Patent Publication Number: US-2023139060-A1

Title: Valve actuator with dc braking system

Description:
BACKGROUND 
     Technical Field 
     The present disclosure generally relates to valve actuator assemblies. 
     Description of the Related Art 
     Valve actuators are often used to open and close valves. Valve actuators can be used in a wide range of settings, including in watercraft, wastewater treatment plants, refineries, power plants, and factories. Valve actuators apply force to operate valves along a range of motion from an open position to a closed position and vice versa. The force applied to the valve by the valve actuator may be a force to create linear movement of the valve, or torque applied to a shaft or other rotating part coupled to the valve to create rotational movement of the valve. 
     The valve actuator closes the valve by driving the valve to a position that prevents the flow of fluid in a fluid channel. In the example of a gate valve and various other types of valves, the valve actuator drives an end of the valve gate from a first side of the fluid channel to a second side of the fluid channel until the end of the valve gate contacts the second side of the fluid channel, nests it in a valve seat, and completely prevents fluid flow through the fluid channel. 
     Large forces or torques may be used to drive the valve to the closed position. In some cases, the valve actuator can drive the valve gate with too much force after it has nested in its vale seat far and can damage the fluid channel or the valve. Furthermore, the gate may become lodged in the valve seat and may not be able to be withdrawn. 
     BRIEF SUMMARY 
     Embodiments of the present disclosure provide a valve actuator that can reliably drive a valve to close off a fluid channel without damaging the valve or the fluid channel. Embodiments of the present disclosure provide a valve actuator that includes a DC braking system that assists in ensuring that a valve arrives at the closed position without damaging the valve or the fluid channel. 
     The valve actuator applies an AC voltage to a motor coupled to the valve. The AC voltage causes the motor to drive the valve toward the closed position. When the valve is near the fully closed position, a control system disconnects the AC voltage from the motor. After disconnecting the AC voltage from the motor, the control system connects a DC voltage from the DC braking system to the motor. The DC voltage act as a brake that stops the motion of the motor and the valve. By the time the DC voltage has stopped the valve, the valve is seated in the closed position. This ensures that the valve gate will not be too forcefully driven into the seating surface of the fluid channel. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the embodiments, reference will now be made by way of example only to the accompanying drawings. In the drawings, identical reference numbers identify similar elements or acts. In some drawings, the sizes and relative positions of elements are not necessarily drawn to scale. 
       For example, some of these elements may be enlarged and positioned to improve drawing legibility. 
         FIG.  1    is a block diagram of a valve system  100 , in accordance with one embodiment. 
         FIG.  2 A  is an illustration of a valve system with a valve in an open position, in accordance with one embodiment. 
         FIG.  2 B  is an illustration of the valve system of  FIG.  2 A  with the valve in a closed position, in accordance with one embodiment. 
         FIG.  3    is a circuit diagram of a DC braking system, in accordance with one embodiment. 
         FIG.  4    is a block diagram of a watercraft, in accordance with one embodiment. 
         FIG.  5    is a block diagram of a control system, in accordance with one embodiment. 
         FIG.  6    is a flow diagram of a method for operating a valve actuator, in accordance with one embodiment. 
         FIG.  7    is a flow diagram of a method for operating a valve actuator, in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     For most valves, there is a defined path of travel for the valve between two end points. For example, if the valve is in an open position, a force can be applied to the valve (e.g., via an actuator or hand wheel controlling the valve) to close the valve. The valve is moved from the first open position and travels along its path until it is seated in a second, closed position. The movement of the valve along its path may occur through translation or rotation of the valve. As such, when designing a valve actuator to control a valve, there are several design considerations. For example, more torque or force is generally needed to seat a valve or to remove a valve from its seated position (e.g., to unseat a valve) than is needed to move the valve through most of its travel path due to the forces associated with seating the valve, which may include pressure along the line of fluid in which the valve operates, close tolerances between the valve body or seal and the disk (which prevents leakage of the valve in the closed, seated position), or other issues of the mechanical configuration of the valve. In some cases, the preferred torque to seat the valve can be upwards of 10 times the preferred torque to move the valve along its travel path. 
     In addition, the design specifications of certain systems that utilize valves and valve actuators typically include a preferred time of operation. In some cases, the valve is preferably moved along its travel path from the open position to the closed position in 30 seconds, while in other cases, the operational time of the valve may be as little as 2 seconds or less. 
     Additionally, the inertial properties of the motors or other sources of torque are considered in designing the actuator. For example, when a rotor of a motor is inactivated, the rotor may continue to spin because of the inertia in the rotor, which will continue carrying the valve through its path after the motor is inactive. Such inertial forces can result in damage to the mechanical components (e.g., the valve), or even jamming of the valve in the seated, closed position. 
