Hydraulic cylinder with position sensor

Abstract

 A piston (16) and rod (18) slide in a double acting cylinder with closed end (12) from which extends a bar (60) having a T-shaped cross-section with a cross piece (64) and stiffening web (66), the edges of which are rounded to engage and slide on the wall of a bore (22) in the piston rod (18), thus supporting the bar in a rigid manner. The face (68) of the cross piece (64) supports a potentiometer track which co-operates with a wiper (118) carried by an insulating carrier (110) fitted into the bore (22). The T-shaped bar (60) partitions the bore (22) into three chambers (65, 67, 69) which are in communication at the distal end of the bar. The carrier (110) is shaped to close the chamber (65) containing the wiper so that there is no fluid flow past the wiper as the piston (16) moves.

Description

[0001] This invention relates to a hydraulic cylinder with a position sensor comprising a bar fixed to the closed end of the cylinder and extending into a bore in the piston rod, a potentionmeter element on a support surface of the bar, and a wiper, carrier attached to the piston rod, with a wiper which moves along the potentiometer element as the piston rod moves in the cylinder.

[0002] In hydraulic controls technology, it is often desirable to obtain feedback signals representing the operating position of a hydraulic cylinder. US. Patent No.3,726,191 discloses"one solution to this problem. A resistance and conductor element support bar-are reciprocably received in a piston rod bore. A conductive wiper assembly is carried for movement with the piston and rod. However, the known transducer construction requires a relatively large transducer cross section which displaces a large volume of hydraulic fluid as the cylinder rod moves with respect to the transducer assembly. The bore required in the cylinder rod to accommodate such a transducer of large cross section and to provide an escape path for displaced hydraulic fluid is so large as to prevent its use in many hydraulic cylinders where compact size is required. Furthermore, the wiper contacts are suspended in the fluid escape path, allowing flow forces to vibrate or bend the contact leaf, resulting in operating noise or loss of signal. Finally, the support bar is supported only at one by a plastics sealing element which is subject to mechanical stress and failures.

[0003] The object of the present invention is to overcome these problems and the invention is characterised in that the support bar has a cross-section such as to slidably engage the wall of the bore, thereby to align the support bar relative to the wiper carrier.

[0004] The invention in another respact is characterised in that the wiper arm is so shaped in conjunction with the bar as to prevent fluid flow in the vicinity of the wiper when the piston rod moves relative to the cylinder.

[0005] An advantage of the present invention is that it provides a position-sensing cylinder with good immunity to electrical noise caused by hydraulic flow-induced vibration. Another advantage of the preferred embodiment is that it provides a potentiometer element support rod with good rigidity and low cross-sectional area, and which is mechanically supported along the length of its engagement with the cylinder rod bore.

[0006] A further advantage of the preferred construction is that it provides a position-sensing cylinder with an improved fluid pressure seal which is spaced apart from the support rod and thus, not subject to mechanical stress caused by the support of the support rod.

[0007] Thus in the preferred embodiment, the support rod is a T-section support'rod with one end welded or fixed to a hollow metallic sleeve which is press fitted into a fitting which is screwed into the closed end of the cylinder. A ceramic plug seals the interior of the sleeve but provides no support for and is separate from the support rod. Further support for the potentionmeter support rod is provided- by the sliding engagement between its edges and the wall of the rod bore within which the support rod reciprocates. A wiper carrier co-operates with the bore wall and the potentiometer support to prevent fluid flow in the vicinity of the flexible wiper contacts. The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which:-,

Fig 1 is a cross-sectional view of the hydraulic cylinder for the present invention,

Fig 2 is a partial sectional view of a portion of the cylinder with the potentiometer assembly fitted,

Fig 3 is a view taken on line 3 - 3 of Fig 2,

Fig 4 is a view taken on line 4 -4 of Fig 2,

Fig 5 is a top view of the terminal piece and the transducer support of the potentiometer assembly,

Figs 6a to 6d are unscaled sectional views taken on lines A-A, B-B, C-C and D-D respectively of Fig 5, and

Fig 7 is an end view of the wiper carrier of the assembly.

[0008] A hydraulic cylinder 10 includes a housing 12 having a hollow cylindrical barrel portion 14 within which reciprocates a piston 16 connected to a rod 18. A rod-receiving cap 20 is screwed into the rod end of the housing 12. A blind stepped bore 22 extends into the piston 16 and the rod 18. As best seen in Fig 2, the bore 22 has a large diameter portion 24 with a snap ring groove 26 therein and an annular ridge 28 at one end, a smaller diameter portion 30 and an annular shoulder 32 extending therebetween.

[0009] The housing 12 includes a threaded axial bore 34 at its closed end for receiving a steel outer transducer fitting 36. The fitting 36 has an annular flange 38 for butting against a corresponding housing shoulder 40. An O-ring 42 maintains a fluid- tight seal between the fitting 36 and the housing 12. the fitting 36 also includes a stepped axial bore 44 extending therethrough.

