Detecting displacer position in a cryopump

Abstract

A cryopump (1, fig 1) comprises a vacuum vessel (2, fig 1) with an inlet (3, fig 1) for receiving gas and a refrigeration unit (4, fig 1), which has a cold head (5, fig 1) extending into the vacuum ve

Description

FIELD OF THE INVENTION The field of the invention relates to a cryopump and a method. BACKGROUND 10 CXI 15 Cryopumps typically comprise a refrigerator unit for cooling the cryopump. The refrigerator unit includes a cold head for cooling a cryopanel structure for condensing gas within the cryopump. The cold head is cooled by a refrigerant which is cooled by the movement of a displacer being driven backwards and forwards by a motor. Conventional cryopumps do not measure the position of the displacer during normal operation. However, the position of the displacer may be measured for R&D purposes by installing a shaft within a yoke configured to drive the displacer. The shaft is connected to a linear position sensor configured to measure the position of the shaft. Since the yoke is positioned within the housing of the refrigerator unit, installing the shaft and the linear position sensor requires partial disassembly of the pump, venting helium from the cryopump and gluing the shaft to the yoke. It would be desirable to provide improved ways of monitoring the position of the displacer. 20 SUMMARY According to an aspect of the invention, there is provided a cryopump, comprising: a vacuum vessel comprising an inlet for receiving gas; a refrigeration unit comprising: a cold head extending into the vacuum vessel for cooling a 25 cryopanel structure for condensing said gas; a displacer configured to move to cool a refrigerant for cooling said cold head; a motor for controlling movement of said displacer; a component arranged to move with said displacer between a first position and a second position, said first position and said second position being indicative of a first end point and a second end point of a stroke of said displacer, 30 respectively, a ferromagnetic material forming at least a portion of said component; and a sensor system positioned outside a housing of said refrigeration unit, said sensor system comprising: a permanent magnet component configured to magnetise said ferromagnetic material; a first magnetic sensor positioned adjacent to said first position to detect a strength of a magnetic field of said component indicative of a position of said component; and a second magnetic sensor positioned adjacent to said second position to detect said 5 strength of said magnetic field of said component indicative of said position of said component. 10 CXI 15 It was realised that providing a cryopump with a sensor system capable of measuring the position of the displacer may provide information which may be used, for example, to improve pump control and diagnostics. The position of the displacer may be measured directly or indirectly. When measuring the position of the displacer indirectly, a different component of the cryopump configured to move in time with the displacer may be measured and the position of the displacer inferred from this measurement. The different component may, for example, be a yoke connecting the displacer to a motor configured to drive the displacer or it may be a shaft of the motor. The component being measured is magnetised such that it can be detected using a magnetic sensor. By using magnetic sensors to measure the position of the 20 component, the sensors can be distanced from the component being detected. Therefore, the position of the sensors may be more flexible and importantly may be less invasive than known methods. The component comprises a ferromagnetic material and the sensor system includes a permanent magnet arranged to magnetise the ferromagnetic material. Like the magnetic sensors, 25 this permanent magnet may also be distanced from the component. Therefore, by providing the component with a ferromagnetic material and a sensor system with a permanent magnet, the sensor system may be separate from the component and be positioned outside one or more of a housing of the pump, a housing of the refrigeration unit, a housing of the motor and a housing of the cold 30 head. Accordingly, embodiments may provide a non-invasive way to measure the position of the displacer. Such a non-invasive sensor system may avoid the need to disassemble the cryopump to fit the sensor system, thereby facilitating retrofitting of the sensor system on existing cryopumps. In some embodiments, said first magnetic sensor is positioned such that said 5 strength of said magnetic field of said component detected by said first magnetic sensor is greater in said first position than said second position. In some embodiments, said first magnetic sensor is positioned such that said strength of said magnetic field of said component detected by said first magnetic 10 sensor is weakest when said component is in said second position and strongest when said component is in said first position. In this way, the maximum and CXI 15 minimum points in the detected magnetic field may represent the end points of the stroke of the displacer, thereby allowing the position of the displacer to be readily determined. In some embodiments, said first magnetic sensor is positioned such that said first magnetic sensor cannot detect said magnetic field of said component in said second position. In some embodiments, said magnetic field detected is negligible in said second position. 