Instrument power controller and method for adaptively providing an output voltage and an output current that together maintain a substantially constant electrical output power

An instrument power controller (120) for adaptively providing an output voltage VO and an output current IO that together maintain a substantially constant electrical output power PO is provided. The controller (120) includes inputs (121) for receiving an input power PI, outputs (122) for providing the substantially constant output power PO to a variable impedance load L, and a communication path (126) for receiving a load voltage VL. The instrument power controller (120) is configured to determine an input voltage VI and an input current II, determine an effective resistance RL of the load L and set the output voltage VO and the output current IO based on the input voltage VI, the input current II, and the effective resistance RL. The output voltage VO is substantially independent from the input voltage VI. The output voltage VO and the output current IO are varied to maximize a load power PL while maintaining the substantially constant electrical output power PO.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an instrument power controller and method, and more particularly, to an instrument power controller and method for adaptively providing an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power PO.

2. Statement of the Problem

Flow meters are used to measure the mass flow rate, density, and other characteristics of flowing materials. The flowing materials can comprise liquids, gases, combined liquids and gases, solids suspended in liquids, liquids including gases and suspended solids, etc. A flow meter can be used to measure a flow rate (i.e., by measuring a mass flow through the flow meter), and can further be used to determine the relative proportions of components in a flow stream.

In many process control or industrial automation settings, a bus loop or instrumentation bus is used to connect to various types of devices, such as flow meters, for example. The bus loop is commonly used to deliver electrical power to the various attached instruments or devices. In addition, the bus loop is also commonly used to communicate data both to and from the sensor or device. Therefore, the bus loop is connected to a master device that can provide regulated electrical voltage over the bus and that can exchange communications over the bus. The master device can send commands and/or programming, data, calibrations and other settings, etc., to the various connected devices. The master device can also receive data from the connected devices, including identification data, calibration data, measurement data, operational data, etc.

The master device can further comprise a power supply that is connected to an electrical power source. The master device typically provides electrical power over the bus loop that is current limited, voltage limited, and power limited.

During normal operation of a vibratory flow meter, such as a densimeter or Coriolis flow meter, the current consumption and voltage requirements are relatively stable. However, when the flow meter is initially powered up, vibration of the meter flow tubes gradually increases in frequency and amplitude. Due to the construction and material of the flow tubes and due to the added mass of flow material in the flow tubes, the flow tubes cannot be immediately brought up to a target vibrational amplitude. Consequently, the startup phase will require electrical current above that required for normal operation. Therefore, the electrical current draw at startup is higher than a current draw during normal operation.

A bus loop can comprise a 4-20 milliamp (mA) bus loop, for example. The 4-20 mA bus is a two-wire instrumentation bus standard that is typically used to connect to a single instrument and is further capable of being used to provide communications between an instrument and a host device. Alternatively, the bus loop can comprise other bus protocols or standards.

According to requirements of Intrinsic Safety protection methods, the electrical power delivered by the master device/power supply is strictly limited for purposes of safety. For example, a 4-20 mA bus protocol can be limited to 20 mA of electrical current and can further be limited to 16-32 volts (V). The electrical power available to a device on the bus is therefore limited.

In some operating environments, flow tube startup can be problematic. One result of power limitation at startup time is that flow tube startup time is greatly extended, as excess current is not available for boosting the vibrational amplitude of the flow tube or tubes.

SUMMARY OF THE SOLUTION

An instrument power controller for adaptively providing an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power POis provided according to an embodiment of the invention. The instrument power controller comprises inputs for receiving an input power PI, outputs for providing the substantially constant output power POto a variable impedance load L, and a communication path for receiving a load voltage VLfrom the load L. The instrument power controller is configured to determine an input voltage VIand an input current II, determine an effective resistance RLof the load L and set the output voltage VOand the output current IObased on the input voltage VI, the input current II, and the effective resistance RL. The output voltage VOis substantially independent from the input voltage VI. The output voltage VOand the output current IOare varied so as to maximize a load power PLbeing transferred to the variable impedance load L while maintaining the substantially constant electrical output power PO.

