Diagnostic system, diagnostic method, and diagnostic program

Abstract

To provide a diagnostic system, a diagnostic method and a diagnostic program for reducing the number of parameters to be used for diagnosis as compared with a conventional manner.SOLUTION: A diagnosis system (1) for diagnosing a state of a cam (5) of a hydraulic motor (4), the hydraulic motor (4) including the cam (5) having a wavy profile, a plurality of pistons (62) each having a follower (63) in contact with the cam (5) at a tip thereof, a rotor (6) having the cam (5) or the pistons (62), and a hydraulic pump (7) configured to supply a hydraulic pressure to the pistons (62), the hydraulic motor (4) being controlled such that a relative rotational speed of the rotor (6) with respect to the cam (5) is substantially constant, the diagnosis system (1) including: The calculation device 3 can realize a feature amount specifying function of specifying at least one feature amount selected from the group consisting of a standard deviation, an effective value, and an amplitude with respect to the hydraulic pressure in a predetermined period, and a diagnosis function of diagnosing the state of the cam 5 based on the feature amount.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a diagnostic system, a diagnostic method, and a diagnostic program for diagnosing a hydraulic motor. [Background technology]

[0002] Hydraulic motors are widely used in rotating equipment such as centrifugal extractors. Hydraulic motors can malfunction due to wear of parts during use, and so it is necessary to diagnose the hydraulic motors for maintenance and management of the equipment. For example, Japanese Patent Application Laid-Open No. 2019-194448 (Patent Document 1) discloses a diagnostic method that creates a prediction model that predicts normal values ​​of output parameters of hydraulic equipment, and diagnoses the presence or absence of an abnormality in the hydraulic equipment based on the deviation between the normal value predicted by the prediction model and the actual measured value obtained by actual measurement. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-194448 Summary of the Invention [Problem to be solved by the invention]

[0004] The technology of Patent Document 1 requires the acquisition of various parameters because it is difficult to predict the parameters that the prediction model uses as explanatory variables. As a result, it is necessary to provide instruments to measure parameters that are not actually used in the diagnosis, which may increase the effort and cost required to perform the diagnosis.

[0005] Therefore, it is desirable to realize a diagnostic system, a diagnostic method, and a diagnostic program that use fewer parameters for diagnosis than conventional systems. [Means for solving the problem]

[0006] The diagnostic system according to the present invention is a diagnostic system for diagnosing the condition of a cam in a hydraulic motor including a cam having a wavy contour, a plurality of pistons each having a follower at its tip that abuts the cam, a rotor having the cam or the piston, and a hydraulic pump that supplies hydraulic pressure to the piston, the system being controlled so that the relative rotational speed of the rotor with respect to the cam is approximately constant, the diagnostic system comprising: a hydraulic meter that measures the hydraulic pressure supplied to the piston; and a computing device, wherein the computing device is capable of realizing a feature quantity identification function that identifies at least one feature quantity selected from the group consisting of standard deviation, effective value, and amplitude for the hydraulic pressure over a predetermined period of time, and a diagnostic function that diagnoses the condition of the cam based on the feature quantity.

[0007] A diagnostic method according to the present invention is a diagnostic method for diagnosing the condition of a cam in a hydraulic motor comprising: a cam having a wavy contour; a plurality of pistons each having a follower at its tip that abuts against the cam; a rotor having the cam or the piston; and a hydraulic pump that supplies hydraulic pressure to the piston, the method being controlled so that the relative rotational speed of the rotor with respect to the cam is approximately constant, the diagnostic method comprising: a measurement step of measuring hydraulic pressure supplied to the piston; a feature identification step of identifying at least one feature of the hydraulic pressure over a predetermined period of time selected from the group consisting of standard deviation, effective value, and amplitude; and a diagnostic step of diagnosing the condition of the cam based on the feature.

[0008] The diagnostic program according to the present invention is a diagnostic program for diagnosing the condition of a cam in a hydraulic motor comprising a cam having a wavy contour, a plurality of pistons each having a follower at its tip that abuts the cam, a rotor having the cam or the piston, and a hydraulic pump that supplies hydraulic pressure to the piston, the program being controlled so that the relative rotational speed of the rotor with respect to the cam is approximately constant. When executed by a computer, the diagnostic program is capable of realizing a hydraulic pressure identification function that identifies the hydraulic pressure supplied to the piston, a feature value identification function that identifies at least one feature value selected from the group consisting of standard deviation, effective value, and amplitude for the hydraulic pressure over a predetermined period, and a diagnostic function that diagnoses the condition of the cam based on the feature value.

