GB2408577A

GB2408577A - Tilt sensor for a lifting position monitor

Info

Publication number: GB2408577A
Application number: GB0327346A
Authority: GB
Inventors: John Philip Pelling
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-11-25
Filing date: 2003-11-25
Publication date: 2005-06-01
Also published as: GB0327346D0

Abstract

The tilt sensor includes an elongate housing 1 containing a fixed magnet 3 and a movable magnet 5 which are separated by magnetic repulsion. As the housing is tilted the movable magnet travels along the housing until, at a critical tilt angle, the moving magnet operates a reed switch 7. The sensor is suitable to be mounted in a lifting position monitor which has an elongate case for attachment to a wearer's clothing. When the wearer adopts an unsafe lifting position the critical angle is exceeded causing the switch 5 to operate a warning sounder.

Description

1 2408577

LIFTING POSITION MONITOR AND TILT SENSOR

TECHNICAL FIELD OF THE INVENTION

This invention relates to lifting position monitors and tilt sensors which are suitable for use therewith.

BACKGROUND

Numerous cases of back injury occur every year due to people adopting an incorrect posture when lifting heavy objects. Devices are already known which are intended to provide a warning when a person adopts an incorrect lifting posture. I lowever, such known devices are not very accurate and may be prone to spurious triggering during rapid movements for example. As a result, they may become a nuisance to the user causing them to be ignored or discarded altogether.

The most common form of tilt switch employs a bead of liquid mercury which is contained within a sealed enclosure. In some orientations the bead completes a circuit between a pair of electrical contacts whereas in other orientations the circuit is incomplete. However, a bead of liquid suffers from inertia problems, and the angle of triggering is extremely variable. Mercury is also a potentially hazardous material and its use is generally undesirable in a device which is intended to be carried on the person.

The present invention seeks to provide a new and inventive form of tilt switch which is capable of accurate and reliable results and which is particularly suitable for use in a lifting position monitor.

SUMMARY OF THE INVENTION

The present invention provides a tilt sensor which includes an elongate housing having a longitudinal housing axis which is substantially vertical when the housing is in a datum position, a magnetic element confined within the housing for longitudinal movement therein, means for supporting the magnetic element in such a way that the longitudinal position of the magnetic element varies as the housing is tilted away from the datum position, and a pair of switch contacts mounted adjacent to the housing for the magnetic element such that the switch contacts are operable by the magnetic element to assume open and closed positions depending on the longitudinal position of the magnetic element whereby the switch changes state when the sensor is tilted beyond a predetermined critical angle relative to the datum position.

The means for supporting the magnetic element could take various forms.

For example, the magnetic element could be arranged to float on a liquid so that the longitudinal position of the magnetic element changes as the surface of the liquid tilts. Another possibility would be to support the magnetic element on a weak spring such that the spring is maximally compressed when the housing is vertical. As the housing tilts the vertical weight component acting on the spring is reduced so that the spring extends. However, both of these proposals have drawbacks. In the case of a liquid it is difficult to obtain a reliable amount of movement when the housing is relatively narrow, and in the case of a spring it is difficult to obtain the necessary amount of travel. It has been found that these drawbacks can be overcome by supporting the magnet by means of a second magnet using magnetic repulsion.

The tilt sensor is particularly suited for use in a lifting position monitor for attachment to the clothing of a wearer to provide a warning when the wearer assumes an undesirable lifting posture, the device including an elongate case having a longitudinal case axis, attachment means for securing the case to the clothing of the wearer with the longitudinal case axis in a substantially vertical position, a tilt sensor in accordance with Claim 1 or 2 mounted in the case with its longitudinal housing axis substantially parallel with the longitudinal case axis, electronic circuitry mounted within the case and connected to the switch contacts of the tilt sensor to actuate an alarm when the tilt sensor is moved from the datum position beyond said critical angle.

The alarm may be audible and/or visual and may operate remotely of the housing, e.g. by means of a radio link.