20 The sensor system is positioned outside a housing of said refrigeration unit. In this way, the sensor system may be less invasive enabling the sensor system to be more easily retrofitted to existing cryopumps. 25 The sensor system comprises a second magnetic sensor positioned to detect said strength of said magnetic field of said component indicative of said position of said component, said magnetic sensor and said second magnetic sensor being positioned adjacent to said first position and said second position, respectively. 30 In some embodiments, said second magnetic sensor is positioned such that said strength of said magnetic field of said component detected by said second magnetic sensor is greater in said second position than said first position. ln some embodiments, said second magnetic sensor is positioned such that said strength of said magnetic field of said component detected by said second magnetic sensor is weakest when said component is in said first position and 5 strongest when said component is in said second position. In some embodiments, said second magnetic sensor is positioned such that said second magnetic sensor cannot detect said magnetic field of said component in said first position. In some embodiments, said magnetic field detected is 10 negligible in said first position. In some embodiments, said permanent magnet is positioned adjacent to a middle of said first position and said second position. In some embodiments, said permanent magnet is positioned equidistant from said first position and said 15 second position of said component. In this way, the permanent magnet may be able to magnetise the ferromagnetic material of the component at all times during the stroke of the displacer. In some embodiments, said component is configured to drive said displacer. In 20 some embodiments, said component comprises a yoke or a shaft of the motor. In some embodiments, said yoke is made from ferromagnetic steel. The yoke interconnects the displacer to the shaft of the motor which drives the displacer. Therefore, the yoke and the shaft of the motor move in synchronisation with the displacer. The yoke and / or shaft of the motor may be more accessible than the 25 displacer itself. The yoke is also commonly made from a ferromagnetic steel and surrounded by a non-ferromagnetic housing of the refrigerator unit. The yoke therefore makes a good candidate for being the component being detected because the sensor system could be retrofitted on existing cryopumps without changing the configuration or materials of the cryopump. 30 In some embodiments, said component comprises said displacer. Where the displacer is driven by pneumatic action or by hydraulics rather than a yoke, it may be beneficial to measure the position of the displacer directly (i.e., the displacer is the component being detected). Displacers are not typically formed from ferromagnetic materials. Therefore, to avoid changing the material of the displacer, embodiments in this scenario may be implemented by attaching a 5 magnetic material, such as a permanent magnet, to the displacer so that the magnetic sensors can detect movement of the displacer throughout the stroke of the displacer between the first and second position. In some embodiments, said magnetic sensors comprise hall-effect magnetic field 10 sensors. In some embodiments, said magnetic sensors comprise inductive sensors. CXI 15 20 In some embodiments, said sensor system comprises a sensor body, said magnetic sensors being mounted on said sensor body. In some embodiments, said permanent magnet is mounted on said sensor body. In some embodiments, said sensor body comprises an E-shaped portion comprising a central leg, a first side leg and a second side leg; said permanent magnet being mounted on said central leg, said first magnetic sensor being mounted on said first side leg, and said second magnetic sensor being mounted on said second side leg. In this way, the sensor system may be configured to position the first and second magnetic sensors at each end of the stroke of the component with the permanent magnet in the middle such that it can magnetise the component throughout the entire stroke. This positioning of the permanent magnet and the magnetic sensors may maximise the magnetic field strength. 25 30 In some embodiments, said cryopump further comprises motor control circuitry; wherein said magnetic sensors are configured to transmit signals indicative of magnetic fields detected by said magnetic sensors to said motor control circuitry, said motor control circuitry being configured to control said motor in dependence on said signals. In some embodiments, said motor control circuitry controls a speed of said motor based on feedback signals from said magnetic sensors. In this way, a closed-loop feedback control scheme for the motor may be provided. The closed-loop feedback control scheme for the motor may take into account signals from both sensors. 