Process control transmitter

A transmitter in a process control system includes input/output circuitry for coupling to a process control loop. A first sensor having a first impedance is responsive to a first sensed parameter. A second sensor having a second impedance is responsive to a sensed parameter. First and second excitation signals are applied to the first and second sensors. A summing node sums the outputs of the first and second sensors. An analog to digital converter provides a digital output representative of the summed signals. Digital signal processing circuitry coupled to the analog to digital converter provides an output related to the outputs of the first and second sensors to the input/output circuitry for transmission over the process control loop.

BACKGROUND OF THE INVENTION 
The present invention relates to a process control transmitter having an 
analog to digital converter providing a digital representation of a sensor 
input signal. More specifically, the present invention relates to a 
process control transmitter having a sensor producing a sensor signal 
representative of a sensed parameter which is converted into digital 
representation of the sensor signal. The sensor signal is representative 
of a sensed parameter. 
Transmitters in the process control industry typically communicate with a 
controller over the same two wires from which they receive power. A 
transmitter receives commands from a controller and sends output signals 
representative of a sensed physical parameter back to the controller. A 
commonly used method is a current loop where the sensed parameter is 
represented by a current varying in magnitude between 4 and 20 mA. 
The transmitter includes a sensor for sensing a physical parameter related 
to a process. The sensor outputs an analog signal which is representative 
of one of several variables, depending on the nature of the process to be 
controlled. These variables include, for example, pressure, temperature, 
flow, pH, turbidity and gas concentration. Some variables have a very 
large dynamic range such as flow rate where the signal amplitude of the 
sensor output changes by a factor of 10,000. 
An analog to digital converter in the transmitter converts the analog 
sensor signal to a digital representation of the sensed physical parameter 
for subsequent analysis in the transmitter or for transmission to a remote 
location. A microprocessor typically compensates the sensed and digitized 
signal and an output circuit in the transmitter sends an output 
representative of the compensated physical parameter to the remote 
location over the two wire loop. The physical parameter is typically 
updated only a few times per second, depending on the nature of the 
process to be controlled, and the analog to digital converter is typically 
required to have 16 bits of resolution and a low sensitivity to noise. 
Charge balance converters are used in transmitters to provide analog to 
digital conversions. One such converter is described in U.S. Pat. No. 
5,083,091 entitled "Charged Balanced Feedback Measurement Circuit" which 
issued Jan. 21, 1992 to Frick et al. Sensors in such transmitters provide 
a impedance which varies in response to the process variable. An output 
from the impedance is converted by the charged balance converter into a 
digital representation of the impedance. This digital representation can 
be transmitted across an isolation barrier which isolates the sensor 
circuitry from the other transmitter circuitry. Charge balance converters 
are a type of sigma-delta (.SIGMA..DELTA.) converter. The output of such a 
converter is a serial bit stream having a width of 1 bit. This 1 bit wide 
binary signal contains all of the information necessary to digitally 
represent the amplitude and frequency of the output signal from the sensor 
impedance. The serial format of the output is well suited for transmission 
across the isolation barrier. The sigma-delta converter also provides a 
high resolution output with a low susceptibility to noise. 
SUMMARY OF THE INVENTION 
The present invention provides a technique for multiplexing more than one 
signal onto an analog to digital converter in a transmitter for a process 
control system. These signals may be the outputs from a process variable 
sensor, a reference, or other sensors used for compensation. In general, 
these signals are referred to as sensed parameters. The transmitter 
includes input/output circuitry for coupling to a process control loop. A 
first sensor has a first impedance which varies in response to a sensed 
parameter, for example a process variable of the process. A second sensor 
has a second impedance which varies in response to another sensed 
parameter. A first excitation signal is provided to the first sensor and a 
second excitation signal is provided to the second sensor. Outputs from 
the first and second sensors are responsive to the first and second 
excitation signals and sensed parameters. A summing node sums the outputs 
from the first and second sensors. An analog to digital converter converts 
the summed signals into a digital format. Digital signal processing 
circuitry extracts the sensed parameters from the digital output of the 
analog to digital converter. The digital signal processing circuitry 
provides an output based upon the sensed parameters, to the input/output 
circuitry for transmission over the process control loop.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a simplified block diagram of a transmitter 10 in accordance with 
one embodiment of the present invention coupled to process control loop 12 
at connection terminals 14. Transmitter 10 includes measurement circuitry 
16 and sensor circuitry 18. Measurement circuitry 16 couples to two-wire 
loop 12 and is used for sending and receiving information on loop 12. 
