Low power process measurement transmitter
This is a continuation of application Ser. No. 06/883,982, filed July 10, 1986 (now abandoned). 
    
    
     BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     This invention relates to a transmitter for coupling an output representative of a process variable to a loop. 
     SUMMARY OF THE INVENTION 
     This invention relates to a transmitter for coupling an output representative of a process variable to a loop as a function in a measurement of the process variable utilizing the appropriate parameter or parameters for generating the output. The transmitter comprises computing means coupled to the process variable and the loop. The computing means sense the process variable, compute an output and provide the output to the loop. Receiver means are coupled to the computing means for receiving a parameter or parameters and providing the parameter or parameters to the computing means. Memory means are coupled to the computing means for receiving the parameter or parameters from the computing means and storing the parameter or parameters. The memory means comprises an energization input for selectively energizing the memory means at a first energization rate during a WRITE time interval. The energization requirements of the transmitter exceed the energization available from the loop during the WRITE time interval. Energization means are coupled to the loop for receiving energization from the loop at a second energization rate lower than the first energization rate. During the WRITE time interval, the energization means selectively couples energization at the first energization rate to the memory means. The energization means preferably comprises storage means which store energy for coupling to the memory means during the WRITE interval. In a preferred embodiment, the energization means couples multiple storage means in series to achieve the desired first energization rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a diagram of a transmitter according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a transmitter 10 is shown coupled to a process variable 12 such as a pressure, a temperature, or a flow and to a 4-20 milliampere current loop 14. The loop 14 comprises a battery or power source 16 and a readout device such as a meter or recorder represented schematically as resistance 18 coupled in series to transmitter 10. The loop 14 couples to a pair of terminals 20, 22 in the transmitter 10 and energizes the transmitter 10. The transmitter 10 controls the loop current I to vary between 4 and 20 milliamperes and the magnitude of such loop current I is representative of the sensed process variable 12. The power source 16 typically provides a limited excitation potential, for example, 17 to 30 volts. A voltage drop in the readout device 18 can reduce the excitation or energization potential available at terminals 20, 22 to as little as 12 volts in some cases. It is desired to have the transmitter energized at the 4 mA loop current level and hence there can be as little as 48  milliwatts available for energization of transmitter 10. The loop current has frequency components ranging between a steady or DC level up to 100 Hertz, for example, and hence it is a low frequency signal. 
     The transmitter 10 comprises a sensor 24 coupled to sense the process variable 12. The sensor 24 can comprise a capacitive or strain gauge pressure sensor for sensing process fluid pressure, for example. Sensor 24 couples a sensor signal along line 25 to an analog-to-digital converter or ADC 26 which converts the sensor signal to a digital signal representative of the sensed process variable 12. A line, such as line 25, as used in this specification can comprise a bus or multiple conductors. The converter 26 couples the digital signal representative of the process variable along line 27 to a computer 28 which can comprise a low power microprocessor, read-only-memory, random access memory, a clock circuit for timing the microprocessor, input-output circuitry and other circuitry useful for performing computations and control functions. The computer 28 calculates a desired magnitude for the output current I which represents the process variable 12 and couples a signal representing the desired magnitude of the current I on line 30 to a digital-to-analog converter or DAC 32. Converter 32 couples a signal on a line 34 to amplifier 36 for controlling the loop current I. The amplifier 36 couples a control signal to a current control network 38 comprising resistors 40, 42, 44, 46 and diode 48 and transistors 50 and 52 connected in a Darlington configuration for controlling the loop current I. The network 38 is coupled to terminals 20, 22 and controls the flow of current I. A sense resistor 54 is coupled in series between the network 38 and terminal 22. The potential across sense resistor 54 is representative of the loop current I and this potential is coupled along line 56 to a high impedance input 58 in converter 32. The converter 32 compares the signal representative of the desired magnitude of loop current received on line 30 from computer 28 to the signal representative of actual loop current received on line 58 and controls the current control network 38 such that the loop current I is substantially the desired value representative of the process variable 12. The sensor 24, converter 26, computer 28, converter 32, amplifier 36, network 38, and resistor 54 comprise a computing means for sensing the process variable 12 and for computing and providing the output loop current I. 