[0010] A steel terminal sleeve 46 is press fitted into the fitting bore 44. The sleeve 46 has an annular groove 48 which receives a back-up ring 50 and an 0-ring 52 for maintaining a fluid seal between the fitting 36 and the terminal 46. A plug of ceramic material 54 is formed in the hollow interior of the sleeve 46 to form a hermatic seal. The sleeve 46 is formed as a separate piece from the fitting 36 so that, when the ceramic material 54 is heated within the terminal 46, the fitting 36 is not degraded by the high temperatures to which it would otherwise be exposed if the fitting 36 and the terminal 46 were a single piece.

[0011] A steel support bar 60 extends into the bore 22 from an end 62 received by and welded to the sleeve 46. Thus, the end 62 of the support 60 is rigidly supported with respect to the housing 12 by metallic pieces 36 and 46, whereas other position sensing cylinders, such as that described in US. Patent No 3,726,191, have supported the transducer support element by press fitting the support into a plastics insulating sleeve, which is, in turn,received by a metallic threaded fitting. In such an arrangement, the transducer support is less rigidly supported at its supported end and the plastics insulating material is subject to stress failures, such as cracking, which then degrades its sealing properties. Furthermore, in the present invention, the ceramicseal 54 is subject to very little mechanical stress caused by supporting the support 60. Instead, the ceramic seal 54 need only withstand the hydraulic pressures which it is designed to withstand, with the result that the embodiment of the invention has increased integrity with respect to.-fluid pressure sealing.

[0012] The support bar 60 has a substantially T-shaped cross-section (as best seen in Figs, 3 and 4) formed by a cross piece 64 and from which perpendicularly extends a stiffening web 66 perpendicular thereto. The face of the cross piece 64 which is opposite the web 66 forms a planar support surface 68. A circular edged notch 70 is formed in the end 62 of the support "60, as best seen in Figs. 2 and 4.

[0013] The cross piece 64 and the web 66 divide the bore 22 into three fluid chambers 65, 67 and 69, of which chambers 65 and 67 are visible in Fig 2. As best seen in Figs 2 and 3, curved outer edges 59,61 and 63 of the support 60 slidably engage the wall of the bore 22. This engagement adds rigidity to the support 60 and helps to prevent deformation of the support 60 and reduces vibration to which the support 60 would be subject if it were cantilevered, ie, supported merely at one end.

[0014] As best seen in Figs 5 and 6a-d, a resistance or potentiometer assembly 72 is placed on the planar support surface 68 of the cross piece 64. The assembly includes an electrically insulating base 74 fixed to the support surface 68 and which may be formed of material such as "Kapton", ( a Trade Mark of DuPont ) .

[0015] The assembly 72 also includes a conductive plastics potentiometer element 76, of which the largest elongated central portion thereof is laid directly upon the insulating base 74. The ends 78 and 80 of the potentiometer element are overlaid on to portions of copper or metallic electrical conductive ground and power strips 82 and 84, respectively. The potentiometer element 76 terminates at edges 79 and 81, respectively.. The • Conductive ground strip 82 terminates at edge 83, under the end 78 of the potentiometer element 76, as shown in dashed line Fig.5. A conductor 86 is soldered to the other end of the ground strip 82. The conductive power strip 84 terminates at an edge 85 under end 80 of the potentiometer element 76, as also shown in dashed line in Fig. 5. The overlaps between the conductive strips 82, 84 and the potentiometer element ends 78,80 need only be sufficient to provide good electrical continuity therebetween. The other end of the power strip 84 is joined via a solder joint to a conductor 88. A metallic or electrical conductive commutator strip 90 extends from an edge indicated by reference numberal 92 to an end 94 to which is soldered to a conductor 96. An overlay strip 98 of conductive plastics material is positioned on top of the commutator strip 90 and extends from edge 99 to an edge also indicated by reference numberal 92. The major portion of the power strip 84 is separated from the commutator strip 90 by the middle or potentiometer strip 76 to reduce the possibility of metallic particles froming undesirable short circuit connections between the power strip 84 and the commutator strip 90. Also note that the transducer element 72 is not received in a recess or channel in the support 60, but rather is positioned on top of the planar support surface 68. The absence of a recess or channel reduces the likelihood of metallic particles forming undesirable short circuit connections between the support 60 and the outer edges of the power strip 84 and the commutator strip 90. For clarity, the thicknesses of the various strips in Figs. 6a-d are exaggerated. The conductors 86,88 and 96 extend from their solder joints, through the notch 70, the terminal cup 46 and the ceramic seal 54 to respective connector terminals 100,102 and 104, two of which are visible in Fig. 2. A conventional female plug-type connector may be coupled to the connector terminals 100,102 and 104.