Measurement circuitry 16 also includes circuitry for providing a power 
supply output for transmitter 10 which is generated from loop current I 
flowing through loop 12. In one embodiment, measurement circuitry 16 and 
sensor circuitry 18 are carried in separate compartments in transmitter 12 
and electrically isolated by isolator 20. Isolator 20 is an isolation 
barrier required for electrically grounded sensors. Sensor circuitry 18 
includes a sensor (shown as impedance) 22 which has a plurality of 
variable impedances responsive to sensed parameters. As used herein, 
sensed parameters include process variables representative of a process 
(i.e. temperature, pressure, differential pressure, flow, strain, pH, 
etc.), reference levels and compensation variables such as sensor 
temperature used to compensate other sensed variables. Excitation signals 
are provided to impedance 22 by excitation input circuitry 24 over the 
electrical connection 26. Other excitation signals could include optical, 
mechanical, magnetic, etc. Impedance 22 produces output signals on output 
27 in response to the excitation input signals from excitation input 24. 
The output signals are variable based upon the sensed parameters. 
In the present invention, impedance element 22 includes one or more 
separate variable impedances coupled to different excitation signals from 
excitation input 24. Each individual impedance provides an output signal 
to conversion circuitry 28 which combines and digitizes the signals into a 
single digital output stream. Conversion circuitry 28 provides an output 
on output line 30 to isolator 20 which electrically isolates conversion 
circuitry 28. Isolator 20 reduces ground loop noise in measurement of the 
sensed parameters. Isolator 20 provides an isolated output on line 32 to 
measurement circuitry 16. Measurement circuitry 16 transmits a 
representation of the digitized signal received from conversion circuitry 
28 on loop 12. In one embodiment, this representation is an analog current 
level or a digital signal. In a preferred embodiment, measurement 
circuitry 16 receives the digital signal and recovers the individual 
signals generated by the separate impedances in impedance element 22. 
Lines 26, 27, 30 and 32 may comprises any suitable transmission medium 
including electrical conductors, fiber optics cables, pressure passage 
ways or other coupling means. 
FIG. 2 is a more detailed block diagram of transmitter 10 which shows 
transmitter 10 coupled to control room circuitry 36 over two-wire process 
control loop 12. Control room circuitry 36 is modeled as a resistor 36A 
and voltage source 36B. Current I.sub.L flows from loop 12 through 
transmitter 10. 
In the embodiment shown in FIG. 2, sensor 22 includes capacitor pressure 
sensors 40H and 40L having capacitance C.sub.H and C.sub.L which respond 
to pressures P.sub.H and P.sub.L, respectively. The capacitance C.sub.H 
and C.sub.L are representative of a sensed pressure of a process, for 
example. Capacitor 40L receives excitation input signal S.sub.1 over input 
lines 26 from input circuitry 24. Capacitor 40H receives excitation input 
signal S.sub.2 over input lines 26 from input circuitry 24. Capacitors 40H 
and 40L responsively generate output signals O.sub.H and O.sub.L on output 
lines 42H and 42L, respectively. Output lines 42H and 42L are coupled 
together at a summing node 44 which couples to conversion circuitry 28 
over line 27. 
Conversion circuitry 28 includes high impedance input amplifier 46. In one 
embodiment, amplifier 46 comprises an operational amplifier 48 having 
negative feedback from an output terminal to an inverting input terminal 
through capacitor 50. The non-inverting input of amplifier 48 is coupled 
to a chassis or earth electrical ground 52. The inverting input of 
operational amplifier 48 connects to summing node 44 through line 27. The 
output from amplifier 46 is provided to sigma-delta conversion circuitry 
54 which operates in accordance with well known sigma-delta conversion 
techniques. For example, the article entitled "The Design of Sigma-delta 
Modulation Analog-to-Digital Converters", Bernhard E. Boser et al., IEEE 
JOURNAL OF SOLID-STATE CIRCUITS, Vol 23, No. 6, December 1988, pgs. 
