Abstract:
A motor control system for deployment in high temperature environments includes a controller; a first half-bridge circuit that includes a first high-side switching element and a first low-side switching element; a second half-bridge circuit that includes a second high-side switching element and a second low-side switching element; and a third half-bridge circuit that includes a third high-side switching element and a third; low-side switching element. The motor controller is arranged to apply a pulse width modulation (PWM) scheme to switch the first half-bridge circuit, second half-bridge circuit, and third half-bridge circuit to power a motor.

Description:
STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH 
     This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the United States Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to an electronic controller for a DC motor, and more particularly to a high-temperature motor controller capable of operation in harsh environments for downhole gas and oil drilling operations. 
     BACKGROUND OF THE INVENTION 
     High-temperature geothermal exploration and drilling operations require a wide array of tools and sensors suitable for instrumentation for monitoring downhole conditions. There are limited options for tools and components with the capability for high temperature drilling and monitoring. 
     Several downhole applications exist in which a small motor may be useful. Applications such as clamping systems for seismic monitoring, televiewers, valve actuators, and directional drilling systems may be able to utilize a robust motor controller capable of operating in harsh environments. 
     Current motor controllers are capable of operating up to 125° C. The development of a high-temperature motor controller capable of operation at temperatures greater than 125° C. and up to 230° C. would significantly increase the operating envelope for next generation high temperature tools and provide a useful component for downhole systems. Recently as motors capable of operating in very high-temperature regimes are becoming commercially available, but motor controls are not available for application in such environments. One method of deploying a motor controller is to use a heat shielded tool and apply low-temperature electronics to control the motor. This method limits the amount of time that controller tool can function in high-temperature environments and does not allow for long-term deployments. Heat shielded tools may be suitable for logging tools that spend limited time in the well. However, a longer-term deployment is not possible, e.g., for a seismic tool which may be deployed for weeks or even months at a time. 
     What is needed is a reliable and robust method for long-term deployments and long-life operations, which uses high-temperature electronics and a high-temperature motor that does not need to be shielded. 
     SUMMARY OF THE INVENTION 
     In one embodiment a motor control system is disclosed for deployment in high temperature environments. The motor control system includes a controller; a first half-bridge circuit that includes a first high-side switching element and a first low-side switching element; a second half-bridge circuit that includes a second high-side switching element and a second low-side switching element; and a third half-bridge circuit that includes a third high-side switching element and a third; low-side switching element. The motor controller is arranged to apply a pulse width modulation (PWM) scheme to switch the first half-bridge circuit, second half-bridge circuit, and third half-bridge circuit to power a motor. 
     In another embodiment a clamping arm apparatus includes a housing, an arm portion, a motor and a control system. The motor is disposed within the housing and rotatably connected to the arm portion. The arm portion is movable in response to rotation of the motor to extend and retract the arm portion from the housing. The control system includes a controller; a first half-bridge circuit that includes a first high-side switching element and a first low-side switching element; a second half-bridge circuit that includes a second high-side switching element and a second low-side switching element; and a third half-bridge circuit that includes a third high-side switching element and a third; low-side switching element. The motor controller applies a pulse width modulation (PWM) scheme to switch the first half-bridge circuit, second half-bridge circuit, and third half-bridge circuit to power the motor. 
     One advantage is to provide a complete high-temperature motor controller capable of operation in environments with an ambient temperature of greater than 125° C. and up to 230° C. for 1000 hours. The motor and controller may be applied to a seismic tool clamping arm system used in such an environment. 
     Another advantage is a motor and controller that can be operated over a two wire cable. 
     Still another advantage is a motor that may be controlled to rotate in both directions, which motor occupies a cylindrical volume no more than about 5.715 cm (2.25 inches) in diameter by about 45.7 cm (18″) high and which generates at least 10.2 Newton-meters (7.5 ft-lb) of torque. 
     Yet another advantage is a control algorithm for the controller that provides the desired performance using a limited selection of parts suitable for high temperature environments. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary schematic diagram of a high-temperature motor controller. 
