GB2318454A

GB2318454A - Adjustable switching mechanism

Info

Publication number: GB2318454A
Application number: GB9621813A
Authority: GB
Inventors: Kevin John Hosking; Christopher John Tolhurst
Original assignee: Amot Controls Ltd
Current assignee: Amot Controls Ltd
Priority date: 1996-10-19
Filing date: 1996-10-19
Publication date: 1998-04-22
Anticipated expiration: 2016-10-19
Also published as: GB2318454B; GB9621813D0

Abstract

An adjustable switching mechanism comprises a movable member 30 which is movable between first and second positions so as to operate an electrical switch 2 and differential adjustment means 25 for adjusting the setting of the first position. The movable member when moved from the first to the second position engages with a first actuator 10 to operate the switch from a first to a second state, but when moved from the second position partly towards the first position disengages with the first actuator, the first actuator leaving the switch in the second state. A second actuator 20 has the differential adjustment means and is biased to engage with the movable member and arranged when the member is moved from the second position to the first position to move and engage with the first actuator to operate the switch to the first state when the member is at the first position.

Description

Adjustable Switching Mechanism The present invention relates to a switching mechanism which may be activated by movement of a movable member, and more particularly to an adjustable switching mechanism with adjustment means to set the points at which the switch operates relative to the movement of the movable member.

Many forms of switch, such as a microswitch, are actuated by movement of an actuator mechanism moved by a movable member in response to a change in a condition being monitored, for example a plunger or pushrod moving in response to a bellows moving according to a pressure or temperature change. The actuator mechanism moves some part of the switch, such as a push button, to move it from the off to the on position and vice versa.

There switch mechanism is usually designed to provide a difference in the positions of the actuator mechanism required to operate the switch from the on to the off position and vice versa. This may be done by amplifying a small mechanical lag or hysteresis within the switch itself, or it may be done by providing a mechanical lag in the actuator mechanism, for example by lost motion in the actuator mechanism for just one direction of travel of the pushrod. This difference can be used to provide a range in the values of the condition being monitored, for example temperature or pressure, over which the switch maintains its on or off position before it operates. This prevents hunting of the switch, which might occur where no differential is provided and the condition being monitored fluctuates continuously. This range will be denoted herein as the differential range of the switch mechanism.

It is often desirable to be able to adjust the value of the variable being monitored, for example the temperature, at which the switch operates. This is typically done by providing a spring bias against which the actuator mechanism bears. The greater the bias, the greater the force the actuator mechanism has to generate to move to operate the switch and hence the greater the pressure or temperature required to operate the switch. Such a mechanism provides a simple adjustment means and is widely used in many forms of switch mechanism.

It is also often desirable to be able to vary the differential range of the switch, for example to increase the temperature range between the on and off actuations of the switch. In one type of adjustable switch, this is done where a spring bias is used to adjust the value of the variable, eg. temperature, at which the switch operates, by providing a second spring bias which operates to oppose the movement of the actuator mechanism towards either direction of its travel, see for example British Patent 1 566 093. By adjusting the second spring bias, the force required for the actuator mechanism to complete its travel against the bias can be selected. Since the condition being monitored provides the force required to move the actuator mechanism, this second bias delays actuation of the switch until the condition has changed sufficiently to generate the larger force required to overcome the bias. This results in a larger differential range. Where the second bias spring assists the movement of the actuator mechanism, increasing the second bias will reduce the differential range.

Such mechanisms ensure that the switch operates at a given point in the travel of the pushrod or other means which is being moved by the condition being monitored. This provides a simple design of the device and any effects due to variations in the geometry of the actuator mechanism at various points in its travel do not have to be taken into account.

However, the adjustment of one bias spring affects the setting of the other bias spring so that, for example, adjustment of the temperature at which the switch operates affects the differential range setting for the switch.

It is then necessary to re-set the differential range bias spring once the temperature of operation has been set.

In turn, adjustment of the differential range bias spring will affect the operation of the operating temperature bias spring so that re-setting of the operating temperature bias spring is required. As a result, it is difficult to ensure accurate setting up of the operating temperature and differential range bias springs.

According to the invention, there is provided an adjustable switching mechanism comprising: an electrical switch; a first actuator which when moved operates the switch between a first state and a second state; a movable member that may be moved between a first position and a second position so as to operate the switch and which when moved from the first position to the second position engages with the first actuator to operate the switch to the second state when the member is at the second position, and which when moved from the second position partly towards the first position disengages with the first actuator, the first actuator then leaving the switch in the second state; a second actuator biased to engage with the movable member and arranged when the member is moved from the second position to the first position to move and engage with the first actuator to operate the switch to the first state when the member is at the first position, the second actuator having differential adjustment means to adjust the setting of the first position.

The first and second positions will be related to some part of the movable member which may be moved under the action of an external stimulus, for example, an end of the member connected to, or biased against a moving object such as a transducer or a bellows.

The switch may be a microswitch, and may be operable via a push button or lever arm extending from the switch which is moved under contact with the first actuator. The switch button or arm may present a varying pressure to operation as the switch is moved between positions. For example, the pressure may rise at a point intermediate the first and second states, and this may permit the first actuator to keep the switch in the first position, yet not permit the first actuator to operate the switch from the second position to the first position.

Preferably the adjustment of the differential adjustment means comprises an adjustment of a gap between the first and second actuators. The first and second actuators may therefore have opposed portions which may come into contact and engage so that the actuators move in tandem.