     It is contemplated in the present disclosure to provide a valve actuator that includes a DC braking system to supplement braking of the motor and the valve as the valve approaches the seated position. The motor may be primarily driven with an AC voltage. The AC voltage is applied to the motor to drive the valve toward the seated or closed position. When the valve is at a threshold distance from the fully seated position, the AC voltage is disconnected from the motor. The DC braking system is then connected to the motor. The DC braking system applies a DC voltage to the motor that rapidly stops rotation of the motor and motion of the valve. The valve arrives at the seated position as the motor comes to a stop. 
     Because the DC braking system can very rapidly stop rotation of the motor and the inertia of the valve, the motor can be utilized to rapidly drive the valve to the closed position, at which point the DC braking system can rapidly stop motion of the motor and the valve. This results in both a decrease in the total time to close the valve and a decrease in the risk that the valve will be forcefully lodged in the seated position. 
     In one embodiment, the motor includes a plurality of stator coils and a rotor. The rotor is driven by applying AC voltages to the stator coils. The rotor is mechanically coupled to the valve such that rotation of the rotor causes movement of the valve. When the motor has driven the valve nearly to the fully seated position, the AC voltage is disconnected from the stator coils. After the AC voltage has been disconnected from the stator coils, the DC braking system provides a DC voltage to one or more of the stator coils. The applied DC voltage quickly stops motion of the rotor and the valve at the seated position. 
     In one embodiment, the valve actuator includes a sensor and a control system. The sensor directly or indirectly senses the position of the valve. The sensor outputs sensor signals indicative of the position of the valve. The valve actuator includes a control system that receives the sensor signals. When the sensor signals indicate that the valve has been driven to a prescribed position, nearly to the fully seated position, the control system disconnects the AC power source from the stator coils of the motor. The control system then connects the DC power source to one or more of the stator coils of the motor providing resistance to rotor rotation as the valve reaches its closed position. 
       FIG.  1    is a block diagram of a valve system  100  including valve actuator  102 , in accordance with one embodiment. The valve actuator  102  controls fluid flow through the fluid channel  104 . More particularly, the valve actuator  102  controls a valve  106  that opens and closes the fluid channel  104 . As will be set forth in more detail below, the valve actuator  102  facilitates efficient and safe seating and unseating of the valve  106  to close the fluid channel  104 . 
     The valve actuator  102  includes a motor  108  coupled to the valve  106 . The motor  108  drives movement of the valve  106 . In particular, when the fluid channel is to be closed, the motor  108  drives the valve  106  into the fluid channel  104  until the valve  106  is seated against a seating surface  124  of the fluid channel  104 . When the fluid channel  104  is to be opened, the motor  108  drives the valve away from the seating surface  124  of the fluid channel  104  to an open position that enables a desired amount of fluid flow through the fluid channel  104 . 
     In one embodiment, the motor  108  includes a rotor  110 . The rotor  110  is mechanically coupled to the valve  106  such that rotation of the rotor  110  causes movement of the valve  106 . The rotor  110  may be coupled to the valve  106  by gears, shafts, or other drivetrain components that enable rotation of the rotor  110  to drive motion of the valve  106 . The rotor  110  can be rotated in a first direction to drive the valve  106  toward the seated or closed position. The rotor  110  can be rotated in a second direction to drive the valve  106  away from the seated or closed position. 
     In one embodiment, the motor  108  includes one or more stator coils  112 . The stator coils  112  can be driven with an AC voltage to generate a magnetic field. The magnetic field causes rotation of the rotor  110 . The motor  108  may include three stator coils  112  that operate in a three-phase configuration. In other cases, the motor  108  includes a single stator coil and operates in a single-phase configuration. While the present disclosure describes embodiments in which a motor  108  includes stator coils  112  and a magnetized rotor  110 , other configurations of a motor  108  can be utilized without departing from the scope of the present disclosure. 
     The valve actuator  102  includes an AC power source  114 . The AC power source  114  can be selectively connected to the stator coils  112  of the motor  108  by one or more first switches or relays S 1 . The AC power source  114  provides an AC voltage to the stator coils  112 . Said another way, the AC power source  114  may drive an AC current through the stator coils  112 . In practice, the AC power source  114  may provide a three-phase AC voltage to the stator coils  112 . For example, there may be three stator coils  112 . The AC power source  114  may provide, to each individual stator core  112 , a respective AC voltage. The respective AC voltages have the same amplitude and frequency, but are mutually 120° out of phase with each other. Alternatively, the AC power source  114  may provide a single-phase AC voltage to a single stator coil  112  or to multiple stator coils  112 . 
     The AC power source may correspond to one or more sets of wires that carry an AC voltage or multiple phases of AC voltage. The AC power source may also include other components facilitate providing the AC voltage or driving an AC current through the stator coils  112  of the motor  108 . 