An electrical power control method for adaptively providing an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power POis provided according to the invention. The method comprises determining an input voltage VIand an input current IIand determining an effective resistance RLof a variable impedance load L. The method further comprises setting the output voltage VOand the output current IObased on the input voltage VI, the input current II, and the effective resistance RL. The output voltage VOis substantially independent from the input voltage VI. The output voltage VOand the output current IOare varied so as to maximize a load power PLbeing transferred to the variable impedance load L while maintaining the substantially constant electrical output power PO.

An electrical power control method for adaptively providing an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power POis provided according to the invention. The method comprises determining an input voltage VIand an input current IIand determining an effective resistance RLof a variable impedance load L. The method further comprises determining whether the effective resistance RLis within a predetermined normal operating range. The method further comprises setting the output voltage VOand the output current IObased on the input voltage VI, the input current II, and the effective resistance RLif the effective resistance RLis not within the predetermined normal operating range. The output voltage VOis substantially independent from the input voltage VI. The output voltage VOand the output current IOare varied so as to maximize a load power PLbeing transferred to the variable impedance load L while maintaining the substantially constant electrical output power PO.

ASPECTS OF THE INVENTION

In one aspect of the instrument power controller, the input voltage VIcomprises a substantially fixed input voltage VI.

In another aspect of the instrument power controller, the effective resistance RLcomprises RL=C1VL.

In yet another aspect of the instrument power controller, the output voltage VOcomprises VO=C2√{square root over (VIIIRL)}.

In yet another aspect of the instrument power controller, the input voltage VIand the input current IIcomply with a bus loop standard.

In yet another aspect of the instrument power controller, the input voltage VIand the input current IIcomply with an intrinsically safe (IS) standard.

In yet another aspect of the instrument power controller, the load L comprises a vibratory flow meter and the load voltage VLis related to a vibrational amplitude of one or more flow conduits of the vibratory flow meter.

In yet another aspect of the instrument power controller, the load L comprises a Coriolis flow meter and the load voltage VLis related to a vibrational amplitude of one or more flow conduits of the Coriolis flow meter.

In yet another aspect of the instrument power controller, the load L comprises a vibratory densitometer and the load voltage VLis related to a vibrational amplitude of one or more flow conduits of the vibratory densitometer.

In yet another aspect of the instrument power controller, the instrument power controller is further configured to determine whether the effective resistance RLis within a predetermined normal operating range and set the output voltage VOand the output current IObased on the input voltage VI, the input current II, and the effective resistance RLif the effective resistance RLis not within the predetermined normal operating range.

In yet another aspect of the instrument power controller, the instrument power controller further comprises a voltage supply configured to set the output voltage VOand a current supply configured to set the output current IO.

In yet another aspect of the instrument power controller, the instrument power controller further comprises a drive voltage converter configured to set the output voltage VO, a drive current source configured to provide the output current IO, and a control coupled to the drive voltage converter, to the load L, and to the loop current control, with the control configured to control the drive voltage converter and the drive current source to generate the substantially constant output power POwhile setting both the output voltage VOand the output current IO.

In one aspect of the method, the input voltage VIcomprises a substantially fixed input voltage VI.

In another aspect of the method, the effective resistance RLcomprises RL=C1VL.

In yet another aspect of the method, the output voltage VOcomprises VO=C2√{square root over (VIIIRL)}.

In yet another aspect of the method, the input voltage VIand the input current IIcomply with a bus loop standard.

In yet another aspect of the method, the input voltage VIand the input current IIcomply with an intrinsically safe (IS) standard.

In yet another aspect of the method, the load L comprises a vibratory flow meter and the load voltage VLis related to a vibrational amplitude of one or more flow conduits of the vibratory flow meter.

In yet another aspect of the method, the load L comprises a Coriolis flow meter and the load voltage VLis related to a vibrational amplitude of one or more flow conduits of the Coriolis flow meter.

In yet another aspect of the method, the load L comprises a vibratory densitometer and the load voltage VLis related to a vibrational amplitude of one or more flow conduits of the vibratory densitometer.