         FIG. 2  is an exemplary half bridge circuit. 
         FIG. 3  shows an auxiliary circuit to perform signal conditioning on the back EMF data to the analog-to-digital (ADC) converter for the high-temperature motor controller of  FIG. 1 . 
         FIG. 4  is an exemplary clamping arm for a seismic tool or sensor. 
         FIG. 5  is a graph of test results for a brushless DC motor at PWM duty cycle ranges from 60% to 100%. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A brushless DC (BLDC) motor requires a complex commutation scheme, which necessitates a significant amount of computation. The computational and temperature requirements limit the selection of the electrical components. According to an embodiment of the invention, a field programmable gate array (FPGA) is used to control a BLDC motor at a high ambient temperature for extended periods. As used in this application, high-temperature refers to temperatures greater than 125° C. unless otherwise indicated. In an embodiment, the controller is capable of operating at a temperature up to 230° C. for 1000 hours. In another embodiment, the controller is capable of operating at a temperature up to 225° C. for 1000 hours. The FPGA does not require auxiliary hardware to operate, as is the case with high temperature microcontrollers, making the FPGA more suitable to the control application. In addition, an FPGA is not limited by an instruction set architecture (ISA) as is the case with a microcontroller, and allows for parallel computation. 
       FIG. 1  is an exemplary schematic diagram of an embodiment of a high-temperature motor controller (controller)  50  according to the present invention. The controller  50  includes an FPGA  10  that controls operation of three half-bridges  12 ,  14  and  16 . FPGA  10  monitors output voltage of each half-bridge via an analog-to-digital (ADC) converter circuit  28 . Half-bridge components  12 ,  14  and  16  provide the power to drive a BLDC motor  18 . Each half-bridge  12 ,  14  and  16  includes switching elements  20 ,  22 , a high-side driver  24  and a low-side driver  26 . High-side driver  24  and low-side driver  26  include supporting electronics (see, e.g.,  FIGS. 2 &amp; 3 ) for switching elements  20 ,  22 , which allows FPGA  10  to control switching elements  20 ,  22 . 
     A charge pump  31  provides a control voltage greater than the rated power supply voltage (VCC) for the operation of high-side driver circuit  24 . VCC is the DC voltage that goes to the collector which provides bias power to switching element  20 . In one embodiment, switching elements  20 ,  22  may be a silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistor (MOSFET) transistor. For example, the SOI MOSFET may be manufactured by Cissoid S.A. of Belgium. SOI transistors may be characterized by high power ratings up to 230° C., although significant resistance while in the ON state may cause the transistor to dissipate excessive heat when conducting high current. Thus, for high temperature applications, a heat sink may be provided. 
       FIG. 2  is an exemplary schematic of a half bridge circuit  19  according to the present invention. As can be seen in  FIG. 2 , the half bridge circuit  19  includes a high-side driver circuit  24  connected to transistor  20  of the half bridge circuit  12 ,  14  or  16 . The high-side driver circuit  24  is powered by charge pump  31  which provides a 20 VDC source. FPGA  10  provides an input signal to high-side driver circuit  24  at input terminal  70 . Input terminal  70  is connected to a 5V logic supply voltage through a resistor R 30 . R 30  may have a resistance of 10 kΩ or other suitable resistance value. Input terminal  70  is also connected in series with a resistor R 31  to a transistor M 2 . R 31  may have a resistance value of 500Ω. Transistor M 2  may be a high voltage 80V N-channel small-signal MOSFET with an operating temperature range from −55° C. to 230° C., and drain voltage up to 80V with a typical output current of 300 mA at 230° C. Transistor M 2  is connected to charge pump  31  through a resistor R 28 , with a resistance of 500Ω, and to transistor  20  through another resistor R 29  with a resistance of 500Ω. 