5 In some embodiments, said cryopump further comprises diagnostic circuitry; wherein said magnetic sensors are configured to transmit signals indicative of magnetic fields detected by said magnetic sensors to said diagnostic circuitry, said diagnostic circuitry being configured to analyse said signals for indications of deterioration or faults during operation of said cryopump. Pauses during the 10 stroke of the displacer or other unexpected movements may be indicative of deterioration in or a fault with the cryopump. By analysing the signals from the sensors, embodiments may help determine when a cryopump needs maintenance or when it should be shut down in case of an emergency. LO CM 15 According to another aspect of the invention, there is provided a method, comprising: fitting a sensor system to a housing of a refrigeration unit of a cryopump, said cryopump comprising: a vacuum vessel comprising an inlet for receiving gas; and said refrigeration unit comprising: a cold head extending into said vacuum vessel for cooling a cryopanel structure for condensing said gas, a 20 displacer configured to move to cool a refrigerant for cooling said cold head; a motor for controlling movement of said displacer; and a component arranged to move with said displacer between a first position and a second position, said first position and said second position being indicative of a first end point and a second end point of a stroke of said displacer, respectively, a ferromagnetic 25 material forming at least a portion of said component; said sensor system comprising a permanent magnet component configured to magnetise said ferromagnetic material; a first magnetic sensor; and a second magnetic sensor; wherein when the sensor system is fitted to the housing, the first magnetic sensor is positioned adjacent to said first position to detect a strength of a magnetic field 30 of said component indicative of a position of said component and the second magnetic sensor is positioned adjacent to the second position to detect said strength of said magnetic field of said component indicative of said position of said component. In this way, a sensor system may be retrofitted to an existing cryopump. The 5 sensor system may be entirely located outside a housing of the refrigerator unit making the sensor system particularly easy to install. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be 10 combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that 15 function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: 20 Figure 1 illustrates a cryopump according to an embodiment; Figure 2 illustrates a portion of a cryopump comprising a sensor system according to an embodiment; Figure 3 illustrates the portion of a cryopump of Figure 2 mid-stroke; Figure 4 illustrates the portion of a cryopump of Figure 2 at the bottom of a 25 stroke; Figure 5 illustrates example signals from the magnetic sensors in the embodiment of Figure 2; Figure 6 illustrates a method according to an embodiment; Figure 7 illustrates a cold head for a cryopump; and 30 Figure 8 illustrates an example signal from the magnetic sensor of Figure 7. DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. Embodiments propose a cryopump comprising a sensor instrument for externally LO CXI 5 measuring a cryopump displacer position. The instrument comprises magnetic sensors, for example, Hall effect magnetic field sensors, which are used to detect the position of a component, for example, a yoke, a displacer or a motor shaft of a cryopump refrigerator. The position of the yoke and the motor shaft are directly indicative of the position of the displacer because they are configured to drive the 10 displacer. The magnetic sensors are positioned outside the refrigerator and detect the magnetisation of the component. The component comprises a ferromagnetic material which is magnetised by a permanent magnet mounted outside the refrigerator housing on the sensor instrument. In this way, embodiments may provide a sensor system located entirely outside the 15 refrigerator that is capable of monitoring the position of the component and consequently the position of the displacer. In some embodiments, the cryopump may not include a yoke, for example, in a pneumatic or hydraulic refrigerator. In this case, the sensor instrument may be 20 arranged to detect the position of the displacer directly rather than the position of a component indicative of the position of the displacer. In some embodiments, the signals from the magnetic sensors are used in a closed-loop feedback control system for the motor which drives the displacer. In 25 some embodiments, the signals from the magnetic sensors are used for pump diagnostics to help determine problems with the pump such as helium escape or the displacer sticking at a certain point during its stroke. Figure 1 shows a cryopump 1 according to an embodiment. The cryopump 1 30 comprises a vacuum vessel 2 having an inlet 3 for receiving gas. The cryopump 1 further comprises a refrigeration unit 4 including a cold head 5, extending into the vacuum vessel 2, configured to cool a cryopanel structure 6 for condensing gas within the vacuum vessel 2. The cold head 5 comprises a 2-stage cold head including a 1st stage 7 and a second stage 8. Each stage comprises a displacer 9 configured to move backwards and forwards to cool a refrigerant for cooling the cold head 5 as is known in the art. A motor 10 is configured to drive the motion of the displacers 9. It will be appreciated that some embodiments may relate to a single stage cold head cryopump. The cryopump 1 further comprises diagnostic circuitry 30 and motor control circuitry 40 discussed in more detail below. 