     The valve actuator  102  includes a DC braking system  116 . The DC braking system  116  can be selectively connected to one or more of the stator coils  112  of the motor  108  by one or more second switching devices such as a switches, relays, FETs, or other switching devices S 2 . The first and second switching devices S 1  and S 2  are controlled such that the AC power source  114  and the DC braking system  116  are never both coupled to the stator coils  112  at the same time. The DC braking system  116  provides a DC voltage to one or more of the stator coils  112  of the motor  108 . Applying the DC voltage to the one or more stator coils  112  has the effect of rapidly stopping motion of the rotor  110 . Accordingly, the DC braking system  116  acts as a brake for the rotor  110  in order to rapidly stop motion of the rotor  110 . Because the rotor  110  is mechanically coupled to the valve  106 , the DC braking system  116  acts as a brake for the valve  106 . The DC voltage may be applied to one of the stator coils  112 , to all of the stator coils  112 , or to a subset of the stator coils  112 . Further details regarding the DC braking system  116  are provided below. 
     The valve actuator  102  includes a control system  118 . The control system  118  controls the operation of the AC power source  114  and the DC braking system  116 . Though not shown in  FIG.  1   , the control system  118  may be coupled to and may control the operation of the first and second switching devices S 1  and S 2 . The control system  118  may selectively connect and disconnect the AC power source  114  and the DC braking system  116  from the motor  108 . 
     The control system  118  may include one or more processors and one or more memories. The one or more memories may store software instructions related to the overall operation of the valve actuator  102 . The software instructions may include instructions for controlling the AC power source  114  and the DC braking system  116 . The memory may include data values related to positions and functions of the valve  106 , the motor  108 , the AC power source  114 , and the DC braking system  116 . The one or more processors may execute the software instructions and may otherwise read data from the one or more memories. Execution of the software instructions causes the control system  118  to control the motor  108 , the AC power source  114  and the DC power source  116 . 
     In practice, the control system  118  may include multiple individual control systems or control modules. Each individual control system or control module may control a component of the valve actuator  102 . For example, a portion of the control system  118 , such as a control unit, may be included in the DC braking system  116 . 
     The valve actuator  102  includes a sensor  120 . The sensor  120  is configured to sense a position of the valve  106 . The sensor  120  generates sensor signals indicative of the position of the valve  106 . 
     The sensor  120  is coupled to the control system  118 . The sensor  120  can provide sensor signals to the control system  118 . The control system  118  can analyze the sensor signals and can control connection and disconnection of the AC voltage and the DC braking voltage from the stator coils  112  responsive to the sensor signals provided by the sensor  120 . 
     In one embodiment, the sensor  120  is a potentiometer coupled to the valve  106 . The resistance of the potentiometer changes based on the position of the valve  106 . In some cases, the resistance of the potentiometer may have a one-to-one correspondence with the position of the valve  106 . The resistance of the potentiometer affects the output of a voltage divider which is used to indicate the current position of the valve  106 . 
     In one embodiment, the control system  118  includes a lookup table. The lookup table includes a plurality of potentiometer-driven voltage values and a plurality of valve position values. Each potentiometer-driven voltage value is paired with a respective valve position value. Accordingly, as the control system  118  receives potentiometer-driven voltage values, the control system  118  refers to the lookup table and compares the current potentiometer-driven voltage value to the potentiometer resistance value stored in the lookup table. The control system  118  may identify in the lookup table the potentiometer resistance value that most closely matches the current potentiometer resistance value. The control system may then refer to the corresponding valve position value to determine or estimate the current position of the valve  106 . In this way, the control system  118  can determine the current position of the valve based on the current potentiometer resistance value. 
     When referring to the lookup table, the control system  118  may be configured to identify in the lookup table the closest potentiometer-driven voltage value that is greater than the current potentiometer-driven voltage value. Alternatively, the control system  118  may be configured to identify in the lookup table the closest potentiometer-driven voltage value that is lower than the current potentiometer-driven voltage value. 
     The sensor  120  can include sensors other than potentiometers. The sensor  120  can include any suitable type of sensor that can determine the current position of the valve  106  and can output sensor signals, or can otherwise provide an indication of the current position of the valve  106 . 
     In one embodiment, the control system  118  includes a threshold valve travel distance. When a command is received to close the valve  106 , the control system  118  connects the AC power source  114  to the stator coils  112  of the motor  108 . The control system  118  monitors the position of the valve based on the sensor signals from the sensor  120 . When the sensor signals indicate that the valve  106  has traveled a threshold distance toward the closed position, the control system  118  disconnects the AC power source  114  from the stator coils  112 . After the control system  118  disconnects the AC power source  114  from the stator coils  112 , the control system  118  connects the DC braking system  116  to the stator coils  112 . The control system  118  causes the DC braking system  116  to apply a DC voltage to one or more of the stator coils  112  of the motor  108 . The application of the DC voltage causes the motor  108  to stop. More particularly, the DC voltage causes the rotor  110  to stop. When the rotor  110  stops, the valve  106  also stops. 
