GB2164154A

GB2164154A - Magnetic velocity sensor

Info

Publication number: GB2164154A
Application number: GB08422279A
Authority: GB
Inventors: Gwyn Morgan; Allan Reginald Grantham
Original assignee: Martonair Ltd
Current assignee: Martonair Ltd
Priority date: 1984-09-04
Filing date: 1984-09-04
Publication date: 1986-03-12
Also published as: DE8524086U1; GB8422279D0; GB2164154B

Abstract

In a fluid-operable device, such as a piston 2 and cylinder 1, including a sensor responsive to motion of the piston 2, the sensor comprises a permanent magnet 4 mounted on the piston 2 within the cylinder 1 and magnetically linked to one end of the cylinder and two co- axial windings 8, 9 on or within the cylinder 1, each extending throughout substantially the whole axial length of the permitted travel of the piston 2, one winding 8 being a control winding which, in use, carries a constant D.C. current and the other winding 9 providing an output signal in response to variation of the magnetic field produced by the permanent magnet 4 proportional to the instantaneous velocity of the piston 2. <IMAGE>

Description

SPECIFICATION Fluid-operable device including sensor responsive to motion of a movable element of the device The invention relates to a fluid-operable device including a sensor responsive to motion of a movable element of the device. The fluid-operable device is particularly, but not exclusively, a piston and cylinder device, such as a fluidoperable actuator, and the sensor is responsive to the linear motion of the piston in the cylinder.

It is known to control the motion of a piston in a cylinder by using a suitable electronic controller. However. additionally it is necessary to provide a feed-back device to give information to the electronic controller responsive to the instantaneous position or motion of the piston. Such a feed-back device may be mounted externally of the cylinder or it may be mounted internally, for example by using a hollow piston rod and a sensor extending axially therewithin. This type of feedback device is usually complex and expensive.

Where it is mounted externally of the cylinder it would be liable to be damaged and where it is mounted internally modification of the cylinder may be required and the feed-back device may be difficult to maintain serviceable.

Known feed-back devices usually give a direct indication of position of the piston and so velocity and acceleration would be derived within the electronic controller by differentiation circuitry. alternatively, the feed-back device may provide a velocity or acceleration response and so the position of the piston would be derived within the electronic controller by integration circuitry. velocity responsive transducers are known. These usually comprise a magnetic element movable within a tube provided with external windings. In such transducers the magnetic element is axially long and two external windings would be required to ensure that the electrical output signals are independent of the axial position of the magnetic element within the tube. It is not possible to use such a velocity responsive transducer in a piston as the piston has a short axial length.

An object of the invention is to provide in a piston and cylinder device, such as a fluidoperable actuator, a motion responsive sensor which does not require a feed-back device as do known electronic controllers and which does not have the aforesaid disadvantages of known velocity responsive transducers.According to the invention, a fluid-operable device includes a sensor responsive to motion of a movable element of the device, the sensor comprising a permanent magnet mounted on the movable element within a housing therefor and magnetically linked to one end of the housing and two co-axial windings on the housing, each extending throughout substantially the whole axial length of the permitted travel of the movable element, one winding being a control winding which, in use, carries a constant D.C. current and the other winding providing an output signal in response to variation of the magnetic field produced by the permanent magnet mounted on the movable element and proportional to the instantaneous velocity of the movable element.

Where the fluid-operable device is a piston and cylinder device, the permanent magnet is mounted on the piston and the latter is connected magnetically through the piston rod to one end of the cylinder, the cylindrical wall of the cylinder being non-magnetic and carrying the control and output windings, the axial length of the two windings each being substantially the same as the permitted travel of the piston within the cylinder.

The control and output windings may be wound on the outside of the non-magnetic cylindrical wall of the cylinder or the non-magnetic cylindrical wall may be of hollow construction and have the two windings wound therewithin. alternatively one winding may be wound within a hollow cylindrical wall and the other winding may be wound on the outside of the cylindrical wall.

The magnetic path between the permanent magnet mounted on the piston may comprise a flux plate of magnetic material mounted on or within the piston adjacent the permanent magnet, a flux plate of magnetic material mounted on said one end of the cylinder and a piston rod of magnetic material magnetically connecting the two flux plates, the piston and the ends of the cylinder, except for the flux plates being made of non-magnetic material.