1298-1308 describes design of sigma-delta converters. Sigma-delta 
conversion circuitry 54 should be constructed to have a sufficiently high 
sampling rate and resolution for the particular sensor used for sensor 22 
across the dynamic range of the sensor output. Sigma-delta conversion 
circuitry 54 provides a bit stream output having a width of a single bit 
on line 30. This digital output contains all of the information necessary 
to digitally represent the amplitude phase and frequency of the input 
signal provided by amplifier 46. 
Excitation signals S.sub.1 and S.sub.2 from excitation input circuitry 24 
may be generated using any appropriate technique. In the embodiment shown, 
signals S.sub.1 and S.sub.2 are generated using a digital signal generator 
60 which provides digital signal outputs D.sub.1 and D.sub.2 to a digital 
to analog converter 62. Digital to analog converter 62 responsively 
generates analog signals S.sub.1 and S.sub.2. Generator 60 is coupled to 
conversion circuitry 54 and provides clock signal to circuitry 54. In one 
preferred embodiment, signals S.sub.1 and S.sub.2 are sinusoidal signals 
having a frequency of about 10 Hz to about 100 H.sub.z and a relative 
phase shift of 90.degree.. In one embodiment, the output of signal 
generator 60 is adjusted to compensate for manufacturing process 
variations in capacitors 40H and 40L. For example, phase, frequency, 
waveshape and amplitude can be adjusted. Signal generator 60 receives 
clock and communication signals through isolator 20B. The clock signal is 
also used by power supply 61 to generate an isolated supply voltage 
V.sub.SI which powers circuitry 18. 
Measurement circuitry 16 includes a microprocessor/digital signal processor 
70 which receives the output from sigma-delta conversion circuitry 54 
through isolator 20A and decimating filter 72. In one embodiment, the 
output of filter 72 carried on data bus 73 is 16 to 24 bits in width 
having 24 bits of resolution. Decimating filter 72 reformats the single 
bit wide data stream on line 32 having a lower data rate digital into a 
byte-wide data stream for use by microprocessor 70. Microprocessor/digital 
signal processing circuitry 70 also receives an input from input circuitry 
24 which provides a reference signal relative to excitation input signals 
S.sub.1 and S.sub.2. Microprocessor 70 processes the digitized signal and 
extracts the signals generated from each of the individual capacitors 40H 
and 40L. Typically, the two different signals are extracted using 
information indicating the phase, frequency and amplitude of excitation 
signals D.sub.1 and D.sub.2. Microprocessor 70 calculates absolute 
pressure sensed by capacitor 40H, absolute pressure sensed by capacitor 
40L and differential pressure. Microprocessor 70 provides this information 
to input/output (I/O) circuitry 74 over data bus 76. I/O circuitry 74 
couples to processor control loop 12 through terminals 14 and receives 
loop current I.sub.L. I/O circuitry 74 generates a power supply voltage 
V.sub.S for powering circuitry 16 transmitter 10 from current I.sub.L. I/O 
circuitry 74 transmits information related to sensed pressure to control 
room 36 over loop 12. Transmission of this information is through control 
of current I.sub.L, by digital transmission or by any suitable 
transmission technique. 
FIG. 3 is a vector diagram signals O.sub.H, O.sub.L, and O.sub.H +O.sub.L. 
FIG. 3 shows the combination of O.sub.H +O.sub.L generated by the analog 
summation at summing node 44. The individual signals O.sub.H and O.sub.L 
can be recovered by determining amplitude at +45.degree. and -45.degree., 
respectively. This allows the pressures P.sub.H and P.sub.L sensed by 
capacitors 40.sub.H and 40.sub.L to be determined. The phase shift of the 
combined O.sub.H +O.sub.L signal, .theta..sub.R, can be measured in the 
time domain in order to determine P.sub.H -P.sub.L with maximum accuracy 
and resolution. 
The technique shown in FIG. 2 is useful for transmitting a number of 
different channels of information across a single isolator in a 
transmitter. For example, the sensor circuitry of a transmitter may 
measure any sensed parameter such as differential pressure, absolute 
pressures, change in temperature, absolute temperature and sensor 
temperature. Additional parameters are used to compensate differential 
pressure and absolute pressure readings. In the present invention, 
capacitor sensors may be employed for all channels of information and 
excited using signals of differing frequencies, phases, amplitudes, or 
wave shapes. Outputs of these capacitor sensors are summed in the analog 
domain and digitized using an analog to digital converter. The digital 
signal is then transmitter across the isolator to the measurement 
circuitry where the individual signals are identified using digital signal 
processing. These signals may be compensated and used in computations 
prior to transmission over the process control loop. The digital signal 
processing computes the amplitude and phase of each frequency component. 