In yet another aspect of the method, the method further comprises determining whether the effective resistance RLis within a predetermined normal operating range and setting the output voltage VOand the output current IObased on the input voltage VI, the input current II, and the effective resistance RLif the effective resistance RLis not within the predetermined normal operating range.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a flow meter5comprising a flow meter assembly10and meter electronics20. The meter electronics20is connected to the meter assembly10via leads100and is configured to provide measurements of one or more of a density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information over a communication path26. It should be apparent to those skilled in the art that the present invention can be used in any type of Coriolis flow meter regardless of the number of drivers, pick-off sensors, flow conduits, or the operating mode of vibration. In addition, it should be recognized that the flow meter5can alternatively comprise a vibratory densitometer.

The flow meter assembly10includes a pair of flanges101and101′, manifolds102and102′, a driver104, pick-off sensors105-105′, and flow conduits103A and103B. The driver104and the pick-off sensors105and105′ are connected to the flow conduits103A and103B.

The flanges101and101′ are affixed to the manifolds102and102′. The manifolds102and102′ can be affixed to opposite ends of a spacer106. The spacer106maintains the spacing between the manifolds102and102′ in order to prevent undesired vibrations in the flow conduits103A and103B. When the flow meter assembly10is inserted into a conduit system (not shown) which carries the flow material being measured, the flow material enters the flow meter assembly10through the flange101, passes through the inlet manifold102where the total amount of flow material is directed to enter the flow conduits103A and103B, flows through the flow conduits103A and103B and back into the outlet manifold102′, where it exits the meter assembly10through the flange101′.

The flow conduits103A and103B are selected and appropriately mounted to the inlet manifold102and to the outlet manifold102′ so as to have substantially the same mass distribution, moments of inertia, and elastic modules about the bending axes W--W and W′--W′ respectively. The flow conduits103A and103B extend outwardly from the manifolds102and102′ in an essentially parallel fashion. The flow conduits103A and103B are driven by the driver104in opposite directions about the respective bending axes W and W′ and at what is termed the first out of phase bending mode of the flow meter5. The driver104may comprise one of many well known arrangements, such as a magnet mounted to the flow conduit103A and an opposing coil mounted to flow conduit103B. An alternating current is passed through the opposing coil to cause both conduits to oscillate. A suitable drive signal is applied by the meter electronics20to the driver104via the lead110.

The meter electronics20receives sensor signals on the leads111and111′, respectively. The meter electronics20produces a drive signal on the lead110which causes the driver104to oscillate the flow conduits103A and103B. The meter electronics20processes the left and right velocity signals from the pick-off sensors105and105′ in order to compute a mass flow rate. The communication path26provides an input and an output means that allows the meter electronics20to interface with an operator or with other electronic systems. The description ofFIG. 1is provided merely as an example of the operation of a Coriolis flow meter and is not intended to limit the teaching of the present invention.

FIG. 2shows a bus system200according to an embodiment of the invention. The bus system200includes a loop power supply110connected to a bus loop112. The flowmeter environment100can further include an instrument power controller120coupled to the bus loop112. In some embodiments, the instrument power controller120adaptively provides an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power PO. The output power PO can be provided to a variable impedance load L, such as the vibratory flow meter5or other bus instrument5.

The loop power supply110can have a fixed or limited voltage output and a fixed or limited current output. In an IS power supply, the provided electrical power (including specifically the provided electrical current) is limited in order to prevent combustion or explosion when used in a hazardous environment. In some embodiments, the loop power supply110comprises an Intrinsically Safe (IS) power supply. Therefore, the loop power supply110can comply with a specific safety standard.

Various devices connected to the bus loop112can send data to and otherwise communicate with the loop power supply110. Where the loop power supply110is behind an IS barrier, for example, the loop power supply110can relay the data and communications on to other devices, including monitoring and/or recording equipment, a transmitter for communicating with other devices, an operator display, etc.

The bus loop112can receive one or more bus instruments5and can provide electrical power to the one or more bus instruments5. The bus loop112can transfer communications between the loop power supply110and the one or more bus instruments5. The bus loop112can include multi-device buses (such as Foundation fieldbus, for example), as well as single device loops (such as a 4-20 mA bus, for example).

The instrument power controller120includes inputs121and outputs122. The instrument power controller120receives electrical power at the inputs121from the loop power supply110via the bus loop112. The instrument power controller120provides electrical power to the one or more connected bus instruments5at the outputs122and additionally can transfer communications between a connected bus instrument5and the loop power supply110. The instrument power controller120provides a substantially constant electrical power to the one or more connected bus instruments5.