The output winding output may be monitored by appropriate electronic circuitry or by appropriate "software" whereby the velocity signal is convertible into piston, or other movable element, displacement or acceleration signals, or both.

By way of example, a linear actuator or other cylinder and piston device incorporating a velocity sensor in accordance with the invention is now described with reference to the accompanying drawing which is an axial section through the actuator.

The actuator comprises a cylinder comprising a non-magnetic cylindrical wall or barrel 1 and two end plates 10, 11, also made of non-magnetic material. Operating fluid, e.g., compressed air or oil or other hydraulic fluid, is applied through a control port in or adjacent one end plate and is exhausted through a port in or adjacent the other end plate and vice versa. The cylinder contains a sealed piston 2, also of non-magnetic material, mounted for reciprocation within the cylinder on a piston rod 3 of a magnetic material extending through the end plate 11 to provide an output reciprocating motion.

The piston 2 has mounted therein an annular permanent magnet 4 or a plurality of separate magnets which are magnetically connected to a flux plate of a magnetic material which is magnetically linked to the piston rod 3. The end plate 11 is also faced at its inner end with a flux plate of a magnetic material.

The magnet 4 therefore produces through the flux plates 5, 6 and the piston rod 3 a magnetic field indicated by chain line 7.

The cylindrical wall 1 has two co-axial windings 8, 9 wound thereon or is hollow and has at least one of the windings wound therewithin. The axial length of each winding 8, 9 is substantially the same as the effective travel of the piston within the cylinder. The winding 8, in use, has a substantially constant D.C. current passed through it and acts as a control winding. The other winding 9 provides an output voltage which is proportional at any instant to the linear velocity of the piston 2.

As the piston moves within the cylinder the length in the axial direction of the cylinder of the magnetic field changes and so the output voltage available from winding 9 changes. The function of the control winding 8 is to make tne output voltage signal from the winding 9 effectively independent of the instantaneous axial position of the piston in the cylinder and to produce an output voltage signal which is proportional to the instantaneous linear velocity of the piston. Without the provision of the control winding 8 the output voltage from winding 9 would be a complex function of piston position and velocity and so would be difficult to interpret.

The output voltage from the winding 9 may be monitored directly to give the instantaneous linear velocity of the piston or by providing appropriate circuitry or "software" to a computer, the output voltage from the winding 9 may be processed to give piston displacement or acceleration signals or both.

Claims

1. A fluid operable device including a sensor responsive to motion of a movable element of the device, the sensor comprising a permanent magnet mounted on the movable element within a housing therefor and magnetically linked to one end of the housing and two coaxial windings on the housing, each extending throughout substantially the whole axial length of the - permitted travel of the movable element, one winding being a control winding which, in use, carries a constant D.C. current and the other winding providing an output signal in response to variation of the magnetic field produced by the permanent magnet mounted on the movable element and proportional to the instantaneous velocity of the moveable element.

2. A device as claimed in Claim 1 comprising a piston and cylinder in which the permanent magnet is mounted on the piston and the latter is connected magnetically through the piston rod to one end of the cylinder, the cylindrical wall of the cylinder being nonmagnetic and carrying the control and output windings, the axial length of the two windings each being substantially the same as the permitted travel of the piston within the cylinder.

3. A device as claimed in Claim 2 in which the control and output windirpgs are wound on the outside of the non-magnetic cylindrical wall of the cylinder. I

4. A device as claimed in Claim 2 in which the non-magnetic cylindrical wall is of hollow construction and the two windings are wound therewithin.

5. A device as claimed in Claim 2 in which the non-magnetic cylindrical wall is of hollow construction and one winding is wound within the hollow wall and the other winding is wound on the outside of the cylindrical wall.

6. A device as claimed in any one of Claims 2-5 in which the magnetic path including the permanent magnet mounted on the piston comprises a flux plate of magnetic material mounted on or wifhin the piston adjacent the permanent magnet, a flux plate of magnetic material mounted on said one end of the cylinder and a piston rod or magnetic material magnetically connecting the two flux plates, the piston and the ends of the cylinder, except for the flux plates, being made of nonmagnetic material.

7. A device as claimed in any preceding claim in which the output of the output winding is monitored by electronic circuitry or by using appropriate software, whereby the velocity signal is convertible into piston, or other movable element, displacement or acceleration signals, or both.

8. A linear actuator or other cylinder and piston device incorporating a velocity sensor substantially as described herein and with reference to the accompanying drawing.