the obtained first output value.
[0014] According to yet another aspect of the invention, the count
may be in a phase locked loop with respect to data bits of said
digital data encoded within the frequency modulated signal such
that the count is synchronized with the data bits encoded within
the frequency modulated signal. The count may be incremented in a
loop such that the count loops from a maximum count value to a
minimum count value at a transition between data bits encoded in
the electrical signal.
[0015] According to still another aspect of the invention, a count
associated with the first sample set may indicate that the first
sample set is associated with a substantially middle portion of a
data bit encoded within the frequency modulated signal. The first
sample set may be selected from a plurality of sample sets because
the count associated with the first sample set indicates that the
first sample set is associated with a substantially middle portion
of a data bit. In this way, the first output value is determined
based upon the substantially middle portion of the data bit thereby
reducing the impact of errors in detecting transitions between data
bits encoded in the frequency modulated signal.
[0016] According to another embodiment of the present invention,
there is provided an apparatus for processing an electrical signal.
The apparatus comprises: a receiver arranged to receive an
electrical signal comprising a frequency modulated signal encoding
digital data; a sampler arranged to sample a first portion of the
electrical signal to provide a first sample set; a memory storing a
plurality of output values, each stored output value indicating a
frequency modulation encoded value; and a processing module to
process the first sample set to determine an intermediate value, to
obtain from the plurality of output values a first output value
based on the intermediate value, the first output value being
associated with the first sample set and to output an indication of
the obtained output value.
[0017] The processing module may be arranged to output a binary
value. For example, the processing module may be arranged to output
a HART data bit.
[0018] The sampler may be arranged to determine a binary value
associated with each of a plurality of first samples in said first
sample set, and the processor may be arranged to determine an
intermediate value such that the intermediate value comprises the
binary values.
[0019] The sampler may be arranged to sample the first portion of
the electrical signal at a sample rate of 83.3 .mu.s.
[0020] The sampler may be arranged to sample the first portion of
the electrical signal by obtaining ten samples from the first
portion such that the first sample set comprises ten samples.
[0021] The sampler may be arranged to sample a plurality portions
of the electric signal and to provide for each sampled portion a
respective sample set.
[0022] The sampler may be arranged to sample a plurality of
contiguous portions of the electrical signal.
[0023] The sampler may be arranged to sample each of the portions
of the electrical signal with respect to a sliding window.
[0024] For example, the sampler may be arranged to move the sliding
window over the electrical signal by a single sample after sampling
each of said portions, such that each sample set will differ from
an immediately preceding sample set by a single sample.
[0025] The apparatus may further comprise a counting module
arranged to associate each of said sample sets with a respective
count. The processing module may be arranged to select the first
sample set from the plurality of sample sets based on a count
associated with the first sample set.
[0026] The counting module may be arranged to associate counts with
sample sets in a phase locked loop with respect to data bits of the
digital data encoded frequency modulated signal such that the count
is updated in synchrony with transitions between data bits of the
digital data encoded within the frequency modulated signal.
[0027] The processing module may be arranged to select the first
sample set based on a first count associated with the first sample
set, the first count indicating that the first sample set is
associated with a substantially middle portion of a data bit
encoded within the frequency modulated signal.
[0028] The apparatus may comprise a filter arranged to filter the
received electrical signal to produce a filtered signal. The
sampler may be arranged to sample a first portion of the filtered
signal. Each of the portions sampled by the sampler may be portions
of the filtered signal. The filter may be a band-pass filter. The
filter may have a pass-band in the range of approximately 1 KHz to
3 KHz.
[0029] According to a another aspect of the invention, there is
provided a computer program comprising computer readable
instructions configured to cause a computer to carry out a method
according to the first aspect.
[0030] According to still another aspect, there is provided a
computer readable medium carrying a computer program according to
the third aspect.
[0031] According to yet another aspect, there is provided a
computer apparatus for processing an electrical signal. The
computer apparatus comprises: a memory storing processor readable
instructions; and a processor arranged to read and execute
instructions stored in the memory, wherein the processor readable
instructions comprise instructions arranged to control the computer
to carry out a method according to the first aspect.