     During application of the DC voltage, the motor  108  and the valve  106  continue to move for a brief amount of time due to the inertia of the motor rotor  1108  and the valve  106 . Accordingly, the threshold gate travel distance can be selected so that the residual motion of the valve  106  during application of the DC braking voltage brings the valve  106  to the fully closed position. The proper threshold distances, AC voltage amplitude, and DC voltage amplitude can be determined through testing and calibration 
     The control system  118  may implement a delay between disconnecting the AC power source  114  and connecting the DC braking system  116 . After the control system  118  disconnects the AC power source  114  from the motor  108 , the control system  118  waits a selected duration of time before connecting the DC braking system  116  to the motor  108 . This delay time can allow the induced current through the stator coils to dissipate and prevent the braking system from creating potentially damaging transient voltages. This delay can also ensure that there are no short circuits between the AC power source  114  and the DC braking system  116 . In one example, the delay between disconnecting the AC power source  114  and connecting of the DC braking system  116  is between 10 ms and 500 ms, though other delay values can be utilized without departing from the scope of the present disclosure. 
     In one embodiment, the control system  118  implements a delay between connecting a first rail of the DC braking system  116  to the stator coils  112  and connecting a second rail of the DC braking system  116  to the stator coils  112 . For example, after the initial delay subsequent to disconnecting the AC power source  114 , the control system  118  may connect a high DC rail of the DC power source  116  to the stator coils  112 . The control system  118  may then wait for a second delay period before connecting low DC rail of the DC power source  116  to the stator coils  112 . Alternatively, the control system  118  may first connect the low DC rail and may then connect the high DC rail after the second delay period. The second delay period may be between 10 ms and 100 ms, though other delay values can be implemented without departing from the scope of the present disclosure. 
     The control system  118  may implement multiple steps of driving the motor  108  and braking in order to close the valve  106 . In this case, the control system  118  may implement multiple threshold travel distances. The control system  118  may connect the AC power source  114  to the stator coils  112  with a first voltage amplitude until the valve  106  has traveled to the first threshold travel distance. The control system  118  may then apply the DC braking system  116  to the motor  108  until the sensor  120  indicates that the valve  106  has stopped. The control system  118  may then connect the AC power source  114  to the stator coils  112  at a second AC voltage amplitude lower than the first AC voltage amplitude, until the sensor signals indicate that the valve  106  has passed the second threshold distance. The control system  118  may then connect the DC braking system  116  to the motor  108  until the motor  108  has stopped. This may continue through multiple different threshold stopping distances until a final threshold stopping distance has been surpassed or achieved. The control system  118  may apply the DC braking system  116  for a final time. 
     In one embodiment, the control system  118  implements multiple threshold travel distances. Rather than continuously monitoring the position of the valve  106 , the control system  118  applies the AC power source  114  for a selected period of time and then applies the DC braking system  116 . After applying the DC braking system  116 , the control system  118  monitors the sensor  120  to determine if the valve  106  has passed the first threshold travel distance. If the valve  106  has not passed the first threshold travel distance, then the control system  118  applies the AC power source  114  at the first AC voltage amplitude to the motor  108  for a selected period of time. The cycles of applying the first AC voltage amplitude and then applying the DC braking system  116  continue until the sensor  120  indicates the valve  106  has passed the first threshold distance. After the valve  106  has passed the first threshold distance, the control system  118  applies the AC power source  114  at a second, lower AC voltage amplitude for a selected duration of time. The control system  118  then connects the DC braking system  116  and checks whether the valve  106  has passed a second threshold travel distance. The control system  118  may implement multiple cycles of the second AC voltage amplitude and DC braking until the second threshold travel distance has been crossed. The cycles of reduced AC voltage amplitudes and braking may continue until a final threshold travel distance has been crossed. 
     The control system  118  can also implement the DC braking system  116  when moving the valve  106  from the closed position to an open position to enable fluid flow through the fluid channel  104 . The control system  118  can implement the DC braking system  116  in substantially the same manner as described above. This can help ensure that the valve  106  is not driven forcibly beyond the desired open position. 
     The control system  118  can also implement the DC braking system  116  when moving the valve  106  to a desired intermediate position. In some cases, it may be beneficial to move the valve  106  to a position that is neither fully open nor fully closed. Such a position may be used when a particular reduced flow rate or increased pressure is desired within the fluid channel  104 . The control system  118  can utilize the DC braking system  116  as described above in moving the valve  106  to the desired intermediate position. 