For example, digital filters may be employed to separate the signals. The 
outputs can be further processed to measure amplitude and phase. A 
discrete fourier transform DFT implemented with a fast fourier transform 
FFT may be used to provide a spectrum of the signal which is examined to 
determine the magnitude of the individual signals at desired frequencies. 
In one embodiment, analog filters are used to recover the individual 
signals, however, analog filters may have limited resolution. 
In one embodiment, excitation signals are signals of different frequencies 
generated relative to the frequency of a system clock. Digital signal 
processing circuitry uses the clock signal as a reference to identify 
signals generated in response to the different excitation signals. In 
other embodiments, differing phases or amplitudes of the excitation 
signals may be used. 
FIG. 4 is a simplified electrical diagram of sensor circuitry 150 in 
accordance with another embodiment. Sensor circuitry 150 includes 
capacitor sensors 152, 154, 156, 158 and 160. Capacitor sensor 152 
measures pressure P.sub.1, capacitor sensor 154 measures pressure P.sub.2 
and the combination of sensors 156 and 158 measure pressures P.sub.1 
-P.sub.2. Capacitor sensor 180 provides a calibration capacitance which is 
used to calibrate the system and measure system errors. Variable 
resistances 162 and 164 vary in response to temperatures T.sub.1 and 
T.sub.2 and are coupled to the non-inverting input of operational 
amplifier 166 which is connected with negative feedback and provides a 
buffer. The output of amplifier 166 is connected to capacitor 168. 
Variable impedances 152 through 164 are connected to signal sources 172, 
174, 176, 178, 180 and 182 which provide excitation signals e.sub.1, 
e.sub.2, e.sub.3, e.sub.4, e.sub.5 and e.sub.6, respectively. FIG. 4 also 
shows the waveforms of signals e.sub.1 through e.sub.6 adjacent each 
signal generator 172 through 182. Signal e.sub.1 has a frequency of 
f.sub.1 and 0.degree. of phase shift. Signals e.sub.2 and e.sub.3 are also 
at a frequency f.sub.1 but shifted 180.degree. and 90.degree., 
respectively, in phase. Signal e.sub.2 is at a second frequency f.sub.2 
which is shown in the example as being equal to f.sub.1 /2. Signals 
e.sub.5 and e.sub.6 are shown at a third frequency f.sub.3 which is shown 
as 2.times.f.sub.1. Signal e.sub.6 is shifted 180.degree. relative to 
e.sub.5. In embodiments in which the excitation signals are 180.degree. 
apart, signal processing circuitry will not be able to isolate the 
individual excitation signals. 
Outputs from capacitors 152 through 160 and 168 are connected to summing 
node 170 at the inverting input of amplifier 184. Amplifier 184 is shown 
as operational amplifier 186 having negative feedback through an 
integrating capacitor 188 given as: 
##EQU1## 
where: e.sub.n =excitation signals from 172-182; 
c.sub.n =capacitor values 152-160 and 168; and 
C.sub.I =capacitor value of 188. 
Amplifier 184 provides an output to analog to digital converter 190 which 
is representative of a summation of the outputs from capacitors 152 
through 160 and 168. 
Temperature is sensed by resistors 162 and 164 which vary in resistance in 
response to temperatures T.sub.1 and T.sub.2. Resistors 162 and 164 
selectively weight signals e.sub.5 and e.sub.6 in a mixing operation and 
provide the mixed signals to capacitor 168 through amplifier 166. Digital 
signal processing circuitry (not shown in FIG. 4) identifies outputs from 
capacitors 152 through 160 and 168 and determines pressures P.sub.1, 
P.sub.2, P.sub.1 -P.sub.2, reference capacitance C.sub.R and differential 
temperature T.sub.1 -T.sub.2. All of these are representative of sensed 
parameters. In one embodiment, the sensed parameter C.sub.R which is 
representative of a reference capacitance is used to compensate and 
determine errors in other measurements. 