[0032] Generally it will be appreciated that embodiments of the
invention can be implemented in any convenient form. For example
aspects of the invention can be implemented by suitable computer
programs. Aspects of the invention may provide transitory and/or
non-transitory carrier media and computer readable media carrying
such computer programs. Further aspects of the invention may
provide apparatus arranged to carry out the methods described
herein. Such apparatus may take the form of a general purpose
computer system comprising a memory storing processor readable
instructions and a processor arranged to read and execute those
instructions, the instructions comprising instructions controlling
the processor to carry out methods described herein.
[0033] It will also be appreciated that features presented in the
context of one aspect or embodiment of the invention in the
preceding and following description can equally be applied or
combined with other aspects or embodiments of the invention.
[0034] These and other advantages and features of the invention
will become apparent to those skilled in the art from the detailed
description and the accompanying drawings. It should be understood,
however, that the detailed description and accompanying drawings,
while indicating preferred embodiments of the present invention,
are given by way of illustration and not of limitation. Many
changes and modifications may be made within the scope of the
present invention without departing from the spirit thereof, and
the invention includes all such modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Various exemplary embodiments of the subject matter
disclosed herein are illustrated in the accompanying drawings in
which like reference numerals represent like parts throughout, and
in which:
[0036] FIG. 1 is a schematic illustration of a decoding apparatus
according to some embodiments described herein;
[0037] FIG. 2 is a representation of a filter response of a filter
of the apparatus of FIG. 1;
[0038] FIG. 3 is a flowchart showing processing carried out by the
decoding apparatus of FIG. 1 to determine an output value;
[0039] FIG. 4 is a schematic representation of a method for
populating a LUT used in the apparatus of FIG. 1;
[0040] FIG. 5 is a schematic illustration of an architecture of a
distributed industrial process control system in which embodiments
described herein may be utilized;
[0041] FIG. 6 is a schematic illustration of a controller of the
industrial process control system of FIG. 5;
[0042] FIG. 7 is a schematic illustration of a possible
configuration of a controller;
[0043] FIG. 8 is a schematic, illustration of possible
configurations of an input assembly and an output assembly of FIG.
5;
[0044] FIG. 9 is one exemplary illustration of a possible
configuration of a two-out-of-three voting arrangement;
[0045] FIG. 10 is another exemplary illustration of a possible
configuration of a two-out-of-three voting arrangement;
[0046] FIG. 11 is a schematic illustration of an input module;
[0047] FIG. 12 is a circuit diagram of an analogue input
termination assembly; and
[0048] FIG. 13 is a schematic illustration of a HART module.
[0049] In describing the various embodiments of the invention which
are illustrated in the drawings, specific terminology will be
resorted to for the sake of clarity. However, it is not intended
that the invention be limited to the specific terms so selected and
it is understood that each specific term includes all technical
equivalents which operate in a similar manner to accomplish a
similar purpose. For example, the word "connected," "attached," or
terms similar thereto are often used. They are not limited to
direct connection but include connection through other elements
where such connection is recognized as being equivalent by those
skilled in the art.
DETAILED DESCRIPTION
[0050] 4-20 mA analogue electrical signals are used to connect
industrial devices comprising sensors and transmitters to
controlling equipment within industrial process control systems.
The 4-20 mA signal communicates conditions sensed by the sensor
such as pressure, temperature differential pressure, etc. Benefits
of the 4-20 mA standard include the ability to detect open circuit
conditions by a signal dropping to 0 mA, and that use of a current
signal is robust over long distances given that current signals are
generally resilient to noise interference and voltage drop. The
4-20 mA signal is generally referred to as the primary value
(PV).
[0051] Many industrial devices that utilize the 4-20 mA standard
also comprise a microcontroller to control their operation. As many
industrial devices are installed in hostile or inaccessible
environments, the Highway Addressable Remote Transducer (HART)
protocol is used to remotely communicate and configure a
microcontroller. HART superimposes a Frequency Shift Keying (FSK)
signal on the 4-20 mA signal, arranged so that the FSK signal can
be filtered out of the received signal, allowing both the FSK
signal and the original analogue 4-20 mA signal to be read.
[0052] An additional benefit of the HART protocol is that
additional data can be gathered from an industrial device during
operation. For example, serial numbers, fault data, calibration
data, and more accurate digital values can be read. Additionally,
HART provides for the reading of more than one value. For example,
a pressure sensor may provide a pressure signal over a 4-20 mA PV
and, through HART, may additionally provide a process
temperature.