10 CXI 15 20 25 30 Figures 2, 3 and 4 illustrate a portion of a cryopump according to an embodiment. The cryopump comprises a yoke 20 configured to connect the motor 10 to the displacer 9 to drive movement of the displacer 9. The yoke 20 is therefore a component which moves in time with the displacer 9 meaning that the position of the yoke 20 is indicative of the position of the displacer 9. The cryopump further comprises a sensor system 11 positioned adjacent to the yoke 20 but outside of the refrigerator unit housing 22. The sensor system 11 is arranged to detect the position of the yoke 20, thereby indirectly measuring the position of the displacer 9. The housing 22 of the refrigerator is made from a non-ferromagnetic material, such as aluminium, so that it does not interfere with the sensor system. In Figure 2, the yoke 20 is in a first position at the top of a stroke, i.e., at one end of the to and fro movement of the displacer 9. By comparison, Figure 3 shows the yoke 20 in the middle of a stroke and Figure 4 shows the yoke 20 in a second position at the bottom of the stroke, i.e., at the opposite end of the stroke to Figure 2. It will be appreciated that the strokes of the yoke and the displacer are synchronised because the yoke drives the displacer. Hence, when the yoke is at the top of its stroke, the displacer will also be at the top of its stroke. Similarly, when the yoke is at the bottom of its stroke, the displacer will be at the bottom of its stroke. The sensor system 11 is mounted adjacent to the yoke 20 outside the housing 22 of the refrigerator unit 4. In some embodiments, the sensor system 11 is mounted to the outside of the refrigerator housing 22. The sensor system 11 comprises a permanent magnet 14 and first and second magnetic sensors 12a, 12b mounted on an E-shaped sensor body 16. It will be appreciated that other sensor body shapes may be used. The sensors 12a, 12b may be any sensor capable of measuring the magnetic field of the component being measured which, in this particular embodiment, is the yoke 20. For example, the sensors 12a, 12b may be Hall-effect sensors or inductive sensors. 10 CXI 15 20 25 30 The yoke 20 of the cryopump comprises a ferromagnetic material. In this particular embodiment, the entire yoke is made from a ferromagnetic material, for example, a ferromagnetic steel. The permanent magnet 14 is mounted on the central leg of the sensor body 16 such that it is positioned adjacent to the yoke 20. The permanent magnet 14 is arranged such that it magnetises the yoke 20 (represented by arrows 14a). By arranging the permanent magnet 14 equidistantly between the first and second positions of the yoke 20 and adjacent to a middle of the first and second positions, the permanent magnet 14 is suitably placed to magnetise the yoke 20 throughout the entire course of a stroke of the yoke 20 as shown across Figures 2, 3 and 4. The strength and position of the permanent magnet may be selected appropriately based on a number of factors including but not limited to the distance to the component to be magnetised, the thickness of the refrigerator housing, the strength of the magnetisation required, the magnetic sensors used and the type of ferromagnetic material used. The magnetic sensors 12a, 12b are mounted on a respective side leg of the sensor body 16 such that they are positioned to detect the magnetic field of the magnetised yoke 20 through the refrigerator housing 22 as represented by arrows 20a. The first magnetic sensor 12a is positioned adjacent to the first position such that the magnetic field of the yoke 20 detected by the first sensor 12a is strongest at the top of the stroke and weakest at the bottom of the stroke. The first magnetic sensor 12a may be able to detect the magnetic field of the yoke 20 through the middle of the stroke as shown in Figure 3, but the first sensor 12a cannot detect the magnetic field of the yoke 20 or only detects a negligible or minimal magnetic field at the bottom of the stroke as shown in Figure 4. The opposite is true for the second sensor 12b which is mounted on the other side leg of the sensor body 16 such that it is positioned adjacent the second position. In this position, the strength of the magnetic field of the yoke 20 detected by the second magnetic sensor 12b is weakest in the first position and strongest in the second position. The second magnetic sensor 12b may be able to detect the magnetic field of the yoke 20 through the middle of the stroke as shown in Figure 3. However, the second sensor 12b cannot detect the magnetic field of the yoke 20 or only detects a negligible or minimal magnetic field at the top of the stroke as shown in Figure 2. 