     A low-side driver circuit  26  is connected to transistor  22  of the half bridge circuit  12 ,  14  or  16 . The low-side driver circuit  24  is powered by charge pump  31  which provides a 5 VDC source. FPGA  10  provides an input signal to low-side driver circuit  26  at input terminal  70 . Input terminal  72  is connected to a 5V logic supply voltage through a resistor R 33 . Resistor R 33  may have a resistance of 10 kΩ or other suitable resistance value. Input terminal  72  is also connected in series with a resistor R 35  to a transistor M 4 . R 35  may have a resistance value of 500Ω. Transistor M 4  may be a high voltage 80V N-channel small-signal MOSFET with an operating temperature range from −55° C. to 230° C., and drain voltage up to 80V with a typical output current of 300 mA at 230° C. Transistor M 4  is connected to charge pump  31  through a resistor R 32 , with a resistance of 1000, and to transistor  22  through another resistor R 34  with a resistance of 500. Motor  18  is connected to the outputs of transistors  20 ,  22  respectively. 
     Referring next to  FIG. 3 , auxiliary circuits are shown to perform signal conditioning on the back EMF data to the ADC  28 . ADC  28  includes a multiplexer circuit  30 . Multiplexer circuit  30  includes analog switches, or inputs, S 1 , S 2 , S 3  and S 4  for multiplexing signals to ADC  28 . Analog switches S 1 , S 2 , and S 3  each is connected to a half-bridge  20 ,  22  through an RC filter circuit  32 . RC filter circuit  32  filters the back electromotive force (EMF) and divides the voltage to attenuate the signal to the proper level for digitization by ADC  28 . Switch S 4  is connected to a 15 VDC source. In the disclosed embodiment capacitors C 1 , C 2 , C 6  and C 9  are rated 1 microfarad, and are connected in parallel with resistors R 2 , R 4 , R 6  and R 11 , respectively. Parallel resistors R 4 , R 6  and R 11  are rated 1 kΩ, and R 2  is rated 200Ω. Series resistors R 1 , R 3 , R 5  and R 10  are rated 10 kΩ and are connected in series with an output connector that connects multiplexer circuit  30  to half-bridges  12 ,  14 ,  16 . ADC  28  is connected to multiplexer circuit  30  at a terminal D to sense the output voltage of the half-bridges  12 ,  14 ,  16 . 
     The six switching elements  20 ,  22  from each half-bridge  12 ,  14  and  16 , may be attached directly to one or more heatsinks. In one embodiment, the transistors may be equally spaced on one or more heat-sinks, and the average temperature of the heat sink may rise by 1° C. for every 451 joule (J) of resistive heat dissipated by switching elements  20 ,  22 . In one exemplary embodiment, where the power dissipated by switching elements  20 ,  22  is 20 watts (W) and a maximum temperature differential, or ΔT, of 10° C., the allowable continuous run time of the system is approximately four minutes. 
     In an embodiment, the controller may be used to operate a clamping mechanism for a seismic sensor. In this application, the motor operates periodically and prolonged continuous operation is not required. To mitigate the resistance heat buildup from the high-temperature power transistors  20 ,  22 , a heat sink may be used to store heat sufficient to prevent the transistors from overheating. In one embodiment the heat sink may be comprised of a brass block with dimensions 22.86 cm×5.08 cm×1.27 cm (9″×2″×0.5″), and 1.225 kg (2.7 lb). Brass is a preferred material for the heat sink, due to its thermal properties, and because brass is easy to machine. Other heat sink materials having comparable thermal properties and machinability may also be used, e.g., copper, bronze and aluminum. 
     Referring again to  FIG. 1 , a control algorithm is implemented in FPGA  10  to provide the input signals to half-bridge drivers  20 ,  22 , in order to operate the BLDC motor  18 . Commutation is achieved by applying drive pulses to each of the half-bridge drivers in such a way as to create a rotating magnetic field in the motor which applies torque to the motor shaft. The control algorithm includes six discrete steps, or phase indexes, in one revolution of the BLDC motor  18 . Each phase index corresponds to driving each of the three motor windings either to positive voltage, ground, or left unconnected. The order in which these voltages are applied to the motor windings determines the direction of the magnetic field generated within motor  18 . FPGA  10  accesses a look-up table programmed with the correct steps required to create a rotating magnetic field. FPGA  10  drives the motor windings in this order via half-bridge driver circuits  24 ,  26 . 