[0053] The generation, and superimposition onto 4-20 mA signals, of
HART FSK signals is well known in the art and as such is not
described herein, it will be appreciated, however, that suitable
4-20 mA signals comprising HART FSK signals may be generated using
any suitable method. For example, HART signals may be generated as
described in co-pending European Publication No. EP2413300. In
general terms, however, each HART message (or packet) is formed
from a plurality of data bytes (8-bits), including data bytes
indicating a start of a message (i.e. a preamble), data bytes
containing the data to be transmitted, and a data byte, in the form
of a checksum, indicating an end of a message. At a HART
transmitter, each HART data byte is transmitted in an 11-bit
character, each 11-bit character comprising a start bit, the data
byte, a parity bit and a stop bit. A logic `1` (or mark) is
represented by a 1200 Hz periodic signal, while a logic `0` (or
space) is represented by a 2200 Hz periodic signal by modulating
the 4-20 mA signal through continuous phase FSK modulation.
[0054] At a receiver, the FSK signal is converted back into a
serial bit stream. Each 11-bit character is identified from the
serial bit stream (the pre-amble being identified first), the data
byte of that character obtained, and the HART message determined
from the contents of the received data bytes. Methods for decoding
received 4-20 mA signals to convert received FSK signals into a
serial bit stream without the use of a dedicated HART modem are now
described.
[0055] In FIG. 1 a decoding apparatus 1 for processing a received
analogue signal to obtain HART data is shown. An input signal 2 is
received from a HART transmitter (not shown), the input signal 2
comprising a 4-20 mA analogue signal on which has been superimposed
an FSK signal. The signal 2 is passed to a band-pass filter 3 for
filtering. The band-pass filter 3 has a passband of approximately 1
KHz to 3 KHz and outputs a filtered analogue signal 4. The
band-pass filter 3 acts to separate the HART FSK signal from the
input signal 2 and to remove noise and avoid aliasing when the
output signal 4 is sampled as described below. In the presently
described embodiment, the band pass-filter 3 has a frequency
response as shown in FIG. 2 and a pass-hand voltage gain of
approximately 3 dB. It is to be understood, however, that any
suitable filter may be used.
[0056] From the band-pass filter 3, the signal 4 is passed to a
sampler 5. According to one embodiment of the invention, the
sampler 5 is arranged to sample the signal 4 at a sampling rate of
83.3 .mu.s and at a sampling depth of 1-bit. For each sample
obtained by the sampler 5, a value of `1` is assigned if the
sampled voltage is above a threshold voltage, and a value of `0` is
assigned if the sampled voltage is at or below the threshold
voltage. In some embodiments, the threshold against which a sample
is judged is the quiescent point of the signal 4 output from the
band-pass filter 3.
[0057] The output of the sampler 5 is used by the decoding
apparatus 1 to provide a 10-bit sliding window over the signal 4.
The 10-bit sliding window slides by 1-bit to provide a new 10-bit
sample set 6 every 83.3 .mu.s. It will be appreciated that the
sliding window covers a portion of the signal 4 of the same length
as that used by the HART protocol to encode a single HART data bit
(i.e. 833 .mu.s). A sliding window of 10-bits and a sampling period
of 83.3 .mu.s has been found to provide sufficient decoding
performance in the presence of interference to be consistent with
the requirements of the HART physical layer protocol. While
increased decoding performance can be achieved with a higher
sampling rate, or a greater bit depth, this would result in
increased, and therefore more expensive, processing and storage
requirements. As such, a sampling rate of 83.3 us at a bit depth of
1-bit provides a particularly beneficial arrangement.
[0058] It is to be appreciated, however, that in other embodiments,
the sampler 5 may be arranged to sample at a greater bit depth and
at a greater sampling frequency. For example, the sampler 5 may be
configured to sample the signal 4 every 8.33 .mu.s to provide a
100-bit sliding window covering a portion of the signal 4 having
equal length to a portion of the signal 4 used to encode a HART
data bit. Indeed, it will be apparent from the teaching herein that
the sampler 5 may sample at any suitable bit depth and with any
suitable frequency. It will be appreciated that the sampler 5 may
be implemented in any appropriate way.