In the embodiment shown, the connected bus instrument5comprises a vibratory flow meter that is coupled to the instrument power controller120. The bus instrument5can include a meter assembly10and meter electronics20, as previously discussed. However, it should be understood that other bus instruments5can be connected to the instrument power controller120.

The instrument power controller120includes a communication path126. The instrument power controller120exchanges communications with the loop power supply110over the bus loop112. In addition, the communication path126exchanges communications between the bus instrument5and the instrument power controller120. The instrument power controller120can therefore relay communications between the loop power supply110and the bus instrument5. In addition, the instrument power controller120can translate/convert the communications. For example, if the meter electronics20generates digital communication signals, the instrument power controller120can convert the digital measurement signals into analog current levels that are appropriate to the loop current IL.

Communication over the bus loop112according to some protocols entails varying the loop electrical current ILflowing through the bus loop112. According to at least one instrumentation bus protocol, the loop current ILis varied between 4 milliamps (mA) and 20 mA when the bus instrument5is operating and therefore constitutes an analog measurement signal. The meter electronics20will control the loop current ILby signals sent to the instrument power controller120and according to a measured mass flow rate of a flow material through the meter assembly10. When there is no flow through the meter assembly10, or where the bus instrument5is not in an operational mode, the loop current ILcan be held to less than 4 mA, according to a relevant instrumentation bus protocol.

However, an IS compliant bus protocol limits the total power that can be delivered to the bus instrument5, such as a flow meter5, for example. The bus instrument5cannot receive more power (P) than is available over the bus loop112. Electrical power (P) is defined as voltage (V) multiplied by current (I), or:
P=V*I(1)

Vibratory flow meters, such as Coriolis flow meters and vibratory densitometers, oscillate by the application of electrical current to a drive coil mounted to one tube, creating a magnetic field that drives a magnet on the opposite tube. The force (F) between the coil and magnet is proportional to magnetic field strength of the magnet (B), the current (i) in the coil, and the length (L) of the coil, as expressed in the equation:
F=BiL(2)

As tube amplitude increases, a voltage (i.e., EMF) is developed in the coil. The voltage is proportional to the amplitude of flow tube vibration. In order to maintain a particular drive amplitude, the drive voltage must be at least as large as the coil EMF voltage associated with that amplitude. However, in practice the drive voltage must be larger than the coil EMF to overcome the voltage drop due to series resistance of the coil.

The average power consumed by the meter assembly10is the product of the coil drive current multiplied by the coil drive EMF. Coriolis flow meters have typically been designed to produce drive coil EMF voltages in the range of 2V to 5V and to consume a drive current of 1 to 10 mA at the target vibrational amplitude. In contrast, a typical transmitter for a flow meter has been designed to supply 10V at up to 100 mA to the driver104. The excess drive voltage allows for the maximum sensor EMF voltage plus overhead to accommodate series resistance. The excess drive current provides additional energy to the system when adverse process flow conditions consume additional drive power, such as during the occurrence of entrained air, for example. The excess drive current also serves to overcome the inertia of the meter assembly10at startup, allowing the target amplitude to be achieved relatively rapidly, perhaps within one to two seconds, for example.

The limited voltage and current available in an IS bus environment presents several problems for a vibratory flow meter. The power limitations inherent in a bus device powered from a loop constrains the maximum drive current, reducing the capability to maintain the target vibrational amplitude under adverse flow conditions. As a result, the vibration of the flowtubes may not be able to be satisfactorily maintained during adverse flow conditions. For example, where there is entrained air present in the flow material, the flowtubes will naturally vibrate at a higher frequency. The entrained air can comprise bubbles, stratified flow, or slug flow, for example. During slug flow, the vibrational frequency may need to fluctuate rapidly.

Another significant problem in an IS bus environment is the provision of electrical power to the meter assembly10during startup. Vibration of the meter assembly10from rest to a substantially resonant frequency takes time and electrical current to accomplish. The startup time for vibration of the flow conduit or conduits is increased in duration as current capability is decreased. The constrained drive current unavoidably lengthens the time required to achieve the target amplitude at startup, which in a standard topology can be as long as four minutes, depending on flowtube size and other factors. Therefore, a startup time for a flow meter assembly can be greatly increased where electrical current is limited due to IS considerations. A greatly lengthened meter startup time is undesirable or even unacceptable to most flow meter customers.