10 CXI 15 By positioning the magnetic sensors 12a, 12b such that they detect a peak magnetic field at one end of the stroke and a weakest magnetic field at the other end of the stroke, the peaks and troughs in the strength of the magnetic field measured by the sensors 12a, 12b are indicative of the end points in the movement of the yoke and the displacer. In this way, the sensor system may provide a clear indication from the detected magnetic field where the displacer is within a stroke. The detected magnetic field may also show where the movement of the displacer is not smooth or as expected, thereby providing a useful tool for pump diagnostics as discussed below. 20 By positioning the sensor system 10 outside the refrigerator housing 22, the sensor system 10 provides a non-invasive solution for monitoring of the position of the displacer. 25 Figure 5 shows an example signal received at the magnetic sensors 12a, 12b during operation of the pump. In practice, the signal may not be a perfect square wave because the strength of the magnetic field detected at the sensors may gradually increase and decrease as the yoke 20 moves closer to and away from the sensors 12a, 12b, respectively. 30 By providing two sensors, the sensor system may be more costly than a sensor system comprising only one sensor but it may be more resilient to noise as the signal from each sensor may be compared to determine the position of the displacer. In the embodiment shown in Figure 1, the cryopump 1 comprises diagnostic 5 circuitry 30. The diagnostic circuitry 30 is communicatively coupled to the magnetic sensors 12a, 12b. The magnetic sensors 12a, 12b are configured to transmit signals indicative of the magnetic field detected by the magnetic sensors 12a, 12b to the diagnostic circuitry 30. The diagnostic circuitry 30 is configured to analyse these signals in search for indications of deterioration or faults in the io operation of the cryopump 1. For example, an unexpected signal that is repeatedly measured at a certain point during the stroke of the displacer may indicate that the movement of the displacer is deteriorating or faulty. For example, the diagnostic circuitry 30 may be configured to detect helium escape or the displacer sticking during its stroke. The results from the diagnostic circuitry 15 may be used to determine when maintenance of the cryopump should be scheduled or when the cryopump should be shut down. In the embodiment shown in Figure 1, the cryopump 1 comprises motor control circuitry 40. The motor control circuitry 40 is communicatively coupled to the 20 magnetic sensors 12a, 12b and the motor 10. The magnetic sensors 12a, 12b are configured to transmit signals indicative of the magnetic field detected by the magnetic sensors 12a, 12b to the motor control circuitry 40. The motor control circuitry 40 is configured to use these signals to control the motor 10. In other words, the motor control circuitry 40 controls a speed of the motor 10 based on 25 feedback signals from said magnetic sensor. Therefore, embodiments can provide closed-loop feedback control of the motor. In some embodiments, the cryopump may not include a yoke, for example, for a hydraulically driven displacer or a displacer moved by pneumatic action. In these 30 cases, the sensor system may be adapted to detect the position of the displacer directly. In some embodiments, the component being monitored is the shaft of the motor which is configured to drive the movement of the displacer. A sensor positioned outside of the motor is configured to detect the varying magnetic field which provides an indication of the position of the motor shaft which in turn may be 5 used to infer the position of the displacer. Since rotation of the motor shaft drives the displacer stroke, there will be two rotational positions or orientations throughout a full rotation of the motor shaft which are indicative of the end points of the linear movement of the stroke of the displacer. 10 Figure 6 shows the steps of a method for fitting a sensor system to a cryopump according to an embodiment. Step S100 includes providing a cryopump comprising a vacuum vessel having an inlet for receiving gas and a refrigeration unit. The refrigeration unit comprises a cold head extending into the vacuum vessel for cooling a cryopanel structure for condensing the gas. The refrigeration 15 unit further comprises a displacer configured to move to and fro to cool a refrigerant for cooling the cold head. The refrigeration unit also comprises a motor for controlling movement of the displacer and a component arranged to move with the displacer. In some embodiments, the component comprises a yoke for driving the displacer. In other embodiments, the component comprises the 20 displacer itself. In some embodiments, the component comprises a motor shaft. Step S200 includes fitting at least a portion of a sensor system to a housing of the refrigeration unit. The sensor system comprises a permanent magnet component configured to magnetise the ferromagnetic material of the component. 