[0059] Each 10-bit sample set 6 is used as an index into a lookup
table (LUT) 7. The LUT 7 comprises one-thousand-twenty-four
entries, one entry for each of the possible patterns of bits in the
10-bit sample set 6. Within the LUT 7, each possible 10-bit sample
set 6 is associated with a value indicating a HART data bit. Upon
receipt of each 10-bit sample set 6 from the sampler 5 (i.e. every
83.3 .mu.s), the LUT 7 is used to provide an output 8 indicating
whether the portion of the signal 4 corresponding to the 10-bit
sample set 6 is most likely to encode a mark (`1`) (or transition
from a space to a mark) or most likely encodes a space (`0`) (or
transition from a mark to a space). For example, the output 8 of
the LUT 7 may be implemented as a single bit, but may equally be
implemented in any other way.
[0060] In every 833 .mu.s period, the LUT 7 outputs ten
indications, while in general only a single HART data bit 10 is
output in the same period by the decoding apparatus 1. In some
embodiments, a HART data bit for a particular 833 .mu.s period is
the value of a predetermined one of the ten indications output by
the LUT 7 in that period. A transition reference count (not shown
in FIG. 1) is maintained by the decoding apparatus 1, the
transition reference count being synchronized with the HART data
bit period of the signal 4. That is the transition reference count
is in Phase Locked Loop with the HART signal encoded in the signal
4.
[0061] In more detail, the decoding apparatus 1 is configured to
increment the transition reference count by a value of one, in a
loop between values of zero and nine, in response to each output of
the LUT 7. Table 1 below illustrates the state of the transition
reference count following respective outputs from the LUT 7, where
each output from the LUT 7 indicates that the portion of the signal
4 being processed most likely encodes a space (`0`) (or a
transition from a mark to a space).
TABLE-US-00001 TABLE 1 LUT output value Transition Reference Count
0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 0
[0062] When synchronized with the HART data bit period of the
signal 4, a transition reference count value of zero indicates that
the most recent sample in a 10-bit sample set 6 is likely to be a
first sample of a new HART data bit. A transition reference count
value of nine indicates that the most recent sample in the 10-bit
sample set 6 is likely to be the last sample of a HART data bit. A
transition reference count value between zero and nine indicates
that the most recent sample in the 10-bit sample set 6 is likely to
have been taken from the middle portion of a HART data bit encoded
in the signal 4.
[0063] In some embodiments, to synchronize the transition reference
count with the HART data bit period, the transition reference count
is reset upon detection of a transition between a space and a mark
within the signal 4, as illustrated in Table 2.
[0064] In Table 2, a first four outputs of the LUT 7 indicate a
space. In response to each of the first four outputs from the LUT
7, therefore, the transition reference count is incremented by one.
A fifth output of the LUT 7 indicates a mark. That is, the fifth
output of the LUT 7 indicates a possible transition between a space
and a mark in the signal 4. Upon detecting the possible transition,
the transition reference count is reset to zero. While each output
of the LUT 7 continues to indicate a mark, the transition reference
count is incremented by one for each output from the LUT 7.
TABLE-US-00002 TABLE 2 LUT output value Transition Reference Count
0 0 0 1 0 2 0 3 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9
[0065] In this way, as described above, a transition reference
count value of zero indicates the start of a HART data bit, while a
transition reference count value of nine indicates the end of a
HART data bit.
[0066] Detection of transitions between HART data bits may not
always be entirely accurate. In some embodiments, therefore, each
HART data bit value 10 output by the decoding apparatus 1 is
selected from one or more of the LUT output values 8 between a
transition reference count value of zero and a transition reference
count value of nine. For example, it may be desirable to select a
HART data bit 10 from a LUT output value 8 generated by a central
portion of a HART data bit period (e.g. a LUT output value 8 having
a transition reference count value of four, or five). In some
embodiments, therefore, the processor 9 is configured to output a
HART data bit 10 corresponding to the output 8 of the LUT 7 having
a corresponding transition reference count of a particular
predetermined value. In one particular embodiment, the processor 9
is configured to output a HART data bit 10 having the value of the
LUT output 8 with a corresponding transition reference count of
4.
[0067] By outputting a HART data bit 10 with a value selected from
a value output by the LOT 7 in response to processing a middle
portion of a HART data bit of the signal 4, the decoding apparatus
1 is more robust to false transitions. That is, while it is
possible to output a HART data bit 10 immediately upon detecting a
transition, noise in the signal 4 may result in false detections of
HART data bit transitions.