     Transmitter 10 has operating parameters which can be adjusted from time to time to suit the transmitter&#39;s application. Transmitter 10 can be a pressure transmitter and pressure transmitter parameters such as zero, span and damping can be adjusted. In some applications, it is desirable to have a linear relationship between process pressure and loop current I, but in other applications a square root relationship between process pressure and loop current I is desirable. Such relationships are transmitter parameters which can be adjusted by altering data or constants stored in a memory 60. The memory 60 preferably comprises a non-volatile memory and thus adjustable data or constants which determine transmitter parameters are stored during periods of time when the transmitter is not energized. The memory 60 is further an electrically alterable memory and hence the data determining transmitter parameters which are stored in memory 60 can be adjusted by an electrical signal applied to the transmitter 10. Parameters of transmitter 10 can thus be adjusted without opening a sealed housing 199 enclosing the transmitter circuitry. 
     A modulator/demodulator 62 can be coupled to the loop 14 at connections 64 and 66 for changing or altering a parameter of the function executed on transmitter 10 or changing the function selected to be executed. A blocking capacitor 68 is coupled in series with the modulator/demodulator 62 for preventing flow of low frequency loop current I through the modulator/demodulator 62. The modulator/demodulator 62 transmits and receives high frequency serial FSK (frequency shift keyed) signals on the loop 14. The frequency components of the signals coupled through the modulator/demodulator 62 are high frequency, preferably in the range of 1.2 Kilohertz or higher so that such high frequency communication signals do not substantially disturb the low frequency loop current I. The high frequency communication signal and the low frequency loop current are simultaneously superimposed on one another in the loop 14. The frequency separation between the communication signal and the loop current and filtering reduce interference between the two signals in the transmitter 10. An operator enters data which represents desired operating parameters into modulator/demodulator 62 and such data is included in the high frequency communication signal transmitted along loop 14 to transmitter terminal 20. The high frequency signal is coupled from terminal 20 along line 70 to a second modulator/demodulator 72. A filter network 74 comprising capacitors 130, 132, 134 and 136 and resistors 138 and 140 in line 70 blocks the flow of low frequency loop current I to the second modulator/demodulator 72 and thus provides filtering. The second modulator/demodulator 72 comprises demodulator means for demodulating the signal received from line 70 and Universal-asynchronous receiver/transmitter or UART means for coupling the received data included in the high frequency signal to the computer 28 along line 76. Computer 28 transfers the data received from second modulator/demodulator 72 to memory means 60 along line 59 during a WRITE time interval. A control circuit 57 in the computer 28 is coupled along a line 55 to the memory means 60 for controlling the memory means 60. The second modulator/demodulator 72 further comprises a modulator coupled to the UART for communicating data from the transmitter 10 to the modulator/demodulator 62. A modulated, high frequency signal is coupled from modulator/demodulator 72 through a filtering network 73 to amplifier 36. Amplifier 36 controls network 38 to superimpose the high frequency signal on the loop current I. The modulator/demodulator 62 receives the high frequency signal from loop 14 and demodulates and decodes the signal to provide a readout of data communicated by the computing means, such data can include the transmitter&#39;s operating parameters such as span, zero and damping. 