During the flow meter startup, the output voltage VOcan be kept just slightly above a response voltage level from the load (i.e., the bus instrument5). Accordingly, the output current IOcan be a maximum at the beginning of the flow meter startup, as a lower output voltage VOenables a higher output current IOto be produced by the instrument power controller120. As the vibrational amplitude of the meter assembly10increases, the output voltage VOcan be increased and the output current IOcan be decreased.

In the prior art, these drawbacks have led to the applied power at the driver being much less than the available power. The typical prior art approach is to simply limit the output current IOto the bus instrument5while not limiting the output voltage VO. However, the output voltage VOmay be much higher than is necessary, especially where the meter assembly10is below the target vibrational amplitude. Consequently, the applied power is much less than the available power, especially during periods of high current requirements.

The instrument power controller120according to the invention provides a substantially constant output power POto the connected bus instrument5. The instrument power controller120varies both the supplied voltage and the supplied current. In some embodiments, the instrument power controller120increases the output current IOby reducing the output voltage VO. The instrument power controller120therefore optimizes the electrical output power POthat is supplied to the connected bus instrument5. The instrument power controller120can keep the output voltage VOjust slightly higher that the vibrational response amplitude, for example. The lower output voltage VOenables the instrument power controller120to provide a higher output current IO. Consequently, while maintaining a substantially constant output power PO, the instrument power controller120can reduce the flow meter startup time and can increase the ability of the flow meter to adapt to changing flow conditions, including multi-phase flow conditions.

In some embodiments, the output voltage VOand the output current IOcan be varied between fixed, discrete levels. Alternatively, the output voltage VOand the output current IOcan be continuously varied.

The instrument power controller120is depicted as a separate component. However, it should be understood that the instrument power controller120can alternatively comprise a component or portion of the connected bus instrument5, such as an integral portion of the meter electronics20, for example.

FIG. 3shows the instrument power controller120according to an embodiment of the invention. In some embodiments, the instrument power controller120adaptively provides an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power PO. The output power PO can be provided to a variable impedance load L, such as the vibratory flow meter5or other bus instrument5.

In the embodiment shown, the instrument power controller120comprises a voltage controller310and a current controller320. The voltage controller310can vary the output voltage VO. The current controller320can vary the output current IO. The communication line126(not shown) can be coupled to one or both of the voltage controller310and the current controller320. The communication line126can communicate a response voltage level to the voltage controller310and the current controller320. In addition, the communication line126can communicate other information to the voltage controller310and the current controller320.

The voltage controller310and the current controller320both are connected to the communication line126. Consequently, the voltage controller310and the current controller320can vary the output voltage VOand the output current IO, as required. Alternatively, the instrument power controller120of this embodiment can include a processing device or control (not shown) that controls the voltage controller310and the current controller320to vary the output voltage VOand the output current IO.

The voltage controller310can output a varied output voltage VO, as needed. The output voltage VOcan be less than or greater than the input voltage VI. Consequently, in some embodiments the voltage controller310comprises a DC-DC converter that can increase the output voltage VOto be greater than the input voltage VI. The DC-DC converter is also variously referred to as a voltage or charge pump, a buck converter, etc.

The current controller320can regulate and output a varied output current IO, as needed. The current controller320in some embodiments can comprise a variable resistance RV. The current controller320will generate a voltage drop Vcurrent. The load L can comprise any manner of variable impedance device. For example, the load L can comprise a flow meter5, including a vibratory flow meter5. For example, the load L can comprise a Coriolis flow meter5or a vibratory densitometer5. The load L will generate a load voltage VL. The output voltage VOcomprises the current control voltage Vcurrentplus the load voltage VL. Similarly, the output power POcomprises the load power PLplus a current control power PCC.

FIG. 4is a graph of an absolute value of voltage of the driver coil Vemf, the load voltage VL(where VLcomprises Vemfplus a voltage due to a resistance RLof the load/driver), and the output current IO. The load voltage VLcan be obtained as a pickoff voltage VPOwhere the load L comprises a vibratory flow meter5. The graph illustrates the changing nature of a vibratory flow meter as a load during startup of vibration of the flow meter assembly10.