[0068] In some embodiments, the processor 9 is configured to output
an average (e.g. a mode) of a predetermined number of received
outputs 8. For example, the output 8 of the LUT 7 having transition
reference values of three, four and five may be subject to a
two-out-of-three vote, with the result of the vote being provided
as the HART data bit 10.
[0069] In some embodiments, to further mitigate the effects of
noise, the transition reference count is not reset to zero
immediately upon detection of a possible transition in the signal
4. For example, in some embodiments, upon detection of a possible
transition, the transition reference count is adjusted so as to be
closer to zero, without actually being reset to zero. For example,
the transition reference count may be incremented or decremented by
one in dependence upon on whether an increment or a decrement would
result in the transition reference count being closer to zero. In
this way, noise resulting in a false indication of a transition in
the signal 4 does not cause large fluctuations in the transition
reference count (and corresponding loss of synchronization). Table
3 illustrates an example in which the transition reference count is
adjusted towards zero, rather than directly reset upon detection of
a possible transition.
TABLE-US-00003 TABLE 3 LUT output value Transition Reference Count
0 0 0 1 0 2 0 3 1 3 0 4 0 5 0 6 0 7 0 8 1 0 1 1 1 2 1 3
[0070] In Table 3, the first four outputs from the LUT 7 each
indicate that the portion of the signal 4 being processed likely
encodes a space. A fifth output of the LUT 7 indicates a possible
transition between a space and a mark. At the fifth output, in the
absence of a possible transition, the transition reference count
would be incremented to four. In light of the possible transition,
however, the transition reference count is adjusted in the
direction closest to zero. In this case, therefore, the transition
reference count is decremented from a value of four, to a value of
three.
[0071] A sixth output of the LUT 7 indicates that the portion of
the signal 4 being processed likely encodes a space. This indicates
that the fifth output of the LUT 7 did not indicate a genuine
transition. It will be appreciated that because the transition
reference count was only adjusted by a single decrement, any
detrimental effect on the synchronization between the transition
reference count and the HART data bit period of the signal 4 is
reduced.
[0072] An eleventh output of the LUT 7 again indicates that the
portion of the signal 4 being processed indicates a possible
transition between a space and a mark. In this case, the transition
reference count is incremented by one (as in this case incrementing
moves the transition reference count closer to a value of zero). As
such, while the transition reference count would have had a value
of nine, the transition reference count takes a value of zero. The
twelfth, thirteenth and fourteenth outputs of the LUT 7 continue to
indicate a transition towards a mark, indicating that the
transition indicated by the eleventh output was a genuine
transition. As such the adjustment to the transition reference
count in the direction of zero at the eleventh output of the LUT 7
is such that the transition reference count is again synchronized
to the HART data bit period of the signal 4.
[0073] FIG. 3 is a flowchart illustrating processing that may be
performed by the decoding apparatus 1 to select a HART output value
10 based on the outputs from the LUT 7 and the transition reference
count. At a step S1, a 10-bit sample set 6 is obtained from the
outputs of the sampler 5. The 10-bit sample set 6 is used as an
index to the LUT 7 at step S2 to obtain a LUT output indicating
whether the 10-bit sample set 6 indicates a mark or a space.
Processing passes to step S3 at which the transition reference
count is incremented. Processing passes to step S4 at which it is
determined whether the LUT output generated at step S2 indicates a
transition between data bits. Hit is determined at step S4 that the
LUT output of step S2 does indicate a transition, processing passes
to step S5. At step S5 the transition reference count is adjusted
towards zero. For example, if the transition reference count has a
value of `1`, `2`, `3` or `4`, the transition reference count is
decremented, while if the transition reference count has a value of
`5`, `6`, `7`, or `9`, the transition reference count is
incremented. It will be appreciated that if the transition
reference count has a value of `5`, it may be incremented or
decremented. If the transition reference count has a value of `0`,
it is not adjusted. Processing passes from step S5 to step S6.