     During a WRITE time interval, the memory 60 is energized at an energization input 61 for erasing data representative of old parameters and for writing data into memory 60 representative of new parameters. The energization requirements of the memory means 60 are high during the WRITE interval. The combined energization requirements of the memory means 60 during the WRITE interval and the rest of the circuitry in transmitter 10 can greatly exceed the rate of energization available from loop 14, particularly when the desired output current is near 4 mA. The memory 60, for example, can require up to 20 milliamperes at 17 volts during the write time interval, and the energization rate required by the memory means 60 can thus be as high as 340 milliwatts at a time when as little as 48 milliwatts is available from the loop 14. If energization were coupled directly from the loop 14 to the energization input 61, the loop current I would be greatly disturbed and the output of the transmitter would thus be very inaccurate during the WRITE interval. To overcome this problem, the transmitter comprises energization means 78 which couple energization during the WRITE interval at a first energization rate to the energization input 61, and receive energization from the loop 14 at a second energization rate lower than the first energization rate such that the loop current or output is substantially undisturbed by energization of the memory means during the WRITE interval. The WRITE time interval is controlled by the control circuit 57 to a preselected interval, for example a 400 microsecond pulse, such that energization demands of the transmitter 10 do not substantially exceed the energization available from the loop. Actuations of the WRITE time interval can be spaced apart in time to further limit energization requirements from the loop. In one preferred embodiment, for example, actuations of the 400 microsecond WRITE pulses are set at a 4% duty cycle such that only a 500 microampere or less disturbance occurs in the loop current during the WRITE pulse. In another preferred embodiment, the duty cycle of the WRITE pulses is set to 2% to reduce the disturbance in the loop current during the WRITE pulse to such a low level that the 4 mA loop current is undisturbed. Energization is coupled from the loop 14 along line 70 to a voltage regulator 80. Voltage regulator 80 is coupled to a DC common potential 102. Voltage regulator 80 produces a first regulated potential &#34;+9 V&#34; on line 82 and a second regulated potential &#34;+5 V&#34; on line 84 with respect to the DC common potential on line 102. The second regulated potential and the DC common potential are coupled to the computer 28, converters 26, 32, amplifier 36, memory 60 and sensor 24 for energizing these portions of the transmitter 10. The first regulated potential is coupled along line 82 to the energization means 78. The regulator 80 limits the magnitudes of the first and second regulated potentials. A resistor 81 couples the regulator to line 70 and limits the amount of loop current which is drawn from line 70 by the regulator 80. 
     The energization means 78 comprises an energy storage network 86 coupled between lines 82 and 102 for storing energy. Energy storage network 86 comprises a capacitor 88 coupled directly to the line 82 and a capacitor 90 coupled in series with a current limiting resistance 92 to line 82. Capacitors 88 and 90 receive energization from line 82 and store the energy for later coupling to the input 61 during a WRITE interval. Multiple Zener diodes 94 in network 86 provide multiple or redundant limits on the energy stored in network 86 under fault conditions. The energization means 78 further comprises a second energy storage network 95 comprising a capacitor 96 coupled in series with a current limiting resistor 98. Multiple Zener diodes 100 in network 95 provide multiple or redundant limits on the energy stored in network 95 under fault conditions. During normal operation of transmitter 10 when there is not a WRITE interval, network 95 is coupled to a DC common potential 102 through a switch 104; network 95 is also coupled to the +9 V potential on line 82 through switch 106 and current limiting resistor 108. Capacitor 96 in network 95 is thus charged at a controlled low rate through current limiting resistor 108 during normal operation to the +9 V potential and stores energy. The control circuit 57 in computer 28 couples a control signal on line 210 through resistance 112 to the base of a transistor switch 114. The collector of transistor switch 114 is coupled to the line 82 through a resistor 116. The collector of transistor switch 114 is also coupled to the gates of switches 104 and 118. Computer 28 thus controls the conduction of switches 104 and 118 in the energization means 78. During a WRITE interval, the transistor switch 114 conducts and couples a low potential to the gates of switches 104 and 118. The switch 102 is opened and the switch 118 is closed during the WRITE interval. The closure of switch 118 couples a potential through energy storage network 95 to switches 106 and 110 shutting off switch 106 and turning on a switch 110. The switching of switches 104 and 118 couples a signal through network 95 which inherently controls the switching of switches 106 and 110 and hence further control circuitry is not needed for controlling switches 106 and 110. The switching action couples the energy storage networks 86 and 95 in series with the memory 60 such that a potential of approximately twice the second regulated potential is applied along conductor 113 to the memory means 60. The energy storage networks 86 and 95 discharge stored energy at a high rate during the WRITE interval to provide the rate needed by the memory means 60. The pulse applied to the energization means 78 from the control output 210 is of a controlled short duration so that only a limited amount of energy is consumed by the memory means 60 during the WRITE time interval. During the WRITE time interval, the computer 28 couples data representing newly entered parameters along line 59 to the memory means 60. A resistor 122 couples the +5 V potential to the energization input 61 during normal operation of the transmitter. Resistor 122 isolates the +5 V supply from the high energization level at energization input 61 during a WRITE interval. 
     In a preferred embodiment, the memory means 60 comprises an electrically eraseable read only memory or EEPROM such as a part number HI3104P manufactured by Hughes. The switches 118, 106 and 110 are preferably MOSFET transistors such as type VP0104N3 manufactured by Supertex Inc. and the switch 104 is preferably a MOSFET transistor such as a type VN0104N3 manufactured by Supertex Inc.