Abstract:
A current loop drive module includes a drive circuit and a compliance voltage controller. The drive circuit is configured to receive a compliance voltage and operable to generate a current loop signal based on the compliance voltage for receipt by an associated load coupled to the drive circuit. The compliance voltage controller is operable to adjust the compliance voltage based on the associated load. A method for generating a current loop signal includes generating a current loop signal based on a compliance voltage for receipt by an associated load and adjusting the compliance voltage based on the associated load.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   The present invention relates generally to current loop driving circuits and, more particularly, to a current loop drive module with a dynamic compliance voltage. 
   In general, two wire, loop circuits are used to provide signals for a variety of devices, for example, valve actuators or meters. Loop circuits typically include a current loop driving circuit which varies the current in the loop, generally from 0 to 20 mA, according to a received driving signal. For example, when the loop circuit is used to control a valve actuator, a controller provides a voltage proportional to the current that should be supplied to the controlled actuator. A current loop drive circuit, including a voltage-to-current converter that may contain a current mirror circuit, generates the 0–20 mA signal based on the received voltage. 
   The voltage supply provided to the current loop drive circuit, commonly referred to as the compliance voltage, is selected such that it is sufficient to drive the current loop signal across the range of expected loads. For example, the loads may range in resistance from ˜750 ohms for a solenoid valve to ˜0 ohms for a panel meter. To provide adequate range, typical loop driving circuits are provided with compliance voltage sources of approximately 24V. 
   The power dissipated in the current loop system is determined by the current signal and the compliance voltage (i.e., P=0.02 A×24V=0.48 W), not the resistance of the load. In cases where the load has a relatively low resistance, most of the power is dissipated as heat in the current loop drive circuit and not the load. Typical drive modules include circuitry for controlling multiple current loop channels. Due to the potential for significant power dissipation in the drive module, the number of channels a drive module can support for a given volume is limited. Also, the components used in the drive circuitry must be sized appropriately to handle the heat. 
   Given the restrictions imposed by the power dissipation requirements, it is difficult to reduce the cost per channel and increase the density of the analog outputs. The number of channels provided in a given module is typically limited based on the worst case power dissipation scenario. Alternatively, a user may be provided with guidelines that allow the determination of acceptable loads for a given compliance voltage and the number of loads that may be simultaneously active. Such restrictions limit the range of applications that may be served by a drive module. If a lower compliance voltage is provided, sufficient voltage may not be present to drive the current loop signal over its full range. Increasing the compliance voltage increases the range, but limits the number of channels for a given volume. 
   Therefore, there is a need for a current loop drive module that has adequate range to drive a variety of load types, but that can reduce the power dissipated in the drive module to allow a higher channel density. 
   This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
   BRIEF SUMMARY OF THE INVENTION 
   The present inventors have recognized that a current loop drive module may be constructed with a dynamic compliance voltage that adjusts based on the nature of the driven load to reduce the amount of power dissipated in the drive module. Reducing the power dissipated in the drive module allows for higher channel density and reduced system cost. 
   One aspect of the present invention is seen in a current loop drive module including a drive circuit and a compliance voltage controller. The drive circuit is configured to receive a compliance voltage and operable to generate a current loop signal based on the compliance voltage for receipt by an associated load coupled to the drive circuit. The compliance voltage controller is operable to adjust the compliance voltage based on the associated load. 
   Another aspect of the present invention is seen in a method for generating a current loop signal. The method includes generating a current loop signal based on a compliance voltage for receipt by an associated load. The compliance voltage is adjusted based on the associated load. 
   These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
       FIG. 1  is a schematic diagram of a control system in accordance with one embodiment of the present invention; 
       FIG. 2  is a simplified block diagram of a current loop drive module in the control system of  FIG. 1 ; 
       FIG. 3  is a simplified block diagram of a compliance voltage controller in the current loop drive module of  FIG. 2 ; and 
       FIG. 4  is a circuit diagram of a boost circuit and a rectifier/filter circuit in the current loop drive module of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   One or more specific embodiments of the present invention will be described below. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers′ specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
   Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1 , the present invention shall be described in the context of a control system  100 . The control system  100  includes one or more controllers  110  providing signals to control one or more loads  120 , each load  120  having an associated channel  130 . The loads  120  may vary widely depending on the particular implementation. For example, the loads  120  may be solenoid valves, valve actuators, panel meters, etc. Generally, the controller  110  generates an output drive signal (Vout) for each load  120  to affect a positioning thereof. Although a unitary controller  110  is illustrated, separate controllers  110  may be used. In one example, the control system  100  may be used to control a multi-axis machine, with each channel  130  being associated with a different control axis. The controller  110  may have a local connection to the current loop drive module  140 , or it may communicate over a communication network with the current loop drive module  140 . For example, the controller  110  may be a networked computer workstation. 