[0074] If, on the other hand, it is determined at step S4 that the
LUT output of step S2 does not indicate a transition, processing
passes from step S4 to step S6. At step S6 it is determined whether
the transition reference count has a predetermined value. In the
example of FIG. 3, the predetermined value is `4`, though it will
be appreciated that the predetermined value may be any
predetermined value between `0` and `9` in the example of FIG. 3,
and any value more generally depending upon the implementation of
the transition reference count. If it is determined at step S6 that
the transition reference count is at the predetermined value,
processing passes to step S7 at which the LUT output of step S2 is
provided as a HART data bit. That is, if the LUT output of step S2
indicates a mark, the HART data bit output at step S7 is a mark,
while if the LUT output of step S2 indicates a space, the HART data
bit output at step S7 is a space.
[0075] Processing passes from step S7 to step S8 at which the
sliding window is moved by one sample before processing passes back
to step S1. If, on the other hand, it is determined at step S6 that
the transition reference count is not at the predetermined value,
processing passes directly from step S6 to step S8.
[0076] It will be appreciated that in other embodiments,
adjustments to the transition reference count may be made other
than those described above. For example, upon detection of a
possible transition the transition reference count may be
incremented or decremented in the direction of zero by a value
greater than one.
[0077] A method for populating the LUT 7 is now described with
reference to FIG. 4. Generally, the LUT 7 is populated by
determining, for each possible 10-bit sample set 6, whether the
HART signal that results in that 10-bit sample set 6 (i.e. the HART
signal which causes the sampler 5 to output that 10-bit digital
sample set) represents a mark or a space.
[0078] The population of a single row of the LUT 7 is schematically
illustrated in FIG. 4. From a 10-bit digital value 11, a frequency
distribution 12 is calculated. The frequency distribution 12 may be
calculated by applying a Fourier transform (e.g. using a Fast
Fourier Transform (FFT) algorithm) to the 10-bit value 11, for
example. In other embodiments, the frequency distribution 12 may be
calculated by applying a Fourier transform to an electrical signal
that samples to the 10-bit value 11.
[0079] From the frequency distribution 12 a HART data bit 13 is
determined in accordance with the HART protocol. In particular, if
the frequency distribution 12 is centered on or around 1200 Hz, a
mark (`1`) data bit 13 is recorded in the LUT 7, while if the
frequency distribution 12 is centered on or around 2200 Hz, a space
`0` data bit 14 is recorded. Together, the 10-bit digital value 11
and the selected HART data bit 13, 14 constitute a single row of
the LUT 7.
[0080] The LUT 7 need only be populated once, and may be populated
"offline" (i.e. before the decoding apparatus 1 is used to decode
signals in a live system). In this way, minimal "online" processing
is required to determine a HART value from a received signal 2. As
such, operations such as Fourier transforms need not be performed
online.
[0081] It is described above that the LUT 7 contains 1024 values.
It will be appreciated that where the sampler 5 samples at a
greater bit-depth or a greater bit-rate, the number of entries in
the LUT 7 will be greater.
[0082] The serial output 10 of the processor 9 may be passed to a
further processor (not shown) arranged to process the output to
determine a complete HART message from the outputs 10. While the
decoding apparatus 1 is schematically illustrated and described
above as comprising a separate filter, sampler, LUT and processor,
it will be appreciated that the decoding apparatus 1 may be
implemented in any way. For example, the decoding apparatus 1 may
be implemented with one or more dedicated hardware devices and/or
in software/firmware.
[0083] Embodiments for decoding analogue current signals which may
be used in industrial process control systems were described above.
An example of an industrial process control system in which
embodiments described above may be used is schematically
illustrated in FIG. 5, in which a distributed architecture is
designed to be used in different SIL (Safety Integrity Level)
environments, so that if a high SIL is required it can be provided,
but if a low SIL is all that is needed the system can be reduced in
complexity in order to reduce unnecessary extra costs.
[0084] In FIG. 5, an exemplary industrial process control system
100 comprises a workstation 102, one or more controllers 104, and a
gateway 106. The workstation 102 communicates with the controllers
104 and the gateway 106 via Ethernet connections 108 to one or more
control networks 103. Multiple Ethernet connections 108 provide
redundancy to improve fault tolerance. The workstation 102 may be
connected via a conventional Ethernet connection 101 to another
external network 105.
[0085] An example implementation of a controller 104 is described
in more detail with reference to FIGS. 6 and 7. Referring to FIG.
