Abstract:
A process instrument includes a transducer, a two wire interface, a microprocessor, a digital to analog converter, a first control circuit, and a second control circuit. A current passing through the two wire interface indicates a condition of the transducer. The microprocessor is interfaced with the transducer. The digital to analog converter receives a signal from the microprocessor indicating a current value. The first control circuit is coupled to the digital to analog converter and adapted to control the current passing through the two wire interface to the current value. The second control circuit is coupled to the digital to analog converter and supplies current to a secondary load.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This divisional application claims priority from application Ser. No. 12/925,201, filed Oct. 15, 2010, entitled DYNAMIC POWER CONTROL FOR A TWO WIRE PROCESS INSTRUMENT, which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Many industrial process instruments operate on a two wire control loop with a current that varies from 4-20 mA based on a sensor reading or a desired actuator condition. In the case of a sensor, a host connected to the process instrument determines the measured value by measuring the control loop current. In the case of an actuator, the control room supplies a current to the process instrument which indicates a desired actuator condition. 
         [0003]    The host is located in a control room and supplies approximately 24V DC to the two wire device. For a sensor, simple diagnostics can be accomplished by measuring an out of range current such as 3.5 mA or 20.5 mA. There may be as much as a mile or more of cable between the control room and the device causing a small voltage drop from the resistance of the wires. Electronics in the device regulate the voltage to a nominal value such as 12V to power a sensor and a microprocessor. 
         [0004]    The microprocessor makes the sensor measurement and determines the necessary current value. It uses a digital to analog converter (DAC) to control a control amplifier and control transistor to consume current through a shunt resistor such that the total current draw of the electronics and the shunt resistor is the proper value. A feedback loop is completed using a high precision sense resistor that measures the total current usage of the process instrument to be sure an accurate value is reported. 
         [0005]    Traditional design techniques for process instruments specified that only functions which could be accomplished with the minimum current (3.5 mA) would be implemented. This is due to the nature of the environment in which a process instrument operates. These devices are very low power, often installed in remote locations, and could bring entire operations to a halt if they fail. Therefore, it is essential that the process instrument be fully functional at the lower limit of the available power. 
         [0006]    Although the device must operate at 3.5 mA, it may be operating as high as 20 mA. This means that 16.5 mA or more of available power is being thrown away in the shunt resistor. 
         [0007]    One use for this current is to provide LED backlighting for the process control instrument. A past approach to provide this feature was to replace the shunt resistor with an LED. While this does provide for backlighting, there is no control of the intensity of the backlight. At 4 mA, the backlighting is dim, while at 20 mA, it can be overly bright. 
       SUMMARY 
       [0008]    A method for controlling power consumption for the process instrument includes determining a desired total power consumption for the process instrument and calculating a value for a control signal related to the desired total power consumption. The control signal is supplied to a primary power control circuit and a secondary power control circuit. Using the secondary power control circuit, power consumption of a secondary load is adjusted to a portion of the desired total power consumption. Using a primary power control circuit, power dissipation in a shunt resistor is adjusted to cause the total power consumption for the process instrument to equal the desired total power consumption. 
         [0009]    A process instrument comprises means for determining a total power consumption for the process instrument based, at least in part, on a process variable measured by the process instrument. The process instrument further includes means for adjusting power consumed by a secondary load to a portion of the desired total power consumption, and means for adjusting power dissipated in a shunt resistor to cause the total power consumption for the process instrument to equal the desired total power comsumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a diagram of a process instrument including a second control amplifier for driving a secondary load. 
           [0011]      FIG. 2  is a diagram of a variation of the process instrument of  FIG. 1  including a second control amplifier with an adjustable gain for driving a secondary load. 
           [0012]      FIG. 3  is a diagram of a variation of the process instrument of  FIG. 1  including a second control amplifier for increasing the clock speed of the microprocessor. 
           [0013]      FIG. 4  is a diagram of a variation of the process instrument of  FIG. 1  including a second control amplifier for increasing the power supplied to the transducer. 
           [0014]      FIG. 5  is a block diagram of a process instrument including multiple secondary loads. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    According to one embodiment,  FIG. 1  is a diagram of process instrument  10  including a second control amplifier for driving a secondary load. Transducer  12  is connected to transmitter electronics  14 . Transmitter electronics  14  may include components such as A/D converter  16  and isolation  18  to condition the output of transducer  12  to be read by microprocessor  20 . Microprocessor  20  determines the necessary total loop current based on the process variable measured by transducer  12  and provides a signal to digital-to-analog converter (DAC)  22  correlated with the appropriate total loop current. For a 4-20 mA loop current, typical DAC output values are 1-3V. The output of DAC  22  is connected through feedback circuitry  23  to control amplifier  24 . 
