Systems and methods for controlling downhole linear motors
Systems and methods for controlling downhole linear motors to minimize connections to surface equipment. In one embodiment, an ESP system is coupled by a power cable to equipment at the surface of a well. The ESP system includes a linear motor and a reciprocating pump. The motor has a set of position sensors that sense the position of a mover in the motor. Combining circuitry (E.G., XOR gate) combines the outputs of the position sensors into a single composite signal in which signal components corresponding to the position sensors are indistinguishable. A single channel carries the composite signal from the ESP system to the surface equipment. A control system determines a starting position of the motor and determines its subsequent position based on transitions in the composite signal. The motor is then operated based on the position determined from the composite signal.
1. A method comprising:
providing a system including a drive controller and an electric submersible pump (ESP), wherein the ESP has a linear motor coupled to drive a reciprocating pump, wherein the linear motor has a plurality of position sensors located at different positions along a stroke of a mover within the linear motor, wherein each position sensor of the plurality of position sensors outputs a signal indicating when the mover of the linear motor is in a corresponding different position within the linear motor, wherein the ESP includes circuitry that combines the outputs of the position sensors into a composite signal in which a source of each signal component is indistinguishable;
receiving the composite signal in the drive controller, wherein an absolute position of the mover within the linear motor is not received by the drive controller; and
tracking a position of the mover in the drive controller by counting transitions in the composite signal and producing output power in dependence on the transitions in the composite signal and thereby driving the linear motor, including:
producing an initial power stroke voltage;
monitoring the composite signal;
determining from the composite signal whether the mover has moved in response to the initial power stroke voltage;
if the mover has moved in response to the initial power stroke voltage, continuing to produce the initial power stroke voltage; and
if the mover has not moved in response to the initial power stroke voltage, increasing the power stroke voltage and continuing monitoring of the composite signal and determining from the composite signal whether the mover has moved in response to the increased power stroke voltage.
2. The method of claim 1 , further comprising:
performing an initialization procedure in the drive controller and thereby identifying a starting position of the mover in the linear motor; and
producing output power in dependence on the identified starting position of the mover in the linear motor and the transitions in the composite signal.
3. The method of claim 2 , wherein performing the initialization procedure comprises producing an output voltage that is adapted to move the mover in a return stroke direction, monitoring the composite signal, and determining from the composite signal when the mover has moved to an end of a return stroke.
4. The method of claim 3 , wherein producing output power and thereby driving the linear motor based on the identified starting position of the mover in the linear motor comprises, after determining that the mover has moved to the end of the return stroke, producing an output voltage that is adapted to move the mover in a power stroke direction.
5. The method of claim 4 , further comprising
counting events detected in the composite signal corresponding to movement of the mover;
determining whether a number of counted events has reached a predetermined maximum number;
if the number of counted events has reached the predetermined maximum number, producing an initial return stroke voltage; and
if the number of counted events has not reached the predetermined maximum number, continuing to produce the power stroke voltage.
6. The method of claim 5 , further comprising comparing a frequency of the linear motor to a power stroke profile and adjusting the power stroke voltage based on the comparison.
7. The method of claim 1 , further comprising carrying the composite signal from the ESP system to the drive controller on a single channel coupled between the ESP system and the drive controller.
8. The method of claim 1 , wherein producing the output power comprises controlling a variable speed drive (VSD) using the drive controller.
9. The method of claim 1 , wherein each signal component of each of the outputs of the position sensors is individually indicated in the composite signal.
10. The method of claim 1 , wherein each signal component comprises either a rising edge or a falling edge of a signal output by a corresponding one of the position sensors.