[0009] The inventors discovered that when a hydraulic motor equipped with a cam mechanism and a hydraulic pump is controlled so that the relative rotational speed of the rotor with respect to the cam is approximately constant, the behavior of the hydraulic pressure changes when a malfunction such as wear occurs in the hydraulic pump, and they utilized this to make it possible to diagnose the condition of the cam of the hydraulic motor, thereby completing the present invention. With the above configuration, the condition of the cam of the hydraulic motor can be diagnosed by measuring at least the hydraulic pressure, so the number of parameters used for diagnosis can be reduced compared to conventional methods.

[0010] Preferred embodiments of the present invention will be described below, but the scope of the present invention is not limited to the preferred embodiments described below.

[0011] In one aspect of the diagnostic system according to the present invention, the hydraulic motor is preferably configured such that the cam is connected to the bowl of a centrifugal extractor and the rotor having the piston is connected to the screw of the centrifugal extractor.

[0012] According to this configuration, the present invention can be used to diagnose a centrifugal dehydrator.

[0013] Further features and advantages of the present invention will become more apparent from the following description of exemplary and non-limiting embodiments, which is given with reference to the drawings. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram showing a configuration of a centrifugal dehydrator according to an embodiment. [Figure 2] 1 is a diagram illustrating a configuration of a hydraulic motor and a diagnostic system according to an embodiment. [Figure 3] FIG. 4 is a diagram schematically illustrating the relationship between the cam and the rotor in a state where there is no wear. [Figure 4] FIG. 10 is a diagram showing an example of the behavior of oil pressure in a state where there is no wear. [Figure 5] FIG. 10 is a diagram schematically illustrating the relationship between the cam and the rotor in a state where wear exists. [Figure 6] FIG. 10 is a diagram showing an example of the behavior of oil pressure in a state where wear is present. DETAILED DESCRIPTION OF THE INVENTION

[0015] A diagnostic system, a diagnostic method, and a diagnostic program according to an embodiment of the present invention will be described below with reference to the drawings. In the following, a diagnostic system 1 that diagnoses a hydraulic motor 4, which is a differential speed device of a centrifugal dehydrator 100, will be described as an example.

[0016] [Configuration of centrifugal dehydrator and hydraulic motor] Prior to describing the diagnostic system 1 according to this embodiment, we will explain the configuration of the hydraulic motor 4 that is the target of diagnosis by the diagnostic system 1. In addition, to help understand the function of the hydraulic motor 4, we will also explain the configuration of a centrifugal dehydrator 100 that uses the hydraulic motor 4 as a differential speed device.

[0017] Centrifugal dehydrator 100 comprises bowl 101 and screw 102 (Fig. 1). More specifically, bowl 101 is a cylindrical body that rotates while accommodating a solid-liquid mixture, such as slurry-like sludge, to be separated, and is the component that performs solid-liquid separation by centrifugal force. Screw 102 is a component that serves to transport the solid-liquid mixture and the solid components after solid-liquid separation. A feed pipe is provided inside shaft 102a of screw 102, and the solid-liquid mixture is supplied to centrifugal dehydrator 100 through the feed pipe.

[0018] The bowl 101 and the screw 102 are journaled by two bearings 103 (103a, 103b) provided at both ends in the extension direction (left-right direction in FIG. 1). The bowl 101 and the screw 102 are rotated together by the driving force of the driver 104, and differential speeds are imparted to them by the hydraulic motor 4. As a result, the bowl 101 and the screw 102 rotate at different rotational speeds. The hydraulic motor 4 is rotated together with the bowl 101 and the screw 102 by the driving force of the driver 104.

[0019] The rotation speed of the screw 102 is about 3 to 5 rpm higher than the rotation speed of the bowl 101. As a result, the screw 102 rotates relatively faster than the bowl 101, and the solid-liquid mixture and solid components are gradually transported downstream (to the right in FIG. 1 ) by the screw 102 inside the bowl 101. The rotation speed of the bowl 101 is set appropriately taking into consideration the properties of the material to be centrifuged and requirements from downstream processes, and can be, for example, 1500 to 3600 rpm. This rotation speed is achieved by the driver 104.