   In the illustrated embodiment, the control system  100  uses current loop signals for communicating with the loads  120 . A current loop drive module  140  receives the output drive signal from the controller  110  and generates a current loop signal (e.g., 0–20 mA) where the magnitude of the current is proportional to the magnitude of the output drive signal. For example, a 20 mA signal may indicate that a valve actuator should be in a fully open position, while a 0 mA signal indicates a fully closed position. Intermediate current values correspond to various intermediate valve positions. In the case of a panel meter, the current signal corresponds to the deflection of the meter. 
   Turning now to  FIG. 2 , a simplified block diagram of the current loop drive module  140  is provided. The current loop drive module  140  includes a plurality circuit components for each channel for driving the multiple current loop circuits. In the illustrated embodiment, the current loop drive module  140  supports  12  channels. As each channel  130  uses similar circuitry, only one channel  130  is described in detail. The current loop drive module  140  includes a compliance voltage controller  200  shared across the channels  130 . However, the compliance voltage controller  200  may generate a different compliance voltage for each channel  130 . A current loop drive circuit  210  receives the output drive signal (Vout) from the controller  110  (see  FIG. 1 ) for its associated channel  130 . The load  120  is coupled to the current loop drive module  140  at terminals  220 . The current loop drive circuit  210  generates the current loop signal (0–20 mA) for the load  120  at a transistor  230 . The construction and operation of the current loop drive circuit  210  are well known to those of ordinary skill in the art, so they are not described in detail herein. For example, a current mirror circuit may be used. 
   The compliance voltage (Vcomp) generated by the compliance voltage controller  200  is received by the current loop drive circuit  210 . Generally, the magnitude of the compliance voltage determines the amount of current the current loop drive circuit  210  can generate for a given load resistance. A lower load resistance equates to a lower compliance voltage requirement to drive the current loop signal, while a higher load resistance requires a higher compliance voltage to drive the current loop signal. 
   The voltage associated with the current loop signal is dropped across the load  120  and the transistor  230 . If the load resistance is near zero, for example, with a panel meter load, essentially the entire compliance voltage is dropped across the transistor  230 , resulting in significant heat dissipation in the current loop drive module  140 . As described in greater detail below, the compliance voltage controller  200  dynamically adjusts the compliance voltage based on the observed characteristics of the load  120  to reduce the amount of power dissipated in the current loop drive module  140  while still maintaining the capacity to drive the current loop signal for the load  120 . 
   The compliance voltage controller  200  provides a control signal (PC 1 ) to a boost circuit  240  that generates an input for a rectifier/filter circuit  250 . The boost circuit  240  and rectifier/filter circuit  250  boost the supply voltage (e.g., 6V) to generate a boost voltage. The rectifier/filter circuit  250  receives the boost voltage, rectifies the boost voltage, and filters the boost voltage to generate the compliance voltage for the current loop drive circuit  210 . A comparator  260  compares the voltage at the transistor  230  to the voltage at the load  120  to determine the relative requirements of the load  120  and generates a digital feedback voltage (Vfbk) for the compliance voltage controller  200 . The voltage at the transistor  230  represents the compliance voltage less the gate-to-source voltage of the transistor  230  and sense resistor contained within the current loop drive circuit  210 . 
   The comparator  260  has analog inputs and a digital output. The comparator  260  outputs a logic “1” if the voltage at the transistor  230  is greater than the voltage at the load  120  and a logic “0” otherwise. If more voltage is dropped across the transistor  230  (i.e., logic 1), the compliance voltage can be lowered without the current loop drive circuit  210  losing the capacity to drive the current loop signal. If the voltage at the transistor  230  drops below the voltage at the load (i.e., logic 0), the compliance voltage controller  200  increases the compliance voltage. 
   Turning now to  FIG. 3 , a simplified block diagram of the compliance voltage controller  200  is provided. In the illustrated embodiment, the compliance voltage controller  200  is implemented using a programmable logic device configured to implement the functions described herein. The compliance voltage controller  200  includes waveform generators  300 ,  310  that generate waveforms to be provided to the boost circuit  240  to affect the magnitude of the compliance voltage. The specific waveforms may vary widely depending on the particular implementation. In the illustrated embodiment, the waveform generator  300  generates a 10% duty cycle square wave that serves to drop the compliance voltage when applied to the boost circuit  240 , and the waveform generator  310  generates a 70% duty cycle square wave that serves to increase the compliance voltage when applied to the boost circuit  240 . A multiplexer  320  receives the feedback voltage signal from the comparator  260  and selects the 10% waveform generator  300  responsive to receiving a logic “1” and the 70% waveform generator  310  responsive to receiving a logic “0”. The waveform signals for different channels may be phase shifted relative to one another to avoid driving all the channels simultaneously, thereby reducing emissions. 