       FIG.  2 A  is a cross-sectional illustration of a valve actuator  102  with a valve  106  at an open position such that fluid can flow through the fluid channel  104 , in accordance with one embodiment. The valve actuator  102  includes a housing  126 . The housing  126  includes a first interior portion  129  and a second interior portion  131 . A motor  108  is positioned within the first portion  129 . A shaft  128  extends through the first portion  129  and the second portion  131 . The motor  108  is coupled to the shaft  128 . Though not shown in  FIG.  2 A , the valve actuator  102  may include a gear train coupled between the motor  108  and the shaft  128 . 
     A valve  106  is positioned at the end of the shaft  128 . The housing  126  includes an aperture through which the valve  106  can pass into the fluid channel  104 . The valve  106  can be driven through the aperture  132  into the fluid channel  104  toward a seating surface  124 . When the end  135  of the valve  106  is positioned at the seating surface  124 , the valve  106  is in the closed position and the fluid channel  104  is closed such that fluid will not flow through the fluid channel  104 . 
     The housing  126  includes a mounting assembly  133 . The mounting assembly  133  can be utilized to mount the housing  126  to a pipe, tube, or other fluid carrying body. Various other configurations of a mounting assembly  133  can be utilized without departing from the scope of the present disclosure. 
     The valve actuator  102  includes a control system  118 , an AC power source  114 , and a DC braking system  116  mounted to the housing  126 . One or more of the control system  118 , the AC power source  114 , and the DC braking system  116  may be positioned in the housing  126 . In some cases, the control system  118 , the AC power source  114 , and the DC braking system  116  may be positioned in a separate housing mounted to the housing  126 . Various other configurations of the control system  118 , AC power source  114 , and the DC braking system  116  can be utilized without departing from the scope of the present disclosure. 
     The valve actuator  102  includes a sensor  120  positioned within the housing  126 . As described previously, the sensor  120  can sense the position of the valve  106 . The sensor  120  can provide sensor signals to the control system  118  indicating the position of the valve  106 . 
     The valve actuator  120  includes a hand wheel  130  coupled to the shaft  128 . The hand wheel  130  can be manipulated manually to move the valve  106  to the open or closed position. 
     As described in relation to  FIG.  1   , the valve actuator  102  can utilize the DC braking system  116  to assist in moving the valve  106  safely and efficiently to the closed position. The valve actuator  102  can also utilize the DC braking system  116  in moving the valve  106  to the open position or to an intermediate position. 
       FIG.  2 A  illustrates one embodiment of a valve actuator  102  and a valve assembly in accordance with principles of the present disclosure. However, various other shapes, configurations, and valve types can be utilized without departing from the scope of the present disclosure. 
       FIG.  2 B  is an illustration of the valve actuator  102  and valve assembly of  FIG.  2 A , except that the valve  106  is in the closed position. In particular, the end  135  of the valve  106  is in contact with the seating surface  124 . The fluid channel  104  is closed. With the help of the DC braking system  116  and the control processes described above, the valve actuator  102  can seat the valve at the seating surface  124  without damaging the valve  106  or the seating surface  124 . 
       FIG.  3    is a schematic diagram of a DC braking system  116 , in accordance with one embodiment. The DC braking system  116  can be implemented on one or more circuit boards or other mounting or housing systems. The DC braking system  116  may include portions of the control system  118 . Alternatively, the DC braking system  116  may include components that communicate with the control system  118 . 
     The DC braking system  116  includes an electrical connection  140 . The electrical connection  140  receives a three-phase voltage from the AC power source  114 , or from another power source. In one embodiment, the electrical connection  140  receives a three-phase voltage via a transformer connected between the AC power source  114  and the DC braking system  116 . The transformer may be a step-up or step-down transformer. 
     The DC braking system  116  includes diodes D 1 -D 6  coupled to the connection circuit  114 . Diodes D 1  and D 4  correspond to a first pair diodes. Diodes D 2  and D 5  correspond to a second pair diodes. Diodes D 3  and D 6  correspond to a third pair diodes. The cathode of the diode D 4  and the anode of the diode D 1  are coupled to a first AC voltage output of the connection circuit  140 . The cathode of the diode D 5  and the anode of the diode D 2  are coupled to a second AC voltage output of the connection circuit  140 . The cathode of the diode D 6  and the anode of the diode D 3  are coupled to a third AC voltage output of the connection circuit  140 . 
     The DC braking system  116  also includes capacitors C 1 -C 3  connected in parallel with each other and with the pairs of diodes. The capacitors C 1 -C 3  and the diodes D 1 -D 6  collectively function as a rectifier that rectifies the three-phase voltage received by the connection circuit  140  and provides a DC voltage. The cathodes of the diodes D 1 -D 3  correspond to a high voltage rail  141  of the DC braking system  116 . The anodes of the diodes D 4 -D 6  correspond to a low-voltage rail  143  of the DC braking system  116 . The voltage between the high and low voltage rails corresponds to the DC voltage of the DC braking system  116 . A variable resistance resistor R 1  is coupled between the high voltage rail  141  and the low-voltage rail  143 . Resistor R 1  dissipates potentially circuit damaging capacitor charge that remains in the circuit after AC power has been switched off. 