[0020] The rotation speed of the screw 102 is set to a value approximately 3 to 5 rpm higher than the rotation speed of the bowl 101, depending on the rotation speed of the bowl 101. For example, if the rotation speed of the bowl 101 is 3000 rpm, the rotation speed of the screw 102 is approximately 3003 to 3005 rpm. This differential speed of approximately 3 to 5 rpm is imparted by the action of the hydraulic motor 4.

[0021] The driving machine 104 is not limited as long as it can rotate the bowl 101, the screw 102, and the hydraulic motor 4 at the required rotational speed (e.g., about 3000 rpm). Non-limiting examples of the driving machine 104 include an electric motor, an engine, etc.

[0022] The driving machine 104 and the hydraulic motor 4 are controlled by a control device 105. Specifically, the control device 105 controls the driving machine 104 so as to obtain a required rotation speed (for example, about 3000 rpm), and also controls the hydraulic motor 4 so that the differential speed between the bowl 101 and the screw 102 is approximately constant (for example, about 3 to 5 rpm).

[0023] The hydraulic motor 4 includes a cam 5, a rotor 6, and a hydraulic pump 7 (FIG. 2). The cam 5 is incorporated into an outer unit 41, which is connected to a bowl 101 of the centrifugal dehydrator 100. The rotor 6 is incorporated into an inner unit 42 housed inside the outer unit 41, which is connected to a screw 102 of the centrifugal dehydrator 100. The hydraulic motor 4 applies a relative rotational force (for example, a differential speed of about 3 to 5 rpm) to the inner unit 42 with respect to the outer unit 41 through the action of the cam 5 and the rotor 6.

[0024] The hydraulic pump 7 is a pump that supplies hydraulic pressure to the pistons 62 of the rotor 6, and a known pump can be used. A distributor 71 that distributes hydraulic oil is incorporated into the outer unit 41. The hydraulic oil driven by the hydraulic pump 7 reaches the distributor 71 via an outgoing path 72 and is distributed to each piston 62. The hydraulic oil discharged from the pistons 62 is collected in the distributor 71 and then returns to the hydraulic pump 7 via a return path 73. The hydraulic oil that reaches the distributor 71 via the outgoing path 72 and before performing work on the pistons 62 is at a relatively high pressure, while the hydraulic oil that has performed work on the pistons 62 and is discharged and passes through the distributor 71 and the return path 73 is at a relatively low pressure. Hereinafter, the former of these will be referred to as the high-pressure side, and the latter will be referred to as the low-pressure side.

[0025] FIG. 3 is a diagram illustrating the relationship between the cam 5 and the rotor 6. The cam 5 has a wavy contour 51 on its inner periphery. The contour 51 has a shape in which portions that are far from the center (referred to as valley portions 52) and portions that are short from the center (referred to as peak portions 53) are alternately connected. In this embodiment, the contour 51 has eight valley portions 52 and eight peak portions 53, and the shapes of the eight valley portions 52 and the eight peak portions 53 are identical to each other. The rotor 6 has a substantially cylindrical main body 61 and pistons 62 radially arranged on the main body 61, and the pistons 62 have followers 63 at their tips. In this embodiment, ten pistons 62 are radially arranged at 36° intervals. The cam 5 and the rotor 6 are positioned such that the followers 63 of the rotor 6 rotate along the contour 51 of the cam 5.

[0026] The hydraulic oil inlet / outlet 64 of the piston 62 opens at the contact surface between the main body 61 and the distributor 71. When the inlet / outlet 64 communicates with the high-pressure oil passage of the distributor 71, hydraulic oil is supplied to the piston 62, causing the piston 62 to extend. At this time, the follower 63 moves radially outward of the rotor 6. The outlet of the high-pressure oil passage in the distributor 71 is located near the rotational position where the follower 63 contacts the peaks 53 of the profile 51. Therefore, the piston 62 extends with the tip of the follower 63 in contact with the peaks 53, and receives a circumferential reaction force from the peaks 53. This reaction force causes the entire rotor 6 to rotate. As the rotor 6 rotates, the follower 63 is guided in a direction from the peaks 53 toward the valleys 52.

[0027] When the inlet / outlet 64 communicates with the low-pressure oil passage of the distributor 71, working oil can flow out of the piston 62. When a force acts in a direction that causes the piston 62 to contract in this state, the follower 63 moves radially inward of the rotor 6. The inlet of the low-pressure oil passage in the distributor 71 is located near the rotational position where the follower 63 contacts the valley portion 52 of the contour 51. When the follower 63 is guided near the valley portion 52, the inlet / outlet 64 communicates with the low-pressure oil passage of the distributor 71, allowing the piston 62 to contract. When the follower 63 rotates further, it is guided in a direction from the valley portion 52 toward the peak portion 53, and receives a reaction force from the peak portion 53 that acts radially inward. This reaction force pushes the piston 62 radially inward.

[0028] That is, the pistons 62 alternately repeat extension, which begins near the position where the followers 63 contact the peaks 53, and contraction, which begins near the position where the followers 63 contact the valleys 52. When extension occurs, the followers 63 receive a circumferential reaction force from the peaks 53, and this reaction force rotates the entire rotor 6. In this embodiment, there are eight valleys 52 and eight peaks 53, and ten pistons 62. Since the numbers of valleys 52 and peaks 53 are different, the followers 63 of all ten pistons 62 do not simultaneously contact the peaks 53 (or valleys 52). Therefore, a state is always realized in which one of the followers 63 receives a circumferential reaction force, and the remaining followers 63 receive a radial reaction force. This continuously generates a driving force that rotates the rotor 6.

[0029] The hydraulic motor 4 is controlled by a control device 105, and the differential speed between the bowl 101 and the screw 102 is kept approximately constant (for example, about 3 to 5 rpm). Because the bowl 101 is connected to the cam 5 (outer unit 41), and the screw 102 is connected to the rotor 6 (inner unit 42), the differential speed between the bowl 101 and the screw 102 is the relative rotational speed (differential speed) of the rotor 6 with respect to the cam 5. Control to keep the differential speed approximately constant in the hydraulic motor 4 can be achieved, for example, by measuring the rotational speeds of the cam 5 and the rotor 6 (bowl 101 and screw 102), and feedback-controlling the output of the hydraulic pump 7 so that the difference remains approximately constant.

[0030] Ignoring the effect of the load on the hydraulic motor 4 from the centrifugal dehydrator 100, the torque of the hydraulic motor 4 must be constant in order to achieve a substantially constant differential speed. This torque is generated by the circumferential reaction force that the follower 63 receives from the ridge 53 when the piston 62 extends.

[0031] [Configuration of diagnostic system] The diagnostic system according to this embodiment includes a hydraulic gauge 2 and a computing device 3. The hydraulic gauge 2 is a hydraulic gauge that measures the hydraulic pressure supplied to the piston 62, and is not particularly limited as long as it measures the hydraulic pressure and transmits the measured value to the computing device 3.

[0032] The arithmetic device 3 is a device capable of executing arithmetic processing related to the diagnosis of the hydraulic motor 4, and may be, for example, a personal computer, a programmable logic controller, or an industrial computer. The arithmetic device 3 is configured to be able to communicate with the hydraulic pressure gauge 2. The hydraulic pressure gauge 2 and the arithmetic device 3 may be directly connected, or may be configured to be able to communicate via a network. Note that, in this embodiment, an example is described in which the arithmetic device 3 constituting the diagnostic system 1 and the control device 105 controlling the centrifugal dehydrator 100 are provided separately, but a configuration in which the functions of both of these devices can be realized by a single arithmetic device may also be used.

[0033] [Functions of the arithmetic unit] The computing device 3 can realize a feature quantity identifying function for identifying a feature quantity related to hydraulic pressure and a diagnostic function for diagnosing a hydraulic motor. Note that each step of the diagnostic method according to this embodiment corresponds to each function described below, and each function of the diagnostic program according to this embodiment corresponds to each function described below.

[0034] (1) Feature identification function The feature quantity identifying function is a function for identifying a feature quantity of the hydraulic pressure for a predetermined period of time, where the feature quantity is at least one value selected from the group consisting of a standard deviation, an effective value, and an amplitude.

[0035] As the piston 62 moves with the rotor 6, it repeatedly expands and contracts. When the piston 62 expands, the hydraulic pressure supplied to the piston 62 is relatively high, and when the piston 62 contracts, the hydraulic pressure supplied to the piston 62 is relatively low. The expansion / contraction movement of the piston 62 is characterized by the profile 51 of the cam 5. Because the shapes of the multiple valleys 52 and peaks 53 in the profile 51 are identical to each other, the expansion / contraction movement of the piston 62 exhibits periodic behavior. Correspondingly, the hydraulic pressure supplied to the piston 62 periodically fluctuates between high-pressure and low-pressure regions. In this embodiment, eight high-pressure and low-pressure regions appear during one rotation of the rotor 6. This corresponds to the fact that the cam 5 has eight valleys 52 and eight peaks 53. That is, the low-pressure regions of the hydraulic pressure correspond to the valleys 52, and the high-pressure regions of the hydraulic pressure correspond to the peaks 53. Note that the standard deviation, the effective value, and the amplitude are all parameters suitable for capturing the characteristics of the periodic behavior of the hydraulic pressure.

[0036] (2) Diagnostic function The diagnostic function is a function for diagnosing the hydraulic motor 4 based on the feature quantities identified by the feature quantity identification function. Since all of the feature quantities listed above have a positive correlation with the amount of wear of the cam 5, the wear state of the cam 5 can be diagnosed based on these feature quantities. The reason why the feature quantities and the amount of wear of the cam 5 have a positive correlation will be explained below.

[0037] When the cam 5 is not worn, the eight peaks 53 have the same shape, and therefore the magnitude of the circumferential reaction force that the follower 63 receives from the peaks 53 is roughly the same for all eight peaks 53. Therefore, the oil pressure in the eight high-pressure zones that appears during one rotation of the rotor 6 is roughly constant (Fig. 4). The oil pressure in the eight low-pressure zones is also roughly constant.

[0038] Next, let us consider the case where the ridges 53 are worn. For example, if one of the eight ridges 53 (referred to as ridge 53a) is worn, the distance from the center of the rotor 6 to the contour 51 at the worn ridge 53a will be longer than the distance from the center of the rotor 6 to the contour 51 at the unworn ridge 53 (FIG. 5). Assuming that the extension state of the piston 62 is the same, the circumferential reaction force that the follower 63 receives at the worn ridge 53a will be smaller than the circumferential reaction force that the follower 63 receives at the unworn ridge 53. Therefore, wear of the ridge 53 can cause the actually generated torque to be smaller than the torque that should be generated.

[0039] In reality, the hydraulic motor 4 is controlled so that the differential speed between the cam 5 and the rotor 6 remains substantially constant, and so the operating state is changed to compensate for the torque shortage caused by wear on the ridges 53. Specifically, the hydraulic pressure supplied to the piston 62 is increased (the output of the hydraulic pump 7 is increased) to increase the extension force of the piston 62, thereby achieving torque equivalent to that obtained when there is no wear. In conjunction with this control, the hydraulic pressure supplied to the piston 62 rises.

[0040] This increase in oil pressure occurs only when the follower 63 is in contact with the worn ridge 53a. Therefore, of the eight high-pressure zones that appear during one rotation of the rotor 6, only the one corresponding to the ridge 53a has a higher oil pressure than the other zones (Figure 8). This phenomenon can be captured as an increase in a feature value. For example, the standard deviation is a feature value that represents the variation in oil pressure values, so it will be larger if the oil pressure in one of the eight high-pressure zones is different than if the oil pressure in the eight high-pressure zones is uniform. Therefore, the hydraulic motor 4 can be diagnosed based on the feature value.

[0041] According to this embodiment, the hydraulic motor can be diagnosed using the hydraulic pressure supplied to the piston as the minimum criteria, so the number of parameters used for diagnosis can be reduced compared to conventional methods. Also, since the hydraulic pressure supplied to the piston of a hydraulic motor is usually measured for the purpose of operating and managing the hydraulic motor, it is often not necessary to obtain new parameters for diagnosis. Due to these features, this embodiment achieves an overall simple diagnosis of the hydraulic pump.

[0042] Other Embodiments Finally, other embodiments of the diagnostic system, diagnostic method, and diagnostic program according to the present invention will be described. Note that the configurations disclosed in the following embodiments can be applied in combination with the configurations disclosed in other embodiments, as long as no contradiction occurs.