The impedance of the load L, where the load L comprises a vibratory flow meter5, will be minimal as the flow meter5is started up (i.e., where the flow meter assembly10is not vibrating or is vibrating at a relatively small amplitude). Conversely, as the flow meter assembly10nears or reaches a target vibrational amplitude, the impedance increases and consequently the current needed to maintain the vibration will decrease. Therefore, larger electrical current levels will be needed at startup of the flow meter5or when adverse flow conditions occur. For example, in cases of high levels of entrained air or slug flow, the vibration of the flow meter assembly10will be heavily damped and the vibrational amplitude may drop precipitously. As a result, during normal operation there may occur time periods when the current demand greatly increases and the output current may need to be correspondingly increased in order to resume or maintain proper vibrational levels.

Conversely, a voltage needed to startup vibration or resume proper vibrational levels in the flow meter assembly10are relatively low. The output voltage requirement will increase as the flow meter assembly10nears a target vibrational amplitude and as the driver coil requires larger voltage levels in order to change direction but yet maintain a drive frequency.

FIG. 5is a flowchart500of a method for adaptively providing an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power POaccording to an embodiment of the invention. In step501, the input voltage VIand the input current IIare determined. The input voltage VIand the input current IIcan be obtained from the bus loop112, for example. The input voltage VIand the input current IIcomprise an available input power PI.

In some embodiments, the bus loop112comprises an Intrinsically Safe (IS) bus loop. Consequently, the input power PIavailable from the bus loop112is typically limited and an output current IOcannot necessarily be increased as needed, at least not without decreasing the output voltage VO.

In step502, an effective resistance RLof a variable impedance load L is determined. The C1term comprises a conversion factor and the load voltage VLin some embodiments comprises a pickoff voltage of a pickoff sensor of the vibratory flow meter. In some embodiments, the effective resistance RLcomprises:
RL=C1VL(3)

The effective resistance RLcan vary over time. As previously discussed, where the load L comprises a vibratory flow meter, for example, the impedance can vary according to the vibration of the flow meter assembly. The vibration can vary during startup and can also vary during adverse or abnormal flow conditions such as gas in a liquid flow (including in the form of bubbles, stratified flow, slug flow, etc.) or other multiphase flows, changes in density of the flow material, etc. The determined effective resistance RLcan therefore comprise a substantially instantaneous impedance or can comprise an at least partially averaged impedance.

In step503, the output voltage VOand the output current IOare set. Assuming one hundred percent efficiency, i.e., no loss in the instrument power controller120, then the output power POwill be equal to the input power PI, where power P=V*I. As a result, the output voltage VOcan be determined according to the formula:
VO=√{square root over (VIIIRL)}  (4)

It should be understood that the output power POwill not be truly equal to the input power PI, as some electrical power will be consumed by the instrument power controller.

In some embodiments, the output voltage VOcomprises:
VO=C2√{square root over (VIIIRL)}  (5)

Here, the C2term comprises a non-ideal power loss factor or efficiency multiplier (i.e., VO<VI). Equations 4 and 5 therefore enable the output voltage VOto be set according to the operating conditions of the load L. Equations 4 and 5 further enable the output power to be maintained at a substantially constant level, even as the effective resistance RLvaries over time. Consequently, the output voltage VOcan be reduced while increasing the output current IO, and vice versa. For example, if the effective resistance RLdrops during operation of the vibratory flow meter, the output voltage VOcan be correspondingly reduced so that the output current IOcan be increased. Conversely, if the effective resistance RLincreases, the output current IOcan be correspondingly reduced so that the output voltage VOcan be increased.

Subsequently, the method loops back to step501and iteratively controls the output voltage VOand the output current IO.

FIG. 6is a flowchart600of a method for adaptively providing an output voltage VOand an output current IOthat together maintain a substantially constant electrical output power POaccording to an embodiment of the invention. In step601, the input voltage VIand the input current IIare determined, as previously discussed.

In step602, an effective resistance RLis determined, as previously discussed.