   The compliance voltage controller  200  also includes broken wire detect logic  330  that monitors the feedback signal from the comparator  260 . In the event that the load resistance is too high or a wire becomes disconnected, the comparator  260  will continually select the 70% duty cycle signal from the waveform generator  310 . The broken wire detect logic  330  includes a flip-flop that is periodically set by the firmware of the programmable logic device and cleared whenever the 10% selection is made. If the flip-flop has remained set just before the periodic action to re-set it, it may be inferred that a 10% selection was not made in the past interval and the compliance voltage controller  200  is attempting to boost the voltage to drive the increased load resistance. The broken wire detect logic  330  may provide a status signal to the controller  110 . The controller  110  may implement a variety of actions based on the broken wire status, such as stopping a process, sending an alert email, etc. 
   Referring now to  FIG. 4 , a circuit diagram of one exemplary embodiment of the boost circuit  240  is provided. Other types of boost circuits are known in the art, and the invention is not limited to the particular circuit described herein. The illustrated boost circuit  240  includes a driver  400  that receives the pulse control signal (PC 1 ) from the multiplexer  320  in the compliance voltage controller  200 . The driver  400  is connected to the gate input of a transistor  410 . An inductor  420  is connected between a voltage supply, Vcc, (e.g., 6V) and the transistor  410 . Applying the pulse train to the transistor  410  to periodically isolate the inductor  420  from ground boosts the voltage from the supply voltage, Vcc, to a higher level. The resulting voltage pulses are provided to the rectifier/filter circuit  250  which regulates the boost voltage signal and stores the voltage to generate the compliance voltage. A higher duty cycle waveform applied to the transistor  410  increases the compliance voltage generated in the rectifier/filter circuit  250  and a lower duty cycle waveform applied to the transistor  410  allows the compliance voltage to decay to a lower level. Because the transistor  410  operates in a switching mode when used to control the boost voltage, only a small voltage is dropped across the transistor  410  while it is active, resulting in little heat dissipation. 
   The rectifier/filter circuit  250  includes a dual diode  430  (e.g., a dual Schottkey diode) that rectifies the boost voltage received from the boost circuit  240 . Capacitors  440 ,  450  and a resistor  460  form an RC filter  470  that generates the compliance voltage, Vcomp, at its output. Of course, other circuit combinations known in the art may be employed to rectify and filter the boost signal to generate the compliance voltage depending on the particular implementation. 
   A zener diode  480  coupled to the dual diode  430  functions as an overvoltage sensor to generate an overvoltage signal if the compliance voltage exceeds a predetermined value (e.g., such as in the case where the load resistance is too high or a broken wire condition occurs). For example, the zener diode  480  may have a breakdown voltage of 20V to limit the value of Vcomp to 20V. When the voltage at the zener diode  480  exceeds its breakdown voltage, it begins to conduct through a pulldown resistor  490  coupled to an enable input  495  of the driver  400 . This voltage turns off the driver  400 , thus isolating the boost circuit  240  from the pulse train provided by the compliance voltage controller  200 . The compliance voltage then decays until it is less than the breakdown voltage of the zener diode  480 , which enables the driver  400  and allows the compliance voltage controller  200  to resume controlling the compliance voltage. 
   The voltage measured at the dual diode  430  approximates the compliance voltage. Of course the zener diode  480  may be connected at various points within the rectifier/filter circuit  250 . If a more direct measurement of the compliance voltage is desired, the zener diode  480  may be connected to the capacitor  450 . Also, the breakdown voltage of the zener diode  480  may be adjusted based on the maximum compliance voltage desired. 
   The current loop drive module  140  of the present invention has numerous advantages. By dynamically adjusting the compliance voltage according to the needs of each load  120 , the amount of power dissipated in the current loop drive module  140  is reduced. This reduction allows more channels to be provided in the current loop drive module  140  for a given volume. Another advantage is that the current loop drive module  140  is able to operate the current loop outputs over a wide load range (e.g., 0–750 ohms) with a wide supply range (e.g., 10V–30V) without user consideration. Covering this range without a dynamic compliance voltage, typically requires a user to calculate the total power dissipated in the current loop drive module  140  or the maximum load resistance that could be handled. For example, a user with 20 mA driving 10 ohm loads on 12 outputs using a 28V supply would normally expect to dissipate (˜28V×0.02)×12=6.7 W inside the current loop drive module  140 , which does not include the power required by the current loop drive module  140  for operation. Employing the current loop drive module  140  with a dynamic compliance voltage reduces the power dissipated in the current loop drive module  140  to around 1.5 W. 
   In addition to power management, the user would also be relieved of the need to calculate the amount of load resistance that can be driven under low supply voltage conditions. For example, if the user supply voltage were from a mobile application where the supply voltage is around 10V, the largest current loop load would be about 300 ohms. With the current loop drive module  140  implementing a dynamic compliance voltage, 750 ohm loads could be driven. 
   The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.