[0043] In the above embodiment, an example has been described in which the diagnostic system 1 is used to diagnose the hydraulic motor 4, which is the differential speed device of the centrifugal dehydrator 100. However, the use of the hydraulic motor to be diagnosed in the present invention is not limited.

[0044] The hydraulic motor to be diagnosed in the present invention is not limited to the above configuration, as long as it includes a cam with a wavy profile, multiple pistons with followers at their tips that abut the cam, a rotor with the cam or pistons, and a hydraulic pump that supplies hydraulic pressure to the pistons, and the rotor rotation speed is controlled to be approximately constant. For example, the number, height, and shape of the cam's peaks and valleys, the number of pistons on the rotor, and the shape of the followers are arbitrary. Furthermore, while the above embodiment describes an example in which the hydraulic motor 1 in which the rotor 6 has pistons 62 is to be diagnosed, the hydraulic motor to be diagnosed in the present invention may also be one in which the rotor has a cam.

[0045] In the diagnostic system, diagnostic method, and diagnostic program according to the present embodiment, it is not prohibited to make a diagnosis taking into consideration parameters other than the characteristic amount of hydraulic pressure.

[0046] Regarding other configurations, it should be understood that the embodiments disclosed in this specification are illustrative in all respects and that the scope of the present invention is not limited thereby. Those skilled in the art will easily understand that appropriate modifications are possible without departing from the spirit of the present invention. Therefore, other embodiments modified without departing from the spirit of the present invention are naturally included in the scope of the present invention. [Industrial Applicability]

[0047] The present invention can be used to diagnose hydraulic motors used in centrifugal dehydrators and the like. [Explanation of symbols]

[0048] 1: Diagnostic system 2: Oil pressure gauge 3: Arithmetic device 4: Hydraulic motor 5: Cam 6: Rotor 62: Piston 63: Follower 7: Hydraulic pump 64: Entrance / exit 100: Centrifugal dehydrator 101: Bowl 102: Screw

Claims

1. A diagnostic system for diagnosing the condition of a cam in a hydraulic motor, the hydraulic motor comprising: a cam having a wavy profile; a plurality of pistons each having a follower at its tip that abuts against the cam; a rotor having the cam or the piston; and a hydraulic pump that supplies hydraulic pressure to the piston, the hydraulic motor being controlled so that the relative rotation speed of the rotor with respect to the cam is substantially constant, a hydraulic pressure gauge for measuring the hydraulic pressure supplied to the piston; a computing device, The computing device a feature quantity specifying function for specifying at least one feature quantity selected from the group consisting of a standard deviation, an effective value, and an amplitude for the hydraulic pressure during a predetermined period; a diagnostic function for diagnosing the state of the cam based on the characteristic amount; and

2. 2. The diagnostic system of claim 1, wherein the hydraulic motor has the cam connected to a bowl of a centrifugal extractor and the rotor having the piston connected to a screw of the centrifugal extractor.

3. A diagnostic method for diagnosing the condition of a cam in a hydraulic motor, the hydraulic motor comprising: a cam having a wavy profile; a plurality of pistons each having a follower at its tip that abuts against the cam; a rotor having the cam or the piston; and a hydraulic pump that supplies hydraulic pressure to the piston, the method being controlled so that the relative rotation speed of the rotor with respect to the cam is substantially constant, the method comprising: a measuring step of measuring hydraulic pressure supplied to the piston; a feature quantity specifying step of specifying at least one feature quantity selected from the group consisting of a standard deviation, an effective value, and an amplitude for the hydraulic pressure during a predetermined period; and diagnosing the state of the cam based on the characteristic amount.

4. A diagnostic program for diagnosing the state of a cam in a hydraulic motor, the hydraulic motor comprising: a cam having a wavy contour; a plurality of pistons each having a follower at its tip that abuts against the cam; a rotor having the cam or the piston; and a hydraulic pump that supplies hydraulic pressure to the piston, the hydraulic motor being controlled so that the relative rotation speed of the rotor with respect to the cam is substantially constant, the program comprising: When executed by a computer, a hydraulic pressure determining function for determining the hydraulic pressure to be supplied to the piston; a feature quantity specifying function for specifying at least one feature quantity selected from the group consisting of a standard deviation, an effective value, and an amplitude for the hydraulic pressure during a predetermined period; a diagnostic program capable of realizing a diagnostic function for diagnosing the state of the cam based on the characteristic amount;

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