     The output of the DC braking system  116  is coupled to a stator coil  112 . More particularly, the high voltage rail  141  and the low-voltage rail  143  are coupled to the stator coil  112 . The high voltage rail and the low-voltage rail may be connected to each of the stator coils  112  of the motor  108  or to a subset of the stator coils  112  of the motor  108 . As will be set forth in more detail below, the DC voltage is selectively applied to the stator coils  112  by operation of relays, transistors, and control signals. 
     The DC braking system  116  includes a switching devices  150  and  152 . The switching devices  150  can selectively connect and disconnect the high voltage rail  141  to the stator coils  112 . The switching devices  152  can selectively connect the low-voltage rail  143  to the stator coils  112 . 
     The DC braking system includes bipolar transistors Q 1  and Q 2 . The DC braking system also includes power MOSFET transistors Q 3  and Q 4 . The transistor Q 4  can be operated to selectively connect the low-voltage rail  143  to the stator coils  112 . In particular, the current path between the low-voltage rail  143  and the stator coils  112  flows through the source and drain terminals of the transistor Q 4 . The source terminal of the transistor Q 4  is coupled to the low-voltage rail  143 . The drain terminal of the transistor Q 4  is coupled to the stator coils  112  via the switching devices  152 . When a high voltage is applied to the gate terminal of the transistor Q 4 , the transistor Q 4  is enabled and the low-voltage rail  143  can be coupled to the stator coils  112  via the relay  152 . The control function of the gate terminal of the transistor Q 4  will be described in greater detail below. 
     The gate terminal of the transistor Q 4  is coupled to the emitter terminal of the bipolar transistor Q 2  via a resistor R 7 . The collector terminal of the bipolar transistor Q 2  is coupled to the high voltage rail  141 . A resistor R 8  is coupled between the gate terminal of the transistor Q 4  in the low-voltage rail  143 . 
     The gate terminal of the transistor Q 3  is coupled to the emitter terminal of the bipolar transistor Q 1  via a resistor R 4 . The collector terminal of the bipolar transistor Q 1  is coupled to the high voltage rail  141 . The source terminal of the transistor Q 3  is coupled to the low-voltage rail  143 . The drain terminal of the transistor Q 3  is coupled to a resistor R 6 . An inductor L 1  and a diode D 9  are coupled in parallel between the resistor R 6  and the high voltage rail  141 . The inductor L 1  corresponds to a relay coil. A resistor R 5  is coupled between the gate terminal of the transistor Q 3  and the low-voltage rail  143 . 
     The DC braking system  116  includes a control circuit  142 . The DC braking system  116  also includes a resistor R 2  coupled to a first output of the control circuit  142 . A second output of the control circuit  142  is coupled to ground. The DC braking system  116  includes a resistor R 3  coupled to a third output of the control circuit  142 . The DC braking system  116  includes a diode D 7  physically adjacent to the base of the transistor Q 1  and having an anode coupled to the resistor R 3  and a cathode coupled to ground. The DC braking system  116  includes a diode D 8  physically adjacent to the base of the transistor Q 2  and having an anode coupled to the resistor R 3  and the cathode coupled to ground. 
     The diodes D 7  and D 8  are photodiodes. The transistors Q 1  and Q 2  include base terminals of a photosensitive semiconductor material adjacent to the light emitting diodes D 7  and D 8 . The diodes D 7  and D 8  and the transistors Q 1  and Q 2  correspond to an optoisolation circuit. 
     When the control system  118  determines that the low-voltage rail  143  should be connected to the stator coils  142 , the control system one  118  causes the control circuit  142  to output a high voltage on the third output terminal. This causes a current to flow through the light emitting diode D 8 . The light emitting diode D 8  outputs light. The base terminal of the transistor Q 2  absorb some of the light output by the light emitting diode D 8 . The absorption of light by the base terminal of the transistor Q 2  causes a current to flow between the collector terminal of the transistor Q 2  and the emitter terminal of the transistor Q 2 . This causes the voltage at the gate terminal of the transistor Q 4  to go high, thereby turning on the transistor Q 4  and coupling the low-voltage rail  143  to the stator coil  146 . 
     The control system  118  can cause the transistor Q 3  to turn on by causing the control circuit  142  to output a high voltage on the first output terminal of the control circuit  142 . When the control circuit  142  outputs a high voltage, a current flows through the light emitting diode D 7 , causing the light emitting diode D 7  to emit light. The photosensitive base terminal of the transistor Q 1  receives the light and turns on, causing a current to flow from the collector to the emitter and causing the gate terminal of the transistor Q 3  to go high, turning on the transistor Q 3 . Turning on the transistor Q 3  enables current to flow through the relay coil L 1 . 