Although the example in FIG. 4 shows sine waves at integral frequency 
multiples, other non-sinusoidal signals could be used and signals which 
are non-integral frequency multiples, aperiodic, random or pseudorandom, 
band limited or any desired combination may be employed. Non-sinusoidal 
signals could be used to generate linear, non linear or logarithmic phase 
outputs. Amplitudes, frequency or phase of the excitation signals could be 
controlled as a function of sensed parameters to generate desired transfer 
functions. Broadband deterministic or random excitation signals can be 
used to increase immunity to narrow band interferences. For example, 
pseudo random sequences can be used as excitation signals. This would be a 
code division multiplexing system similar to that used in the multiuser 
communications systems (CDMA). 
Determination of the sensed parameter may be through any appropriate signal 
processing technique. For example, the instantaneous frequency shift 
associated with a change in phase may be employed to detect change in 
pressure. This is expressed with the following equations that hold true 
during the change: 
##EQU2## 
Where f.sub.EX is the frequency of the excitation signal, f.sub.OUT is the 
output from a capacitor sensor K is a constant and .theta. is the phase 
shift. C is a constant of proportionality which converts 
K.multidot..theta. into change in pressure. 
Distortions to sinusoidal signals may also be employed as excitation 
signals and used to optimize sensitivity of the sensor circuitry. For 
example, FIG. 5A shows a distorted sinusoidal signal and FIG. 5B shows the 
sinusoidal signal of 5A shifted 90.degree. in phase. The distorted sine 
waves shown in FIGS. 5A and 5B increase the sensitivity of the measurement 
circuitry in the region of .DELTA.P=0 (i.e. C.sub.H =C.sub.L). It is also 
possible to adjust the waveform such that there is a logarithmic 
relationship in the output signal and the analog to digital converter does 
not need as large a dynamic range. 
It is also possible to use a reference waveform in the measurements. In 
this embodiment, C.sub.H and C.sub.L are driven with excitation signals 
which are 180.degree. shifted in phase. A reference capacitor is driven 
with a waveform shifted 90.degree. relative to either of the waveforms 
used to drive C.sub.H and C.sub.L. The resulting output amplitude is as 
follows: 
##EQU3## 
Where C.sub.H and C.sub.L are the capacitance values of the high and low 
pressure sensors and C.sub.R is a reference capacitance. In another 
embodiment, phase is measured twice per cycle to eliminate 1/f noise and 
zero offset errors in zero crossing detection. Zero offset errors will add 
and subtract the same amount of phase shift to the two signals and 
therefore cancel each other out. 
The present invention overcomes a number of problems associated with the 
prior art. For example, one prior technique uses time multiplexing which 
increases the possibility of aliasing noise and limits the ability to 
adjust resolution versus response time of the conversion circuitry. Using 
multiple analog to digital converters increases power consumption. 
Further, the converters may interact with in unpredictable ways and 
complicate isolation of the sensor circuitry. In addition, using two 
converters to measure a difference signal doubles the error magnitude. The 
present invention uses a low power technique by utilizing a large portion 
of the available band width of the analog to digital converter, 
particularly a sigma-delta converter. Fewer parts are required because 
only a single converter is utilized. Interactions between various 
components are minimized and are more predictable. Aliasing is limited 
because all of the sensed parameters can be monitored at the high sampling 
frequency of a sigma-delta converter and antialiasing digital filters can 
be incorporated before the microprocessor samples the sensor output. 
Variations on the particular implementation set forth herein are considered 
within the scope of the invention. For example, any or all of the 
functions may be implemented in analog or digital circuitry such as signal 
generation, transmission across an isolator, filtering, signal processing, 
compensation, transmission, etc. These techniques are well suited for 
reducing noise during measurements, even if a single sensed parameter is 
being measured. Further, any appropriate implementation of the various 
features are considered within the scope of the invention. The generation 
of the excitation signal may be through other techniques than those 
disclosed. The particular technique for summing the outputs from the 
impedance elements may be varied, different types of filters or digital to 
analog and analog to digital converters may be employed. Any appropriate 
impedance or any number of elements may be used having an impedance which 
varies in response to a sensed parameter may be employed. Other techniques 
for detecting and identifying individual sensor outputs may be used as 
well as other synchronization or power generation techniques. Signal 
processing techniques such as fuzzy logic, neural networks, etc. may also 
be employed. Other signal processing techniques such as lock-in amplifier 
technology, implemented in either digital or analog technologies may also 
be employed. Lock-in amplifiers are well suited for identifying and 
isolating a signal among other signals using a reference signal. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention.