Abstract:
A positioner for a fluid actuator comprising an electronics assembly designed for setpoint/actual value comparison and for outputting a feedback control signal includes a current/pressure transducer receiving the feedback control signal for controlling the actuator, an analog power supply, particularly in a range 4 to 20 mA, for powering the electronics assembly, and an additional voltage input, particularly in a range of up to 24 V, wherein said voltage input is configured as a further power supply for powering the current/pressure transducer separate from the analog power supply.

Description:
BACKGROUND 
   The present invention relates to a positioner for a fluid, particularly pneumatic, actuator driven by pressurising fluid, as often finds application for actuating translational or rotational final control elements, such as valves, in process and system engineering. A positioner for a fluid actuator has an electronics assembly which via a positional setpoint/positional actual value comparison outputs a signal for feedback control of a current/pressure transducer for controlling the fluid actuator. The current/pressure transducer may comprise a pneumatic pilot stage via which the pneumatic main stage connected to a particularly constant compressed air source may be activated, the main stage driving a final control element of the actuator by pressurising fluid activation. As an alternative the current/pressure transducer may be configured as a structural unit. 
   Published German Application DE 44 29 401 C2 discloses such a positioner for which an inherently safe quick-action ventilating system according to type “e” protection is proposed which is engineered to run the final control element to a fail-safe position when called for. In this known positioner, a feedback control electronics assembly and the current/pressure transducer are powered by an analog power supply defined by 4 to 20 mA and 9.6 V. The electronics assembly is powered directly by the analog power supply. The current/pressure transducer is powered via an electrical conductor from the electronics assembly with both a feedback control signal and with the power of the analog power supply as is non-consumed by the electronics assembly. When the system is down or at fault, the control element of the actuator is run to a fail-safe position by the actuator being dumped. This is achieved by an emergency switch having a spring for automatically open-circuiting the emergency switch in the electrical conductor between the electronics assembly and current/pressure transducer. The emergency switch also has a coil connected to a usually binary emergency voltage input of 12 V via which the coil provides electromagnetic forces in closing the emergency switch by overcoming the spring force. When the voltage is missing—in other words, when the system is down—the emergency switch is open-circuited to break the electrical connection between the electronics assembly and the current/pressure transducer, resulting in the current/pressure transducer for dumping the actuator being out of circuit. 
   The structure of this known positioner is risky in terms of safety in that there is no total separation of the electric circuits of the positioner from its analog power supply and of the dump system from its binary emergency voltage. It has been discovered, namely, that a “residual current”—despite the system being down, in other words with lack of the 24 V emergency voltage—flows in the electrical conductor between the electronics assembly and the current/pressure transducer, inducing stray currents in the coil of the switch. The closing forces induced by these stray currents in the emergency switch may even be sufficient to maintain the emergency switch closed for a time which is at least still risky for safe OFF and during which the actuator cannot be dumped to the fail-safe position instantly when the system is at fault. Furthermore, because of its mechanical spring component, the emergency switch structure may malfunction because of material fatigue, risking the current/pressure transducer not being signalled OFF with the mandatory high assurance. In conclusion, the power supply balance of this known positioner has disadvantages in that, because of having to operate the current/pressure transducer by the analog power supply, only a restricted amount of energy of the analog power supply is available for consumption by the electronics components of the positioner. 
   SUMMARY 
   The following summary is provided to introduce various concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify specific key features of the claimed subject matter, nor is it intended to be used to determine the scope of the claimed subject matter. 
   Embodiments of the invention overcome the foregoing disadvantages of the prior art, particularly by providing a positioner with optimized fail-safe, enhanced reliability and with fewer mechanical components with which particularly higher power electronic components for a higher performance positioner can now be employed. 
   A positioner according to one embodiment of the invention comprises, for a fluid actuator driven by pressurising fluid, an electronics assembly designed for a positional setpoint/positional actual value comparison and for outputting a feedback control signal received by a current/pressure transducer for controlling the actuator, an analog power supply particularly in a range from 4 to 20 mA for powering the electronics assembly, and an additional voltage input particularly in a range up to 24V. Just as possible, however, are corresponding usual standard signals with voltages of 5 V, 12 V or the like at the voltage input. 
   Now, in accordance with one aspect of the invention, the voltage input is configured as a further power supply separate from the analog power supply, more particularly as an extension thereof, to which the current/pressure transducer is connected for its power supply. In other words, the current/pressure transducer is now powered independently of the analog power supply, by an additional power supply formed by the voltage input. The electronics assembly of the feedback control can now be powered preferably exclusively by the analog power supply. 
   A configuration as described above with two separate power supplies optimizes fail-safe functioning of the positioner because any mutual electrical interference of the electronics assembly by the positioning and dumping is now excluded without having to add to the number of mechanical components. Indeed, it has surprisingly been discovered that making the change in accordance with the invention even reduces the number of mechanical components, because there is now no need for a spring-biased switch or relay assembly. Apart from this, the foregoing aspect of the invention now makes it possible to select electronic components that consume a higher amount of power because the analog power supply, as standard for many actuators, is now exclusively available for the feedback control electronics of the positioner, thus granting the designer of the positioner added flexibility in defining the architecture of the electronic components of an even higher performance positioner. 
   It has been surprisingly discovered in application of the invention that a standard binary emergency control voltage of 24 V is sufficient to at least power the current/pressure transducer adequately. Between the electronics assembly and the current/pressure transducer an electrical conductor is installed, the power level of which is simply sufficient for communicating the control signal. 
   In one aspect of the invention, the further power supply is provided with an amplifier, particularly an operational amplifier, downstream of the voltage input. This amplifier serves to amplify the low power control signal to be applied to the current/pressure transducer, the power supply at the voltage input resulting in the feedback control signal being amplified. The amplified feedback control signal is applied to the current/pressure transducer for controlling the particularly pneumatic main control stage. 
   In one preferred embodiment of the invention, the further power supply comprises, downstream of the voltage input, electronic structure for stabilizing and, where necessary, rectifying the output voltage of the voltage input, for example to 5 V. This stabilizer may comprise transistors, zener diodes, integrated circuits or the like. In particular, the stabilizer is provided upstream of the amplifier so that the voltage at the amplifier is substantially always the same. 
   In another aspect of the invention, the power of the feedback control signal supplied to the current/pressure transducer is set, particularly reduced or modulated, so that on failure or a “zero” logic status of the, in particular, binary power supply, the current/pressure transducer is deactivated, and in operation or a “one” logic status of the, in particular, binary power supply, the current/pressure transducer can be operated by control particularly in accordance with the amplified feedback control signal. This ensures on loss of the 24 V emergency voltage that, despite a continuation of the analog power supply from 10 mA to 20 mA or occurrence of a “residual current,” the current/pressure transducer remains deactivated so that the requirement for dumping the actuator and assuming the fail-safe position is automatically satisfied. 
   Preferably, feedback control between the electronics assembly and the current/pressure transducer is provided with a current limiter. This current limiter may be configured as an electric resistor, a metal film resistor or as an optocoupler, for example. 
   In another preferred embodiment of the invention, the electronics assembly and the current/pressure transducer are accommodated in a separate, preferably sealed enclosure. The pneumatic main control stage, such as the pneumatic amplifier, may be arranged outside of the enclosure. 
   Furthermore, the positioner in accordance with the invention may be configured with a diagnostic apparatus. This diagnostic apparatus may include a sensor for sensing the switching condition of the current/pressure transducer, the switching condition signal of the diagnostic apparatus being suitable for communication in particular via a current limiter to the electronics assembly. 
   In yet another aspect of the invention, an additional circuitry for monitoring the switching condition of the voltage input is provided, this monitoring signal being suitable for communication in particular via a current limiter to the electronics assembly. 

   
     DESCRIPTION OF THE DRAWINGS 
     Further advantages, features and properties of the invention will now be detailed by describing preferred aspects with reference to the drawings in which: 
       FIG. 1  is block diagram of a first version of a positioner in accordance with the invention; and 
       FIG. 2  is block diagram of a second version of a positioner in accordance with the invention. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , there is illustrated a positioner  1  constructed in accordance with the invention comprising two power supplies, each separate from the other, i.e. one for the positioner A, one for the emergency air dump or quick-action ventilating system B. The power supplies are defined by a first analog power supply  3  with a current level of 4 to 20 mA for the positioner, and a second further power supply  5  with a standard binary voltage input  4  for an emergency signal of 24 V for the emergency air dump B. 
   Connected to the analog power supply  3  is an electronics assembly  7  constituting the positioner. The electronics assembly  7  comprises an input circuit  9  with a power supply or supply circuit (not shown) as may be configured as a transistor, zener diode or the like. Downstream of the input circuit  9  is a microprocessor  11  operated by the supply output signal  13  of the input circuit  9 . The microprocessor  11  compares a stored setpoint value to an actual positional value sensed by displacement sensor  15 , in conjunction with the microprocessor  11 , as regards the position or angular setting of drive shaft  45 , which is connected to a final control element  47 , the displacement sensor  15  being likewise powered by the power of the power supply  3 . 