In step603, if the effective resistance RLis within a predetermined normal operating range, then the method loops back on itself, otherwise the method proceeds to step604. The predetermined normal operating range corresponds to an optimal or expected vibration of the flow meter assembly10and an optimal or expected vibrational amplitude. The predetermined normal operating range can vary according to a flow meter model and according to a flow material. If the effective resistance RLis within the predetermined normal operating range, then the load L can be considered to be operating satisfactorily and no further action is taken in this iteration of the control loop. Otherwise, if the effective resistance RLis not within the predetermined normal operating range, then the output voltage VOmust be set (i.e., changed). The comparison will typically fail during startup or during some manner of flow anomaly, such as entrained gas in the flow material, for example.

In step604, the output voltage VOand the output current IOare set if the effective resistance RLis not within the predetermined normal operating range, as previously discussed.

FIG. 7shows the instrument power controller120according to an embodiment of the invention. In this embodiment, the instrument power controller120includes a loop current control710, a drive voltage converter715, a controller720, and a drive current control725. The loop current control710can comprise an optional component (see dashed lines) and may be included in embodiments where the input current IIis modulated in order to communicate data over the inputs121. The drive voltage converter715is coupled to the controller720by a line755and is coupled to the loop current control710by a line751. The loop current control710is coupled to the controller720by a line757, is coupled to the drive current control725by a line754, and is coupled to the drive voltage converter715by the line751. The drive current control725is coupled to the controller720by a line756. The lines751and754further comprise the inputs121.

The instrument power controller120in this embodiment is connected to a flow meter sensor730. The flow meter sensor730can include the meter assembly10. In addition, the flow meter sensor730can include the meter electronics20. The flow meter sensor730is coupled to the drive voltage converter715by a line752, is coupled to the drive current control725by a line753, and is coupled to the controller720by the communication path126. The lines752and753further comprise the outputs122. The flow meter sensor730receives electrical power through lines752and753. One or more measurement signals (an optionally other sensor characteristics) are provided to the controller720via the communication path126. For example, a mass flow rate and/or density can be provided to the controller720over the communication path126.

The drive voltage converter715can receive the input voltage VIand can generate an output voltage VOthat is independent of the input voltage VI. The output voltage VOcan be less than, equal to, or greater than the input voltage VI. The drive voltage converter715can create an output voltage VOthat is greater than the input voltage VIprovided by the bus loop112or other power input. The drive voltage converter715can comprise a DC-DC converter, for example. The drive voltage converter715can convert the DC input voltage VIto an AC waveform, can step up the voltage of the AC waveform, and can then convert the AC waveform back to a DC voltage. In this manner, the output voltage VOcan be generated to be greater than the input voltage VI.

In operation, the drive voltage converter715can provide a predetermined output voltage VOto the flow meter sensor730. In addition, the drive voltage converter715can vary the output voltage VOover a predefined voltage range, such as an IS specific voltage range, for example. The output voltage VOcan be varied in order to maximize the output current IOwhile maintaining the substantially constant output power PO, as previously discussed.

The loop current control710can regulate the amount of input current IIthat is provided to the flow meter sensor730. Consequently, the loop current control710can convert at least part of the input current IIinto the output current IO. The output current IOcan be less than or equal to the input current II. In some embodiments, the output current IOcan even be greater than the input current II. However, in other embodiments, unlike the output voltage VO, the output current IOcannot exceed the input current II.

The controller720receives feedback information from the flow meter sensor730via the communication path126. The feedback information can include the load voltage VL, as previously discussed. In addition, the controller720can receive other information, including a response frequency, a phase lag or time delay between pickoff sensor signals, etc. The load voltage VLis related to an amplitude of a vibrational response in the flow meter sensor730. The load voltage VLcan comprise a pickoff voltage in some embodiments. The controller720is coupled to the drive voltage convert715and to the loop current control710and is configured to vary the output voltage VOand the output current IO.

The controller720can be configured to control the drive voltage converter715and the loop current control710in order to generate a substantially constant output power POto the flow meter sensor730while varying both the output voltage VOand the output current IOin relation to the load voltage VLreceived from the flow meter sensor730. Alternatively, the controller720can be configured to control the drive voltage converter715and the loop current control710in order to increase the output current IOand correspondingly decrease the output voltage VOin order to maintain a substantially constant output power POif the load voltage VLis below a predetermined operational threshold (i.e., if the effective impedance RLis not within a predetermined normal operating range).