         [0016]    Feedback circuitry  23  includes resistors  23   a - 23   c  and capacitors  23   d - 23   e . Resistors  23   a  and  23   b  are connected to DAC  22 . Resistor  23   a  is also connected to resistor  23   c  and capacitor  23   d.  The opposite terminal of capacitor  23   d  is connected to resistor  23   b.  Resistor  23   c  is connected to the non-inverting input of control amplifier  24 . Capacitor  23   e  is connected between the inverting input and the output of control amplifier  24 . 
         [0017]    The output of control amplifier  24  is connected to control transistor  26 . Control transistor  26  is connected to shunt resistor  28 . Shunt resistor  28  shares ground contact  30  with sense resistor  32 . Sense resistor  32  is connected back to resistor  23   b  and capacitor  23   d  to complete the feedback loop for controlling the loop current (I L ). Terminals  34   a  and  34   b  are connected to control transistor  26  and sense resistor  32  respectively. Power subsystem  36  is also connected to terminal  34   a  and provides the necessary circuitry to regulate and provide the power supply rails used by process instrument  10  (for example 10-15V, 4V, 3V, etc). 
         [0018]    Together, feedback circuitry  23 , control amplifier  24 , and control transistor  26  form a primary power control circuit for adjusting power dissipated in shunt resistor  28 . Alternatively, this primary power control circuit may be a number of other analog control circuits understood by those skilled in the art. 
         [0019]    Pursuant to this embodiment, secondary load  38  is connected to the output of DAC  22 . Switch  40  is included in some embodiments to allow microprocessor  20  to enable or disable secondary load  38  as necessary. DAC  22  is connected to a voltage divider comprising resistors  42  and  44 . The voltage divider is connected to control amplifier  46  which is connected to control transistor  48 . Control amplifier  46  and control transistor  48  form a secondary power control circuit for adjusting power consumed by a secondary load. Alternatively, this secondary power control circuit may be a number of other analog control circuits understood by those skilled in the art. 
         [0020]    In this embodiment, the secondary load is one or more LEDs  50  (for simplicity only one LED is drawn). Control transistor  48  is connected to the positive voltage rail (4V in some embodiments) through LEDs  50  and to ground through resistor  52 . LEDs  50  can be used as a backlight for a display on the process instrument and are one example of a secondary load. 
         [0021]    In a minimum power scenario, process instrument  10  requires a base current requirement of 1.5-2.7 mA to operate transducer  12  and microprocessor  20 . This means that as little as 0.8-2 mA of additional current must either be discharged through shunt resistor  24  or used for a secondary load such as LEDs  50 . In a maximum power situation, this increases to as much as 19 mA. 
         [0022]    Secondary load  38  allows the control of the current through LEDs  50  to be regulated independently. Secondary load  38  accepts the primary analog control signal from DAC  22  to allow independent control of the current through LED  50   s.  This allows LEDs  50  to be operated with a controlled intensity for minimizing flickering. LEDs  50  can also be selectively turned on and off based on measured conditions, a fault condition, available power, or a command from a user interface on the process instrument. 
         [0023]    Independent control of excess available power offers additional advantages for a two wire process instrument. In the past, the design philosophy for two wire process instruments was that if a function could be done at minimum power (3.5 mA less the minimum required base current of 1.5-2.7 mA), then it would not be done at all. This invention allows for the selective control of a secondary subsystem to use available power for increased functionality and processor speed as desired, and can be extended to handle many additional non-critical loads. 
         [0024]    The described architecture allows a design time decision to route a pre-determined portion of the loop current that would otherwise be discharged in shunt resistor  28  to the secondary load. For example, assume a LED current of 1-6 mA is desired for a loop current of 4-20 mA. A resistance of 5 ohms is selected for resistor  52  yielding an input voltage range for control amplifier  46  of 5-30 mV. The output of DAC  22  for a 4-20 mA loop current is 1-3V. Values for resistors  42  and  44  may be 95 k ohms and 5 k ohms, respectively, to yield the desired LED current. 
         [0025]    This approach offers many benefits. LEDs  50  can be turned on and off selectively. In some embodiments, LEDs  50  may be enabled only at certain loop currents. Intensity can be controlled using pulse width modulation of switch  40 . Switch  40  can also be used to flash LEDs  50  to indicate an error condition. 
         [0026]    While a process instrument may have a range of 4-20 mA, the process variable will often be in the middle of its range. Previous designs have focused on providing functionality which could only be accomplished at minimum loop currents. Here, the secondary system can be selectively enabled at typical higher operating currents and disabled at lower loop currents. This allows process instrument  10  to selectively invoke additional functionality when it is possible to support those tasks. 
         [0027]    The LED current is automatically regulated through DAC control making it transparent to microprocessor  20  and the remaining circuitry in process instrument  10 . This eliminates flickering that might be caused by scavenging power from shunt resistor  28 . Failure modes are benign as the LED circuitry is separated from the main process instrument current control loop. This avoids altering a critical part of the circuitry for process instrument  10  (shunt resistor  28 ). Intrinsic safety (IS) problems are avoided by maintaining existing shunt circuit designs. 