     The DC braking system  116  includes a diode having an anode coupled to the high voltage rail  141  and the cathode coupled to the relay  150 . The DC braking system  116  includes a diode D 11  having an anode coupled to the drain terminal of the transistor Q 4  and a cathode coupled to the cathode of the diode D 10 . The DC braking system includes a transmission gate  148  coupled between the relays  150  and  152 . 
     The DC braking system  116  can selectively supply the DC voltage to the stator coils  112 . The DC braking system generates the DC voltage by rectifying the three-phase voltage received by the selection circuit  140 . When the DC voltage is to be applied to the stator coils  112 , the control circuit  118  controls the relay  150  to connect the high-voltage rail  141  to the stator coils  112 . After a brief delay, as described previously, the control system  118  causes the relay  152  to connect the low-voltage rail  143  to the stator coils  112 . The control circuit  118  also causes the control circuit  142  to turn on the transistor Q 4 , in order to supply the low-voltage rail to the stator coils  112 . When the high-voltage rail and the low-voltage rail are both connected to the stator coils  112 , the DC voltage is supplied across the stator coils  112 , creating a static magnetic field within the stator coil, creating drag on the rotor, causing the motor to stop. In this way, the DC braking system  116  can supply the DC voltage to the stator coils  112 , causing the rotor  110  of the motor  108  to stop. Other configurations of a DC braking system  116  can be implemented without departing from the scope of the present disclosure. 
       FIG.  4    is a block diagram of a watercraft  160 , according to one embodiment. The watercraft  160  includes a valve actuator  102 , a fluid channel  104 , and the valve  106 . The valve actuator  102 , the fluid channel  104 , and the valve  106  can include components, functions, structures, and processes as described in relation to  FIGS.  1 - 3   . Watercraft can include a boat, ship, or other type of watercraft capable of carrying people and cargo on the river, a lake, the sea, or an ocean. 
       FIG.  5    is a block diagram of a control system  118 , in accordance with one embodiment. The control system  118  can include components, functions, and processes as described in relation to  FIGS.  1 - 4   . The control system  118  includes processing resources  162 , memory resources  164 , and communication resources  166 . The processing resources  162  can include one or more processors, microcontrollers, microprocessors or other computing resources. The memory resources  164  can include one or more memories, such as computer readable media that store software instructions for executing functions associated with the control system  118  as described previously. The processing resources  162  can execute the instructions stored in the memory resources  164  and can otherwise store and retrieve data from the memory resources  164 . The communication resources  166  can include resources for communicating in a wired manner and/or in a wireless manner between components of the control system  118  and between components and systems external to the control system  118 . 
       FIG.  6    is flow diagram of a method  600  for operating a valve. The method  600  can include components, systems, and processes described in relation to  FIGS.  1 - 3   . At  602 , the method  600  includes driving a valve toward a closed position by applying an AC voltage to a motor. At  604 , the method  600  includes disconnecting the AC voltage from the motor prior to the valve reaching the closed position at  606 , the method  600  includes stopping motion of the valve by applying a DC voltage to the motor after disconnecting the AC voltage from the motor. 
       FIG.  7    is flow diagram of a method  700  for operating a valve. The method  700  can include components, systems, and processes described in relation to  FIGS.  1 - 4   . At  702 , the method  700  closing off a fluid channel of a watercraft by applying an AC voltage to a motor coupled to a valve. At  704 , the method  700  includes sensing a position of the valve. At  706 , the method  700  includes disconnecting the AC voltage from the motor responsive to sensing that the valve has reached a threshold position. At  708 , the method  700  includes stopping a motion of the valve by applying a DC voltage to the motor after disconnecting the AC voltage from the motor. At  710 , the method  700  includes disconnecting the DC voltage from the motor after stopping the motion of the motor. 
     In one embodiment, a method includes driving a valve toward a closed position by applying an AC voltage to a motor, disconnecting the AC voltage from the motor prior to the valve reaching the closed position, and stopping motion of the valve by applying a DC voltage to the motor after disconnecting the AC voltage from the motor. 
     In one embodiment, a valve actuator includes a motor, an AC power source configured to apply an AC voltage to the motor, a DC braking system configured to apply a DC voltage to the motor, and a control system. The control system is configured to control the AC power source to apply the AC voltage to the motor to drive a valve toward a closed position, to control the AC power source to disconnect the AC voltage from the motor before the valve reaches the closed position, and to control the DC braking system to apply the DC voltage to the motor to stop motion of the valve. 