The monitor may include an on/off switch which is operable by the attachment means when the device is attached to the wearers clothing to supply power to the electronic circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a longitudinal section through a tilt sensor for use in a lifting position monitor in accordance with the invention, shown in a vertical datum position; Figure 2 is a view of the tilt sensor when inclined at a critical angle; Figure 3 is a general view of a lifting position monitor incorporating the sensor; Figure 4 is a longitudinal cross sectional view of the lifting position monitor on the line IV-IV of Figure 5; Figure 5 is a longitudinal cross section on the line V-V of Figure 4; Figure 6 is a circuit diagram of the electronic circuitry used in the lifting position monitor; Figure 7 is general view of a person wearing the lifting position monitor, assuming a correct lifting position; Figure 8 is another general view of the same person, assuming an incorrect lifting position; and Figures 9 to 11 show alternative forms of tilt sensor which can be used with the lifting position monitor.

DETAILED DESCRIPTION OF THE DRAWINGS

The tilt sensor shown in Fig. 1 includes an elongate housing 1 which may be formed of glass, plastic, or other non-magnetic material. The housing may be of any cross-sectional shape and has a longitudinal axis 2 which, in the datum position of the tilt sensor, is substantially vertical as shown.

Inside the housing, a first magnet 3 is secured at a lower end of the housing, e.g. by means of thin layer or resin 4. A second magnet 5 is slidably contained within the housing with one of its north and south poles opposed to the same pole of the fixed magnet 3 (S-S or N-N). The two magnets therefore repel each other so that the sliding magnet 5 moves to a position where the weight W1 of the magnet balances the repulsive magnetic force.

Adjacent to the upper end of the housing a reed switch 7 is fixed relative to the housing. Under the conditions shown in Fig. 1 the sliding magnet 5 is spaced sufficiently from the switch 7 that the switch contacts 8 and 9 adopt an open position.

When the housing 1 is progressively tilted away from the datum position as shown in Fig. 2 the weight W1 of the second magnet 5 continues to act vertically so that the weight component W2 acting along the axis 2 becomes smaller. The magnet 5 will therefore move further away from the fixed magnet 3 so that the repulsive force again balances the weight component W2. When the tilt angle of the housing reaches a predetermined critical angle L, shown in Fig. 2, the magnet 5 will exert a sufficient force on the switch to cause the contacts 8 and 9 to close. Returning the tilt sensor towards the datum position causes the switch contacts to open again within a few degrees of the critical angle.

The use of magnetic repulsive force produces very reliable operation of the tilt sensor. The frictional forces acting on the movable magnet 5 are normally small but they may be minimised by forming the housing 1 of a low friction plastic or including a small quantity of lubricant such as oil or graphite dust within the housing. Inertial or oscillatory movements of the magnet 5 are also low.

Fig. 3 shows a lifting position monitor which incorporates the tilt sensor. The monitor has an elongate plastic case 20 which is moulded in two halves 21 and 22 with a longitudinal division. At the upper end of the housing there is a small piezo sounder 23. One half of the housing has a spring clip 25 which extends substantially parallel to the case wall enabling the monitor to be secured to a wearers clothing in an upright position, e.g. clipped inside a shirt pocket.

Referring to the sectional views of Fig.s 4 and 5, the two halves of the case are secured together by screws 27 and 28 which are screwed through aligned pillars moulded on the two halves of the case. The case halves could also be snap-engaged. The clip 25 operates a small on/off switch 24, mounted alongside the sounder 23 at the upper end of the housing, which automatically powers the device when the clip is flexed to secure the monitor to the wearers clothing. The lower end of the case forms a battery compartment containing a pair of small electrical cells 30 and 31, one end of which are electrically connected by spring contacts 32. The upper ends of the cells are disposed adjacent to an elongate printed circuit board 33 to which the ceils supply power via spring contacts 34 and 35. The cells may be of the rechargeable type so that a small charger can be plugged into the monitor, avoiding the need to open the case to replace the batteries. The circuit board carries electrical circuitry, described in detail below. As shown, the housing 1 and reed switch 5 of the tilt sensor are mounted on the board substantially aligned with the longitudinal axis of the case 20 along which the section line V-V extends. However, the housing and the switch may be inclined to the axis of the case should this be necessary to assist actuation of the device.