         [0028]    A typical use for an LED is as a backlight on a display attached to process instrument  10 . This architecture allows for the LED to be added as an optional display module without altering the remainder of the circuitry. Excess power is typically dissipated in shunt resistor  28 . Modifying the shunt traces and resistor element to accommodate an accessory module causes intrinsic safety (IS) issues that require significant design, validation, and certification effort. This construction avoids that problem by retaining the existing shunt circuit designs and does not require the shunt traces to be routed into the display module to allow for the accessory lighting. 
         [0029]    According to another embodiment,  FIG. 2  is a diagram of process instrument  100  which is a variation of process instrument  10  (of  FIG. 1 ) that includes an adjustable gain for driving the secondary load. Similar reference numerals are used in  FIG. 2  to designate similar elements to those shown in  FIG. 1 . Secondary load  102  is connected to the output of DAC  22  through switch  104 . Switch  104  is connected to variable resistor  106 . Microprocessor  20  controls both switch  104  and variable resistor  106 . Together with resistor  108 , variable resistor  106  forms a voltage divider at the input of control amplifier  110 . Control amplifier  110  is connected to control transistor  112 . Control transistor  112  is connected to the positive voltage rail through LEDs  114  and to a ground contact through resistor  116 . 
         [0030]    As current flows through LED  114 , control transistor  112 , and resistor  116 , a voltage will be present on resistor  116 . The feedback loop to control amplifier  110  will ensure that the voltage on resistor  116  matches the voltage on resistor  108 . By altering the value of variable resistor  106 , the operation of the voltage divider and the voltage drop on resistor  108  at any given output of DAC  22  will change. In this way, microprocessor  20  can control the current through the secondary load. In the case of an LED as the secondary load, this adjustment may be used for dimming. 
         [0031]      FIG. 3  is a diagram of process instrument  200  which is a variation of process instrument  10  (of  FIG. 1 ) that includes a second control amplifier for increasing the clock speed of the microprocessor. Similar reference numerals are used in  FIG. 3  to designate similar elements to those shown in  FIGS. 1 and 2 . Secondary load  202  is connected to the output of DAC  22  through switch  204 . Switch  204  is controlled by microprocessor  20  and connected to the voltage divider created by resistors  206  and  208 . Control amplifier  210  is connected to the voltage divider at the input and control transistor  212  at the output. Control transistor  212  is connected to a positive voltage rail and to ground through resistor  214 . The operation of the feedback circuit for the secondary load is the same as that described for the embodiments of  FIGS. 1 and 2 . 
         [0032]    Voltage controlled oscillator  216  provides a clock for microprocessor  20 . Its voltage input is summer  218  which adds the voltage of a positive rail with that of the output of the secondary load control circuit. When the secondary load is enabled, the voltage at oscillator  216  increases which in turn increases the clock speed. This allows microprocessor  20  to selectively increase its processing power based on the loop current. When more power is available, microprocessor  20  can take on additional tasks by increasing its processing capabilities. 
         [0033]      FIG. 4  is a diagram of process instrument  300  which is a variation of process instrument  10  (of  FIG. 1 ) that can provide additional power to the transducer. Similar reference numerals are used in  FIG. 4  to designate similar elements to those shown in  FIGS. 1-3 . Secondary load  302  is connected to the output of DAC  22  through switch  304 . Switch  304  is controlled by microprocessor  20  and connected to a voltage divider created by resistors  306  and  308 . This voltage divider is connected to control amplifier  310 . Control amplifier  310  is connected to control resistor  312 . Control resistor  312  is connected to a positive voltage rail and to ground through resistor  314 . The operation of this control circuit is identical to that described with respect to  FIGS. 1-3 . 
         [0034]    Resistor  314  is also connected through isolation  316  to transducer  12 . This allows microprocessor  20  to selectively supply additional power or activate subsystems such as heaters within transducer  12 . Purposes for doing so include maintenance of, or to complete advanced diagnostics on transducer  12 . This architecture allows the power supplied to transducer  12  to be increased based on the loop current or to supply the power under certain conditions such as a threshold loop current. 
         [0035]    Pursuant to another embodiment,  FIG. 5  is a block diagram of process instrument  400  including multiple secondary load subsystems. Transducer  410  is connected to transmitter electronics  412 . Transmitter electronics  412  is connected to microprocessor  414  which is in turn connected to DAC  416 . DAC  416  is connected to primary power control circuitry  418 . Primary power control circuitry  418  is connected to terminals  420   a - 420   b.  Secondary loads  422   a - 422   c  are each connected to DAC  416  and microprocessor  414 . In this way, any number of secondary loads can be connected to process device  400  and selectively enabled individually or collectively by microprocessor  414  during operation. 
         [0036]    The described embodiments for the secondary load are only illustrative. Any number of possible secondary loads may be used. Further, any number of secondary loads may be included in a single process instrument. The independent control of secondary subsystems such as LEDs for backlighting or increasing processor power for additional tasks allows the process instrument to provide additional functionality. 
         [0037]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.