[0016] A wiper carrier 110 is received by bore portion 24 of the bore 22, as best seen in Fig, 2. The wiper carrier 110 has a generally cylicndrical body 112 of an insulating material such as glass-reinforced nylon with an annular rebate l14 in one end thereof. The rebate 114 registers with the ridge 28 only when the carrier 110 is properly mounted in the bore port;-on 24, otherwise, the annular ridge 28 will prevent the carrier 110 from being inserted far enough into bore portion 24 to allow insertion of snap ring 126 into snap ring groove 26. A wiper support web 116 extends inwardly from a portion of the body 112. A two- pronged conductive wiper 118 is embedded in and supported by the web 116 so that prongs 120 and 122 resiliently and slidably engage the surfaces of the plastics conductive strip 98 above commutator strip 90 and the potentiometer element portion 76, respectively. The wiper 118 is exposed to the fluid in the chamber 65. As best seen in Fig. 7, the prongs 120 and 122 have multiple fingers. A rectangular recess 124 is formed in the inner surface 125 of the carrier web 116 so that the inner surface 125 closely registers with the surface 68 of the support element 60 and with the transducer element 72. The carrier 110 is coupled for movement with the rod 18 and piston 16 between the snap ring 126 and a resilient wave washer 128.

[0017] When the piston 16 and rod 18 move back and forth with respect to he housing 12 and the support 60, fluid flows into and out of the chamber 65, around the end 129 of the support 60 and via chambers 67 and 69. However, the close sliding fit between the inner surface 125 and surface 68 and element 72 substantially prevents fluid flow into or out of the chamber 65 via the wiper carrier 110, thus reducing flow-induced turbulence in the vicinity of the wiper 118. This reduced turbulence prevents turbulence induced vibrations in the wiper 118 which could otherwise introduce noise into the signals picked up by the wiper 118.

[0018] In operation, a voltage is applied across terminals 102 and 100 to apply a. corresponding voltage across the resistance element 76. As the piston 16 and rod 18 move back and forth within the housing 12, the wiper 118 moves with the piston 16 and rod 18 and with respect to the support 60 and the transducer element 72. Thus, the voltage which the wiper 118 taps off varies from ground potential to approximately the voltage at the power strip 84, depending upon the longitudinal position of the wiper l18 relative to the resistance element 76. The voltage on the wiper 118 is communicated to the exterior of the cylinder housing 12 via the commutator strips 98 and 90, conductor 96 and connector terminal 104, from where it can be monitored to indicate the position of cylinder 10.

Claims

1 A hydraulic cylinder with a position sensor comprising a bar fixed to the closed end of the cylinder and extending into a bore in the piston rod, a potentiometer element on a support surface of the bar, and a wiper carrier attached to the piston rod, with a wiper which moves along the potentiometer element as the piston rod moves in the cylinder, characterised in that the support bar (60) has a cross-section such as to slidably engage the wall of the bore (22), thereby to align the support bar relative to the wiper carrier (110).

2 A hydraulic cylinder according to claim 1, characterised in that the bar (60) has a T-shaped cross-section with' a cross piece (64) and a stiffening web (66), the support surface (68) being the face of the cross-piece (64) remote from the stiffening web (66).

3 A hydraulic cylinder according to claim 1 or 2, characterised in that the cross-section of the bar (60) is such as to partition the bore (22) into a plurality of chambers (65,67, 69) the wiper (118) is located in one such chamber (65), and the wiper carrier (110) is shaped to prevent fluid flow into or out of the said one chamber (65), through the wiper carrier.

4 A hydraulic cylinder according to claim 3, characterised in that the chamber(s) (67,69) other than the said one chamber (65) are open through the wiper carrier (110) and are in communication with the said one chamber '(65) at the distal end of the bar (60) .

5 A hydraulic cylinder according to claim 2, 3 or 4, characterised in that the edges (59,61,63) of the cross-piece(64) and stiffening web (66) are rounded to match the curvature of the wall , of the bore (22).

6 A hydraulic cylinder according to any of the claims 1 to 5 characterised in that the end (62) of the bar (6) is fixed to a sleeve (46) which is fitted into a separate fitting (36) removably • fitted into a bore (40) through the closed end (12) of the cylinder (10), the sleeve being closed by a plug (54) of ceramic or other insulation material.

₇ A hydraulic cylinder according to claim 6, characterised in that electrical conductors (100,102,104) extend through the plug (54) to the assembly of the wiper (118) and potentiometer element (76) .

8 A hydraulic cylinder according to claim 6 or 7, characterised in that the plug (54) is axially spaced in the sleeve (46) from the end (62) of the bar (60) fixed to the sleeve.

9 A hydraulic cylinder according to any of claims 1 to 8 characterised in that the wiper carrier (110) is a rebated cylindrical body fitted into the bore (22) which is stepped to provide a locating shoulder co-operating with the rebate.

10 A hydraulic cylinder with a position sensor comprising a bar fixed to the closed end of the cylinder and extending into a bore in the piston rod, a potentiometer element on a support surface of the bar, and a wiper carrier attached to the piston rod, with a wiper which moves along the potentiometer element as the piston rod moves in the cylinder, characterised in that the wiper carrier (110) is so shaped in conjunction with the bar (60) as to prevent fluid flow in the vicinity of the wiper (118) when the piston rod (18) moves relative to the cylinder (10) .

11 A hydraulic cylinder according to claim 10, characterised in that the bar partitions the bore (22) into a plurality of chambers (65,67,69) which are in communication at the distal end of the bar, the wiper (118) being located in one such chamber (65) and the carrier (110) being shaped to prevent fluid flow into or out of the said one chamber (65) through the wiper carrier while allowing flow into and out of at least one other chamber (67,69).

Drawing