POSITION DETECTOR WITH FLEXIBLE MOUNT

The present invention relates to a position detector with a flexible mount. It is particularly suitable for, although not limited to, a position sensing device mounted on a machine tool such as press brake.

Many machines have two parts which are movable relative to one another and for which it is required to measure relative displacement between the two parts. Accordingly, various types of position sensor have been developed which include two members, fixed to the first and second parts of the machine tool respectively, which measure relative displacement between them.

One example is an optical encoder which includes an elongate glass scale along which an optical reader is passed. Another example is an inductive position detector of the type described in GB 1513567 and GB 2377497 for example. This type of detector includes a scale in the form of an elongate magnetic element which has a periodically varying dimension in a direction perpendicular to its longitudinal axis . A transducer which surrounds and travels along the length of the elongate magnetic element is used to induce a magnetic field in the element. The periodic variations in the dimension of the element result in detectable periodic variations in the magnetic field and provide corresponding periodically varying signalling which can be used to determine the relative displacement of the element and the transducer.

Clearly for both these types of detector the reader and elongate scale must have a high degree of alignment to ensure they are freely movable relative to one another and the scale can still be read to provide repeatable and accurate readings. Since the reader and the scale are attached to the relatively movable parts of a machine, those parts would ideally have high tolerances in their relative movement. However, this is not always possible. A typical example is a press brake machine used for forming materials e.g. by bending, trimming or punching. The large forces requires in metal forming operations can lead to deformation of the machine so that its movement is not sufficiently tightly constrained to accommodate the proper functioning of the aforementioned types of position detectors. For example, the body of the machine and the press may not run along truly parallel axes and or may not move in a repeatable fashion.

Therefore, it is necessary to mount the position detector on the machine in such a way to accommodate this movement and allow the detector to function properly.

A conventional system uses two standard ball joints at 90° rotation to one another, connected by a rod. This mechanism is typically used to link the reader head of the detector to the body or fixed part of the machine. The scale is attached to the moving, press member of the machine. The compounded action of the ball joints provides the degree of freedom generally required. However, even using standard parts, such compound ball joints are relatively costly to implement and need to be to be lubricated from time to time, requiring running maintenance.

Conventionally, as mentioned above, the compound ball joint mechanism is used to link the head of the position detector to the machine. This is because, for optical detectors using glass scales, in order to work properly the reader head must remain at a constant height relative to the scale and therefore the reader head mechanism has to be carefully guided and constrained. One prior art example incorporates a linear guide rail to provide this alignment control for the reader head. However, this is a particularly complex and therefore expensive solution.

The present invention provides a position detector for measuring the position of a first body relative to a second body, comprising a reader device mountable on a first body, an elongate element having a longitudinal axis and mountable on a second body, the reader device and elongate element being movable relative to each other, and mounting means to mount the elongate element on the second body, wherein the mounting means comprising a flexible member secured to a first end of the elongate element and securable to the second body, wherein the flexible member is substantially rigid in a direction parallel to the longitudinal axis and flexible in a direction non-parallel to the longitudinal axis such that the flexible member supports the whole weight of the elongate element and allows relative movement between the reader device and the elongate element to occur in a direction parallel to the longitudinal axis.

In this way, a mount is provided which is simple and cost effective to manufacture, install and maintain, while providing a greater freedom of movement over the prior art mounts .

Preferably, the flexible member comprises a portion of cable having a core surrounded by a plurality of twisted strands. Most preferably, the core is of a substantially similar construction, and the strands are stainless steel. This type of cable provides optimum characteristics for the mount .

Conveniently a first mounting block is rigidly secured to a first end of the flexible member, and a second mounting block is rigidly secured to a second end of the flexible member, one block being rigidly mounted to the elongate element and the other block being rigidly mountable to the second body. This provides a simple way to secure the mount in position.

The invention also provides a machine having a position detector mounted thereon by a flexible mount of the aforementioned type.

In use, the position detector may be secured to the machine with the longitudinal axis of the elongate element in a substantially vertical orientation.

In this case, the flexible mount may be positioned beneath the elongate element.

Alternatively, the flexible mount may be positioned above the elongate element and the elongate element is suspended from it.

The machine may comprise a press brake.

The present invention will now be described in detail, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a machine such as a press brake with a position detector attached using a flexible mount in accordance with the present invention;

Figure 2 is an enlarged side view of a flexible mount according to an embodiment of the invention;

Figure 3 illustrates how the mount shown in Figure 2 accommodates tilt and rotation; and

Figure 4 illustrates how the mount shown in Figure 2 accommodates shear.