     In one embodiment, a method includes closing off a fluid channel by applying an AC voltage to a motor coupled to a valve, sensing a position of the valve, and disconnecting the AC voltage from the motor responsive to sensing that the valve has reached a threshold position. The method includes stopping a motion of the valve by applying a DC voltage to the motor after disconnecting the AC voltage from the motor and disconnecting the DC voltage from the motor after stopping the motion of the motor. 
     In one embodiment, a system includes a fluid channel, a valve coupled to the fluid channel configured open and close the fluid channel, and a valve actuator coupled to the valve. The valve actuator includes a motor, an AC power source configured to apply an AC voltage to the motor, a DC braking system configured to apply a DC voltage to the motor, and a control system. The control system is configured to control the AC power source to apply the AC voltage to the motor to drive the valve toward a closed position, to control the AC power source to disconnect the AC voltage from the motor before the valve reaches the closed position, and to control the DC braking system to apply the DC voltage to the motor to stop motion of the valve. 
     In one embodiment, a non-transitory computer-readable medium having contents which configure a valve actuator to perform a method. The method includes, driving a valve toward a closed position by applying an AC voltage to a motor, disconnecting the AC voltage from the motor prior to the valve reaching the closed position, and stopping motion of the valve by applying a DC voltage to the motor after disconnecting the AC voltage from the motor. 
     In the above description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with valve actuator assemblies and methods and electric, hydraulic, or pneumatic motors have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. 
     As used herein, the term “valve” is broadly construed to include, but is not limited to, a device capable of regulating a flow of one or more substances by opening, closing, or partially blocking one or more passageways. For example, a valve can halt or control the flow of a fluid (e.g., a liquid, a gas, or mixtures thereof) through a conduit, such as a pipe, tube, line, duct, or other structural component (e.g., a fitting) for conveying substances. Valve types include ball valves, butterfly valves, globe valves, plug valves, gate valves, guillotine valves, and the like. 
     Further, as used herein, unless the context clearly dictates otherwise, the term “gear” is broadly construed to include a device for transferring force (e.g., torque, etc.) from one object to another, and includes, but is not limited to, devices with structures such as ribs, channels, teeth, splines, protrusions, extensions, projections, or other structural components to accomplish such transfer by meshing with another device having corresponding structures. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” Further, the terms “first,” “second,” and similar indicators of sequence are to be construed as interchangeable unless the context clearly dictates otherwise. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its broadest sense, that is as meaning “and/or” unless the content clearly dictates otherwise. 
     The relative terms “approximately” and “substantially,” when used to describe a value, amount, quantity, or dimension, generally refer to a value, amount, quantity, or dimension that is within plus or minus 5% of the stated value, amount, quantity, or dimension, unless the content clearly dictates otherwise. It is to be further understood that any specific dimensions of components provided herein are for illustrative purposes only with reference to the exemplary embodiments described herein, and as such, the present disclosure includes amounts that are more or less than the dimensions stated, unless the context clearly dictates otherwise. 
     The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Although specific embodiments of and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art. The teachings provided herein of the various embodiments can be applied outside of the valve actuator assembly context, and not necessarily the exemplary valve actuator assembly systems, methods, and devices generally described above. 
     The foregoing detailed description has set forth various embodiments of the devices and/or processes via the use of certain exemplary embodiments. Insofar as such embodiments contain one or more functions and/or operations, it will be understood by those skilled in the art that each function and/or operation within such embodiment can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. In one embodiment, the present subject matter may be implemented via Application Specific Integrated Circuits (ASICs). However, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented in standard integrated circuits, as one or more computer programs executed by one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs executed by on one or more control systems (e.g., microcontrol systems) as one or more programs executed by one or more processors (e.g., microprocessors), as firmware, or as virtually any combination thereof. 
     When logic is implemented as software and stored in memory, logic or information can be stored on any computer-readable medium for use by or in connection with any processor-related system or method. In the context of this disclosure, a memory is a computer-readable medium that is an electronic, magnetic, optical, or other physical device or means that contains or stores a computer and/or processor program. Logic and/or the information can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions associated with logic and/or information. 
     In the context of this specification, a “computer-readable medium” can be any element that can store the program associated with logic and/or information for use by or in connection with the instruction execution system, apparatus, and/or device. The computer-readable medium can be, for example, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus or device. More specific examples (a non-exhaustive list) of the computer readable medium would include the following: a portable computer diskette (magnetic, compact flash card, secure digital, or the like), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM, EEPROM, or Flash memory), an optical disc memory (CD/DVD/Blu-ray), magnetic tape, and other nontransitory media. 
     Many of the methods described herein can be performed with variations. For example, many of the methods may include additional acts, omit some acts, and/or perform acts in a different order than as illustrated or described. 
     The various embodiments described above can be combined to provide further embodiments. To the extent that they are not inconsistent with the specific teachings and definitions herein, all of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.