6, the depicted controller 104 comprises an input assembly 202, a
processor assembly 204, and an output assembly 206. In the
schematic illustration of FIG. 6, the input assembly 204 and output
assembly 206 are on different backplanes but it will be appreciated
that in other embodiments, the input assembly 204 and the output
assembly 206 may equally well share a single backplane.
[0086] Assemblies 202, 204, 206 are created from one or more
communications backplane portions. Each backplane portion comprises
three slots, each slot able to accommodate respective modules,
together with termination assemblies. Each termination assembly
comprises up to three slots, which interface to field sensors and
transducers. A termination assembly may straddle two contiguous
backplane portions. A module comprises a plug-in card with multiple
connectors for plugging onto a communications backplane and a
termination assembly.
[0087] It will be appreciated that while the exemplary arrangement
shown in FIG. 6 comprises three slots per backplane portion, other
arrangements with greater (or fewer) slots per backplane portion
may be provided in other embodiments.
[0088] FIG. 7 illustrates an example physical configuration of the
controller 104. In the example shown in FIG. 7, the input assembly
202, output assembly 206 and processor assembly 204 are physically
separated from one another by grouping the modules of different
types onto separate communications backplanes. That is, input
modules are grouped onto one communications backplane, processor
modules are grouped onto another communications backplane, while
output modules are grouped onto yet another communications
backplane.
[0089] In the example shown in FIG. 7, the input assembly 202
comprises two communications backplane and termination assembly
portions, 202', 202''. The first backplane and termination assembly
portion 202' has a triplex input termination to which are connected
three input modules 202a, 202b, 202c; the second backplane and
termination assembly portion 202'' has a duplex input termination
assembly to which are connected two input modules 202d, 202e. The
processor assembly 204 comprises a single processor backplane
portion 204' having three processor modules 204a, 204b and 204c.
The output assembly 206 comprises two backplane portions 206',
206''. The first backplane portion 206' has a duplex output
termination assembly with two output modules 206a, 206b and the
second backplane portion 206'' has a simplex output termination
assembly with a single output module 206c.
[0090] An alternative embodiment of the input assembly 202 is shown
in FIG. 8. In FIG. 8, the input assembly 202 comprises three
backplane portions/termination assemblies 202' 202'' 202'. For
example, a triplex backplane portion 202' having three modules
202a, 202b, 202c might be used for an application having a
requirement of high availability, a duplex backplane portion 202''
having two modules 202d, 202e might be provided for fault tolerant
applications and a simplex portion 202''' with a single modules
202f might be provided for failsafe applications. The termination
assemblies may be provided with different types of field
conditioning circuits. For example assembly 202' may be provided
with a 24V DC field conditioning circuit 401 assembly 202'' may be
provided with a 120V DC field conditioning circuit 402 and assembly
202''' may be provided with a 4-20 mA field conditioning circuit
403. Similarly possible configurations are shown for an output
assembly 206. It will be appreciated that numerous configurations
of backplane portions and termination assemblies with various
different numbers of modules and various different types of field
conditioning circuits are possible and are in no way limited to
those shown in these examples.
[0091] Where an assembly provides more than one module for
redundancy purposes it is possible to replace a failed module with
a replacement module whilst the industrial process control system
is operational which is also referred to herein as online
replacement (i.e. replacement is possible without having to perform
a system shutdown). Online replacement is not possible for a
simplex assembly without interruption to the process. In this case
various "hold last state" strategies may be acceptable or a sensor
signal may also be routed to a different module somewhere else in
the system.
[0092] The processor assembly configures a replacement processor
module using data from a parallel module before the replacement
module becomes active.
[0093] The field conditioning circuits 401, 402, 403 transform a
signal received from a sensor monitoring industrial process control
equipment to a desired voltage range, and distribute the signal to
the input modules as required. Each field conditioning circuit 401,
402, 403 is also connected to field power and field return (or
ground) which may be independently isolated on a channel by channel
basis from all other grounds, depending on the configuration of the
input termination assembly. Independent channel isolation is the
preferred configuration because it is the most flexible. The field
conditioning circuits 401, 402, 403 comprise simple non active
parts and are not online replaceable.