An embodiment of the present invention is shown in schematic form only in Figure 1, mounted on a machine such as press brake 10. The press brake 10 consists of a fixed machine body 12 and a movable press 14, which moves up and down relative to the body 12. A position detector 16 is fitted to the press brake 10. The position detector 16 consists of a elongate scale 18 and reader head 20.

For example, the position detector 16 may be an inductive position detector of the type described in GB 2377497 in which the scale 18 consists of a tube containing a row of magnetic balls constrained against relative movement and in point of contact with one another. The head 20 consists of a casing containing induction coils to induce a magnetic field in the line of balls and pick-up

coils to detect how the induced magnetic field changes as the reader is moved axially relative to the scale. However, other types of position detector such as magnetic tape or optical detectors using a glass scale and optical reader head may equally be used.

Preferably, the reader head 20 is rigidly mounted to the machine body 12 and thus remains stationary with it . Consequently, the scale 18 is mounted to the press 14 by the flexible mount 22 of the present invention, and moves up and down with it. The mount 22 supports the whole weight of the scale 18 so that no further mounting or support means is necessary. As the scale 18 moves up and down it runs through a channel 24 formed in the reader head 20. Channel 24 acts as a guide for the scale and the flexible mount 22 accommodates any shifting of the scale which is needed. In particular, it allows the scale 18 to move slightly relative to the press 14 as the latter goes up and down, ensuring that the scale 18 can run smoothly through the channel 24 in the reader head 20 without snagging or j amming .

The flexible mount 22 is shown in more detail in Figure 2. The mount 22 consists of a first block 26 rigidly secured to the press 14 in any convenient manner, such as by screws. A second block 28 is rigidly secured to the scale 18 in any convenient manner, such as also by screws. Between the two blocks 26, 28 is a short length of cable 30 is secured. This is preferably a stainless steel stranded core surrounded by a plurality of twisted stainless steel strands .

The cable 30 must be of a material and construction which is stiff enough to support the scale 18 and the second block 28, whilst being flexible enough to accommodate movement of the scale 18 and the second block 28 relative to the first block 26 and the press 14 as described below. Accordingly, the cable 30 is substantially rigid in a direction parallel to its longitudinal axis so that it does not compress or stretch under compression or tension and so is able to support the weight of the scale 18. However, it is sufficiently flexible to accommodate movement in a direction non-parallel to its longitudinal axis . The stainless steel core is preferred in order to provide this functionality, although other materials and constructions are possible. For example, non metallic cores may be used where greater flexibility is required.

As shown in Figure 3, the flexibility of the cable 30 allows the scale 18 and block 28 to tilt relative to the lower block 26 and press 14 (see arrows A) . In addition, it allows rotation of a full 360° about the longitudinal axis of the lower block 26 (see arrows B) . The cable 30 also allows some twist about its longitudinal axis.

As shown in Figure 4, the cable 30 also accommodates shear compliance, which is of particular importance in the press brake field. This occurs when the press 14 is moving up and down along an axis C which is substantially parallel to, but laterally displaced from, an axis D of the machine body 12. As shown, the flexible cable 30 allows the scale 18 to be shifted laterally relative to the longitudinal axis of the lower block 26, whilst remaining parallel to that axis .

Clearly, the flexible cable 30 may accommodate several different types of movement at the same time. Thus, the greatest possible degree of freedom of movement of the scale 18 relative to the fixed block 26 and the press 14 is catered for by a mechanism that is both simple and cost effective to manufacture, install and maintain.

In use, the present invention would typically be installed with the flexible mount 22 at the bottom of a vertically mounted scale 18 and the reader head 20 would be located such that when the press brake 10 is open, the reader head 20 is at the bottom end of the scale 18, close to the flexible mount. Thus, when the press brake 10 is open the reader head 20 and the scale 18 can be aligned during installation to minimise any stress on the flexible mount 22. Optionally, the reader head 20 might be provided with bearings in the channel 24, such as oil impregnated bearings, to aid the alignment and movement of the scale 18.

However, it is not essential for the flexible mount 22 to be installed at the lower end of a vertically mounted scale 18. It is equally possible for the flexible mount 22 to be used at the top, with a vertically mounted scale 18 suspended from it. In either case the mount is sufficient to support the whole weight of the scale 18.