An example of the electrical circuitry which may be employed in the monitor is shown in Fig. 6. The circuitry includes a quad NAND gate, IC1a to IC1d and two timer chips IC2 and IC3. The switch SW1 represents the reed switch 5. When the switch is closed upon tilting of the monitor to or beyond its critical angle capacitor C1 charges through resistor R1 until the threshold voltage of the gate IC1a is reached. This provides a small delay of about 250 milliseconds to prevent false triggering due to contact bounce for example. The output of the gate therefore falls low, and after inversion by ICI b, enables the timer IC2 which is configured as an oscillator to drive the piezo sounder 23 causing it to emit a tone. The rising voltage on capacitor C1 also triggers IC1c which applies a short start pulse via capacitor C3 to the second timer IC3 causing it to commence a timing period of about five seconds set by resistor R7 and capacitor C5. On expiry of the timing period the output of IC3 goes low, turning off the NAND gate IC1a and thereby inhibiting the sounder. The audible warning signal can therefore only be emitted for a maximum of 5 seconds, preventing the battery from running iow if the monitor is placed in a horizontal position for a long period when not in use.

When the monitor is moved back past the critical angle towards the datum position the reed switch SW1 opens allowing the capacitor C1 to discharge through resistor R2. The low input to the gate ICI a causes the input to IC2 to go low and the oscillator IC2 is immediately inhibited even if the 5 second timing period has not expired. IC1d inverts the output of IC1c to apply a reset signal to the timer IC3.

From the foregoing it will be appreciated that when the device 20 is placed in the pocket of a wearer as shown in Fig. 7, provided the wearer maintains a safe lifting posture when lifting a heavy object B the tilt sensor will not reach the critical action and the sounder 23 will not operate. On the other hand, if the wearer should attempt to lift the object from an incorrect lifting position as illustrated in Fig. 8 the monitor will tilt to an angle exceeding the critical angle causing the sounder to emit a penetrating warning signal.

Fig. 9 shows an alternative form of tilt sensor which can be used with the lifting position monitor. The housing 1 is again of elongate form and contains a cylindrical magnet 5 can roll axially within the housing. Normally the magnet rests at the lower end of the housing, but when the housing tilts beyond a critical angle the magnet rolls to the opposite end of the housing where it operates an externally mounted reed switch 7. A disc magnet could also be used in this embodiment. Furthermore, the reed switch could be replaced by a pair of spring contacts mounted in one end of the housing 1 which are bridged by a cylindrical metal element 5 to complete an electrical circuit when the housing is tilted.

Fig.s 10 and 11 show a similar form of tilt sensor which could be used with the lifting position monitor. The housing 1 is also of elongate form but the a cylindrical magnet 5 is contained within an inclined end portion 1a.

Normally the housing is mounted in an upright position as shown in Fig. 10.

The magnet rests at the lower end of the housing, but when the housing tilts beyond a critical angle L as shown in Fig. 11 the magnet rolls to the opposite end of the housing where it operates an externally mounted reed switch7. Again, a disc magnet could be used to achieve a rolling action.

It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

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Claims

1. A tilt sensor which includes an elongate housing having a datum position, a magnetic element confined within the housing for longitudinal movement therein such that the longitudinal position of the magnetic element changes when the housing is tilted away from the datum position, and a pair of switch contacts mounted adjacent to the housing for the magnetic element such that the switch contacts are operable by the magnetic element to assume open and closed positions depending on the longitudinal position of the magnetic element whereby the switch changes state when the sensor is tilted beyond a predetermined critical angle relative to the datum position.

2. A tilt sensor according to Claim 1, in which the housing has a longitudinal housing axis which is substantially vertical when the housing is in its datum position, and the housing contains means for supporting the magnetic element in such a way that the longitudinal position of the magnetic element varies progressively as the housing is tilted away from the datum position.

3. A tilt sensor according to Claim 2, in which the means for supporting the magnetic element includes a magnet which is arranged to support the magnetic element by magnetic repulsion.

4. A lifting position monitor for attachment to the clothing of a wearer to provide a warning when the wearer assumes an undesirable lifting posture, the device including an elongate case having a longitudinal case axis, attachment means for securing the case to the clothing of the wearer with the longitudinal case axis in a substantially vertical position, a tilt sensor in accordance with Claim 1, 2 or 3, electronic circuitry mounted within the case and connected to the switch contacts of the tilt sensor to actuate an alarm when the tilt sensor is moved from the datum position beyond said critical angle.

5. A lifting position monitor according to Claims 4 and 2, in which the tilt sensor is mounted in the case with its longitudinal housing axis substantially parallel to the longitudinal case axis.

6. A tilt sensor substantially as described with reference to the drawings.

7. A lifting position monitor substantially as described with reference to the drawings.

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