25 The sensor system further comprises magnetic sensors positioned to detect a strength of a magnetic field of the component which is indicative of a position of the component and the displacer. Figure 7 shows a two-stage cold head 5 for a cryopump. A permanent magnet 14 30 is mounted to one end of the non-ferromagnetic yoke 20. A magnetic sensor 12, such as a mono or bipolar Hall sensor, is mounted to the outside of the housing of the cold head 5 such that it can detect the magnetic field of the permanent magnet 14. The magnetic sensor 12 is positioned adjacent to a first end point of the stroke of the yoke 20. In this way, the magnetic field detected by the magnetic sensor 12 is at a maximum when the yoke 20 reaches the first end point and at a minimum when the yoke 20 reaches the second end point at the opposite end of 5 the stroke. The permanent magnet 14 and the magnetic sensor 12 are positioned on a central axis of the cold head 5 along which the yoke 20 is configured to move. Figure 8, in the top graph, shows an example signal detected by the magnetic io sensor 12 of Figure 7. The magnetic sensor 12 outputs a voltage, V, over time, t, which is indicative of the position of the displacer of the cryopump. The voltage peaks show when the yoke 20 is in the up position (the first end point of the stroke) and the troughs show when the yoke 20 is in a down position further from the magnetic sensor 12 (the second end point of the stroke). The period of the 15 stroke, T, may be obtained from the time between the peaks or troughs. The bottom graph of Figure 8 shows the output signal from the magnetic sensor 12 in the top graph post processing. The signal processing converts the output signal of the magnetic sensor 12 into a sine wave which provides a more 20 accurate representation of the position of the yoke 20 within its stroke. The position of the yoke 20 may be calculated using the equation Stroke Position = —-—(Cos(2nf(t — dt)) + 1) where f = -. j T 25 In summary, embodiments propose an instrument utilising hall-effect magnetic field sensors to sense the position of the steel yoke in a cryopump refrigerator from outside of the pump. The position of the yoke is directly coupled to the displaced position, motor shaft angle, and valve positions. In current cryopumps on the market, the displacer position is not measured, nor is the motor angle or 30 valve positions. In R&D tests where this information is necessary, a shaft is installed which passes through the refrigerator housing and is connected to a LO CXI linear position sensor. However, this requires venting the helium from the pump and gluing the sensor shaft to the yoke. This is not a solution that can be used for customer pumps or for non-invasive measurements of competitor pumps. Embodiments provide for non-invasive measurement of the yoke position, motor 5 angle and valve position. Embodiments may help detect helium contamination issues and displacer sticking. Embodiments can further allow instantaneous measurements to be taken at the same displacer position as diode temperature oscillates throughout the displacer stroke. Certain measurements, such as helium flow and array temperatures, will fluctuate as the displacer moves between the 10 top and bottom of its stroke causing oscillations in those measurements. If data is only captured at mid-stroke, these oscillations should not be present, thereby displaying the centre line of a sine wave. For example, where it is desired to measure the temperature, measurements of the temperature may be aligned with a certain point during the stroke of the displacer such that fluctuations due to 15 displacer position variation may be eliminated or reduced. Embodiments may further allow closed-loop control of motor RPM. Current cryopumps provide no feedback method for refrigerator motor RPM and the motor runs in an open loop configuration. The signal from the Hall effect sensors can be used for a closed loop motor control scheme. 20 In some embodiments, Hall effect sensors are used to measure the strength of the magnetic field they are in. With the use of a steel or iron core, the magnetic field of a permanent magnet can be directed through the steel yoke of the refrigerator. As the refrigerator runs, the yoke slides back and forth, changing the 25 magnetic field seen by the hall effect sensors. The measured strength of the magnetic field is arbitrary, but there will be peaks at the points where the refrigerator yoke completes the magnetic circuit. By using multiple sensors spaced appropriately, the top and the bottom of the stroke can be measured. As the 1st and 2nd stage displacers are directly coupled to the refrigerator yoke, the 30 positions of these displacers can be determined from the detected position of the yoke. The refrigerator yoke is used to open and close the helium supply and return valves, so their position can also be extrapolated. LO CXI Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and 5 modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. 04 11 25 REFERENCE SIGNS 1 cryopump 2 vacuum vessel 3 inlet 5 4 refrigerator unit 5 cold head 6 cryopanel structure 7 1st stage 8 2nd stage io 9 Displacer 10 Motor 11 Sensor system 12, 12a, 12b magnetic sensors 14 permanent magnet 15 14a magnetic field 16 sensor body 20 Yoke 20a magnetic field 22 refrigerator unit housing