   The further power supply  5  for an I/P-transducer  29  is formed by the voltage input  4 , power supply or supply circuit  19 , and operational amplifier  21 . Applied to the voltage input  4  is the power supply  19  for stabilizing the power supply for an operational amplifier  21  downstream of the power supply  19 . The power supply  19  is superfluous when the operational amplifier is designed immune to voltage fluctuations. The stabilized supply signal  23  is applied to the input  25  of the operational amplifier  21 . The output  27  of the operational amplifier  21  is electrically connected to the I/P-transducer  29 , which is activated and operated by a combined power/feedback control signal  31 . 
   The microprocessor  11  is connected to a control signal input  33  of the operational amplifier  21  via a feedback control line  35  provided with a current limiter  37  which ensures that the power of the feedback control signal  39  is reduced such that, without amplification, it is not strong enough to operate the I/P-transducer  29 . 
   The I/P-transducer  29  is connected to a pneumatic main stage (not shown) which, as controlled by the I/P-transducer  29 , drives a pneumatic actuator  41 . The pneumatic connection to the pneumatic main control stage, as well as the coupling from the current/pressure transducer to the pneumatic main control stage, is indicated by the reference numeral  43 . 
   The electronic components of the positioner A and emergency air dump B, particularly the I/P-transducer  29 , are accommodated in a common fluid-tight enclosure or housing  61 . The displacement sensor  15  senses the positioning of the drive shaft  45  by non-contact means for which a Hall sensor (not shown) may be employed, for example. As an alternative, a potentiometer may be provided which is mechanically linked to the drive shaft  45 . 
   Operation of the positioner  1  will now be detailed: 
   In normal operation of the positioner  1 , the power supply  3  powers all components of the electronics assembly  7  for performing the comparison of the stored positional setpoint value to the actual value as sensed by the displacement sensor  15 . The power of the feedback control signal  39  is reduced by the current limiter  37 . The reduced feedback control signal  35  is applied via the control signal input  33  to the operational amplifier  21  which increases the power of the feedback control signal  35  by means of the voltage input  4 . The combined power/feedback control signal  31  applied to the I/P-transducer  29  prompts operation of the I/P-transducer  29  in accordance with the control. 
   In an emergency situation, as typically defined by lack of the 24 V input voltage, the I/P-transducer  29  is instantly signalled OFF, or at least its power reduced such that any signals—for example stray currents of the electronics assembly  7  gaining access to the I/P-transducer  29  or the continued power supply at the power supply  3 —are too weak to operate the I/P-transducer  29 . 
   With the positioner  1  in accordance with the invention, a fail-safe level is now attained to ensure the final control element being rendered fail-safe with a probability bordering on certainty. 
   Referring now to  FIG. 2 , there is illustrated a further version of a positioner  1  constructed in accordance with the invention. For a better understanding of the description of  FIG. 2 , components of the positioner as shown in  FIG. 2  identified or similar to those of the positioner as shown in  FIG. 1  are identified by like reference numerals. 
   The positioner  1  as shown in  FIG. 2  differs substantially from the positioner as shown in  FIG. 1  by an additional diagnostic apparatus  51  being provided, comprising a sensor for sensing the switching condition of a magnetic valve (not shown) of the I/P-transducer  29 . The diagnostic apparatus  51  is likewise powered by the binary power supply  5  so that the analog power supply  3  is not additionally loaded. The diagnostic apparatus  51  is connected by a communicating line  53  to the microprocessor  11 , the communicating line  53  comprising a current limiter  55 . 
   The embodiments of the positioner  1  as shown in  FIGS. 1 and 2  may also include circuitry for monitoring the voltage input  4 , i.e. monitoring the actual voltage level at the voltage input  4 . Feedback of the monitoring signal as a low strength signal by a current-limiting input to the microprocessor  11  optimizes reliably safe operation of the positioner  1  in accordance with the invention. 
   LIST OF REFERENCE NUMERALS 
     1  positioner 
     3  power supply 
     4  voltage input 
     5  power supply 
     7  electronics assembly 
     9  input circuit 
     11  microprocessor 
     13  power supply output signal 
     15  displacement sensor 
     19  power supply 
     20  operational amplifier 
     23  supply signal 
     25  operational amplifier power input 
     27  operational amplifier output 
     29  I/P transducer 
     31  power and feedback control signal 
     33  control signal input 
     35  feedback control signal line 
     37  current limiter 
     39  feedback control signal 
     41  actuator 
     43  pneumatic connection 
     45  drive shaft 
     47  final control element 
     51  diagnostic apparatus 
     53  communication line 
     55  current limiter 
     61  enclosure 
   A positioning 
   B emergency air dump 
   While various embodiments have been illustrated and described, it will be appreciated that changes can be made therein without departing from the spirit and scope of the invention. The features of the invention as disclosed in the above description as well as in the claims may be substantial to achieving the invention in its various embodiments both individually and in any combination thereof.