[0094] FIGS. 9, 10 illustrate the flexibility of the industrial
process control architecture described herein, each showing an
alternative configuration of a triplex system for generating a
signal with a high availability requirement. Referring to FIG. 9,
an input assembly 501 comprises three replicated input modules
503a, 503b, 503c. The input assembly 501 receives a signal 506 from
a sensor 500 via a field conditioning circuit in termination
assembly 504. The signal may be, for example, a 4-20 mA signal
including a HART FSK signal. The field conditioning circuit in the
termination assembly 504 transforms the signal to a desired voltage
range and distributes the signal to the three replicated input
modules 503a, 503b, 503c. Each input module 503a-c processes the
signal and the results of that processing are sent to a
two-out-of-three voter 502 which generates a result signal 507 in
dependence upon the output of each processing module 503a-c.
[0095] Referring to FIG. 10, three replicated sensors 600a-c, each
send a respective signal to respective simplex assemblies 601a-c
via respective field conditioning circuits in termination
assemblies 604a-c. Each input module 603a, 603b, 603c processes the
signal and provides its output to a two-out-of-three voter 602 to
generate a signal in dependence thereon. It will be appreciated
that many variations and configurations are possible in addition to
those illustrated here.
[0096] FIG. 11 illustrates schematically an input module 700 in
accordance with the present invention:
[0097] An input module 700 comprises eight isolated input channels
701. Each input channel 701 receives signals 702, 703a, 703b from
field conditioning circuits in a termination assembly 704. Each
input channel 701 communicates with a field programmable gate array
(FPGA) 705 which interfaces to a backplane (not shown) via a
non-isolated backplane interface 706. Light emitting diodes (LEDs)
707 may be provided to indicate a status of the input module
700.
[0098] It will be appreciated that having eight channels is merely
one design option and other embodiments may comprise greater (or
fewer) channels.
[0099] Programmable I/O pins of the FPGA 705 are used to directly
drive low power isolated supplies, supplying the channels 701
without the need for additional power amplifiers.
[0100] Referring to FIG. 12, an analogue input field conditioning
circuit for measuring field 4-20 mA analogue current loop signals
from a field device comprises a fuse 1001 in series with sense
resistors 1003, 1004. The use of the fuse means that the sense
resistors 1003, 1004 only need to operate to the maximum rating of
the fuse, which in some embodiments is 50 mA. A blown fuse signal
702 is output to the input module to allow the input module to
sense and report a blown fuse condition.
[0101] The field conditioning circuit shown in FIG. 12 outputs a
primary sense signal 703a and a secondary sense signal 703b for use
by the input channel 701.
[0102] One of the input channels 701 is illustrated in FIG. 13 and
comprises a blown fuse circuit 1011, a primary input circuit 1013
and a secondary input circuit 1012 for detecting the 4-20 mA
signal, which will not be described further here. The input channel
701 also comprises a HART input/output (I/O) module 1018. The HART
I/O module 1018 is connected to an analogue input which bears
secondary signal 703b and is arranged to send and receive HART data
to a field device. Generation and transmission of HART data is not
described in detail herein. In general, however, HART output data
is generated in dependence upon a digital input signal 1014a
received from a microprocessor 1014. Output from the HART I/O
module 1018 is enabled by a HART enable signal 1018a supplied by
microprocessor 1015. A signal 1014b, representing HART input data
is sent to the microprocessor 1014 for decoding. The signal 1014b
may be generated according to the methods described above with
reference to FIGS. 1 to 4.
[0103] It will be appreciated that while particular arrangements
for providing industrial control systems are described above,
embodiments for decoding FSK signals from a received analogue
signal are more widely applicable. Indeed, embodiments described
herein may be used to decode any FSK signal superimposed on a
received analogue signal. Similarly, while embodiments herein are
described with reference to the HART protocol, it will be
appreciated that the methods described herein are not limited to
the HART protocol, but may be used, generally, to decode digital
data encoded within electronic signals.
[0104] It should be understood that the invention is not limited in
its application to the details of construction and arrangements of
the components set forth herein. The invention is capable of other
embodiments and of being practiced or carried out in various ways.
Variations and modifications of the foregoing are within the scope
of the present invention. It also being understood that the
invention disclosed and defined herein extends to all alternative
combinations of two or more of the individual features mentioned or
evident from the text and/or drawings. All of these different
combinations constitute various alternative aspects of the present
invention. The embodiments described herein explain the best modes
known for practicing the invention and will enable others skilled
in the art to utilize the invention.
* * * * *