Claims

1. A cryopump, comprising:a vacuum vessel comprising an inlet for receiving gas;5 a refrigeration unit comprising:a cold head extending into the vacuum vessel for cooling a cryopanel structure for condensing said gas;a displacer configured to move to cool a refrigerant for cooling said cold head;io a motor for controlling movement of said displacer;a component arranged to move with said displacer betweena first position and a second position, said first position and said second position being indicative of a first end point and a second end point of a stroke of said displacer, respectively, a ferromagnetic15 material forming at least a portion of said component; anda sensor system positioned outside a housing of said refrigeration unit, said sensor system comprising:a permanent magnet component configured to magnetise said ferromagnetic material;20 a first magnetic sensor positioned adjacent to said firstposition to detect a strength of a magnetic field of said component indicative of a position of said component; anda second magnetic sensor positioned adjacent to said second position to detect said strength of said magnetic field of said25 component indicative of said position of said component.

2. A cryopump according to claim 1, wherein said first magnetic sensor ispositioned such that said strength of said magnetic field of said component30 detected by said first magnetic sensor is greater in said first position than said second position.

3. A cryopump according to claim 2, wherein said first magnetic sensor is positioned such that said strength of said magnetic field of said component detected by said first magnetic sensor is weakest when said component is in said second position and strongest when said component is in said first position.

54. A cryopump according to any preceding claim, wherein said permanent magnet is positioned adjacent to a middle of said first position and said second position.io 5. A cryopump according to any preceding claim, wherein said component is configured to drive said displacer.

6. A cryopump according to claim 5, wherein said component comprises a yoke or a shaft of the motor.

157. A cryopump according to any one of claims 1 to 4, wherein said component comprises said displacer.

8. A cryopump according to any preceding claim, further comprising motor 20 control circuitry; wherein said magnetic sensor is configured to transmit signals indicative of magnetic fields detected by said magnetic sensor to said motor control circuitry, said motor control circuitry being configured to control said motor in dependence on said signals.25 9. A cryopump according to any preceding claim, further comprisingdiagnostic circuitry; wherein said magnetic sensor is configured to transmit signals indicative of magnetic fields detected by said magnetic sensor to said diagnostic circuitry, said diagnostic circuitry being configured to analyse said signals for indications of deterioration or faults during operation of said cryopump.io10.152025A method, comprising:fitting a sensor system to a housing of a refrigeration unit of a cryopump, said cryopump comprising:a vacuum vessel comprising an inlet for receiving gas; andsaid refrigeration unit comprising:said housing;a cold head extending into said vacuum vessel for cooling a cryopanel structure for condensing said gas,a displacer configured to move to cool a refrigerant for cooling said cold head;a motor for controlling movement of said displacer; anda component arranged to move with said displacer between a first position and a second position, said first position and said second position being indicative of a first end point and a second end point of a stroke of said displacer, respectively, a ferromagnetic material forming at least a portion of said component;said sensor system comprising:a permanent magnet component configured to magnetise said ferromagnetic material;a first magnetic sensor; anda second magnetic sensor;wherein when the sensor system is fitted to the housing, the first magnetic sensor is positioned adjacent to said first position to detect a strength of a magnetic field of said component indicative of a position of said component and the second magnetic sensor is positioned adjacent to the second position to detect said strength of said magnetic field of said component indicative of said position of said component.

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