     The timing that each phase voltage is applied to the windings of motor  18  determines the speed at which the motor shaft turns. In a conventional BLDC control scheme, the rotor position may be sensed directly via Hall effects sensors, and the next phase step can be instantly applied at the optimal timing for the greatest efficiency. However, as the high-temperature motor  18  does not include Hall-effect sensors, a more complex control algorithm is applied to motor  18  via FPGA  10  to electronically control both the speed and torque of the motor. 
     In order to implement such a control algorithm and still meet the size constraints of FPGA  10 , a constant speed algorithm may be implemented, to significantly reduce the size of controller  50 . A constant speed algorithm commutes the motor  18  at a predetermined rate without adjusting to match varying loads. From a stationary position, motor  18  is provided with current sufficient to ensure a predetermined torque output, and motor speed slowly increased to a constant operating speed. By using voltage feedback from the ADC, power output to motor  18  may be adjusted to match the power requirements of the load. Controller  50  applies a switched power supply pulse width modulation (PWM) scheme to control the voltage available to the motor windings. The PWM duty cycle is dynamically adjusted by FPGA  10 , to increase or decrease the torque generated by motor  18 . In addition, the voltage feedback may be used to detect a stall condition, at which point the control algorithm will stop motor commutation, and the controller will determine the remedial commutation sequence, which may vary by application. Such responses may be pre-programmed into FPGA  10 , and may be customized to fit different applications. 
       FIG. 4  illustrates an embodiment of a tool  60  according to the present invention. In an embodiment, the tool  60  may be a seismic tool used in a downhole field. As can be seen in  FIG. 4 , the tool  60  includes a housing  62  and a package  64  located within the housing  62 . The package  64  may be, but is not limited to a sensor, hydraulic pump, actuator, drill, an/or sampler. In an embodiment, the sensor may be a seismic, acoustic, pressure, temperature, chemical, camera, or other environmental measurement device. The tool  60  further includes a protrusion or clamping arm  66  pivotally attached to the housing  62  and motor  18  and motor controller  50  disposed and arranged within a housing  62 . The motor  18  is coupled to the clamping arm  66  by a coupling  67 . When initiated by the controller  50  and motor  18  may be operated to extend, rotate or otherwise manipulate the coupling  67  to extend the clamping arm  66 . In an embodiment, the coupling  67  may be used to extend and retract the clamping arm  66 . In an embodiment, the coupling may be a screw that is rotated in clockwise and counterclockwise directions by the motor  18  to extend and retract, respectively, the clamping arm  66 . 
     The direction of rotation of motor  18  is determined based on the voltage being applied to the system. If the voltage is lower than a certain level motor  18  rotates clockwise; otherwise motor  18  rotates counter-clockwise. To deploy or stow clamping arm  66  the controller  50  will step through a discrete number of steps during which the controller  50  will operate the motor  18  in a constant torque and constant speed mode. The torque is increased and speed is decreased between each step to achieve desired clamping force. Each step consists of startup sequence, normal operation and stall condition. During the startup sequence, the motor  18  is internally aligned and then accelerated to desired speed over a period of 500 ms. While in normal operation mode, the motor  18  maintains the constant output torque and speed until a stall condition is detected and then the controller  50  moves to the next step until desired force is achieved or stops the operation pending power down or reset. 
     Motor controller  50  operates BLDC motor  18  using a sensorless control algorithm. In one embodiment, BLDC motor  18  and motor controller  50  may generate 18.8 ft·lb of torque, which exceeds the torque necessary to clamp a tool or sensing system in a borehole, and which is suitable for periodic operation. PWM duty cycles were tested at between 60% and 100% with torque/speed results as shown in  FIG. 5 . Lower PWM duty cycles may also be useful as well. 
     While the invention has been described with reference to a preferred embodiment, 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 disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.