The adjustment would then be provided between the opposed portions to vary the gap.

It is advantageous if the gap is adjusted by a screw passing through the second actuator, the screw being turned by a linkage comprising a spring in order to set the gap. The spring also provides a bias for the second actuator, which will be described further on. The spring is preferably a coil spring, however other types of spring may be used, such as a leaf spring. Alternatively other compressible or extensible members may be used, such as a flexible or compressible rod or even a piston and cylinder. However, a coil or leaf spring is preferred because this provides some lateral flexibility in the linkage to accommodate movement which is not purely longitudinal, for example rotational movement of the second actuator about a pivot point.

The second actuator may be biassed by the spring in the linkage. Although it would be possible to have a second spring to bias the second actuator, use of the spring in the linkage has the advantage that the compression (or extension) of the spring may not significantly alter as the linkage is turned to adjust the screw and the gap, and this may allow the differential range to be adjusted without affecting one of the set points at which the switch operates.

Because it is often easier to provide rotational movement, as opposed to linear movement, both first and second actuators may move by pivoting about a point. Preferably the first and second actuators are pivotable about a common axis. The actuators may be balanced about the pivot point so that the weight of the actuators does not affect the operation of the switch, however, the biasing means will in general provide a stronger bias than any unbalanced weight, so that the mechanism is not affected by gravity and may be used in any orientation.

The common axis may be defined by a fitting attached to the side of the switch, however, the switching mechanism may comprise a housing to hold the switch and the other components of the switch, in which case the common axis may be supported by the housing.

It will usually be the case that the movable member has set point adjustment means to equally adjust the setting of both the first position and second position.

The housing may also provide support for the differential and set point adjustment means, which may then have rotatable knobs on the outside of the housing to drive the adjustment mechanisms.

The movable member may have a shoulder, flange, groove or other such feature, for engaging with the first and second actuators. The shoulder will then move in conjunction with the movable member, as the movable member moves between the first and second positions.

The set point adjustment means may then move or alter the position of the shoulder with respect to the movable member to adjust the setting of both the first and second positions. The set point adjustment then adjusts points in the travel of the movable member at which the switch operates. The movable member may be a linear actuator which mov-s linearly, for example a pushrod.

The first actuator may be weighted, sprung or otherwise biased to engage with the switch, the biasing being sufficient to retain the switch in the first state, but insufficient to operate the switch from the second state to the first state. The first actuator biasing will therefore usually be much less strong than the second actuator bias, the second actuator bias having the dominant effect when the second actuator moves the first actuator to operate the switch to the first state when the movable member is at the first position.

The first actuator may be biased with a coil spring, or other such means as described above for the second actuator.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a front view of an adjustable switch mechanism according to the invention; Figure 2 is a side view of the switch mechanism of Figure 1; Figure 3 is a top view of the switch mechanism of Figures 1 and 2; Figures 4 to 7 are side views of the switch mechanism showing the mechanism at stages during toggling from a first state to a second state; and Figures 8 to 11 are side views of the switch mechanism showing the mechanism at stages during toggling from the second state to the first state.

Referring first to Figures 1 to 3, a switch mechanism 1 has a microswitch 2 with three electrical contacts 3 switched upon activation of a press button 4. The microswitch is activated between a first state, in which the button is depressed, and a second state, in which the button protrudes, by means of a pivoting first actuator 10 which has an arm 11 at the end of which there is a pad 12 which contacts the button.

The switch mechanism has a second pivoting actuator 20, which is drawn in bolder outline to help distinguish this from the first actuator 10. Both the first and second actuators pivot about a common axis 15. Each actuator has a form which is roughly L-shaped.

Between the actuators 10,20 is a movable member in the form of a pushrod mechanism 30 (omitted for clarity in Figure 3). The pushrod mechanism has a rod 31 which passes through a sleeve 32 arranged to prevent rotation of the rod 31. In use, the sleeve along with the other illustrated components, would be supported and fitted inside a housing, which is omitted from the drawings for clarity.

An end, the upper end as drawn, of the rod 31 is threaded and is screwed inside an engagement member 33 which has an annular flange 34. As indicated by the double headed arrow, the rod may be moved longitudinally from a lower first position to an upper second position, i.e. towards the engagement member 33, and back again by an external stimulus, for example the movement of a bellows. When the rod is moved towards the engagement member, the upper side of the flange 34 comes into contact with a pair of similarly shaped convex surfaces 13,23 one on each of the actuators 10,20, causing each actuator to pivot clockwise to a position as illustrated in Figure 2. This causes the first actuator pad 12 to come away from the button 4, in which case the switch 2 will be in the second state with the button protruding.

A biasing spring 14,24 is connected to each of the actuators 10,20 respectively. The springs are in compression, and so will tend to keep each of the actuators pressing against the flange 34.

The first actuator spring 14 is fixed against a post in the housing (not shown) and is therefore not adjustable.

This spring is weak and is only slightly in compression, and is therefore not strong enough to depress the button when the engagement member 30 is moved sufficiently towards the lower first position. The second actuator spring 24 is stronger than the first spring 14.

The operation of the switch mechanism 1 may be appreciated from Figures 4 to 7. These and subsequent drawings omit the push rod mechanism 30 so that the movement and operation of the actuators 10,20 can be better appreciated. In Figure 4, the push rod mechanism would be in the first state, that is the lowermost position, with the flange 34 just in touching contact with the actuator convex surface 23 of the second actuator, and below and spaced from the convex surface 13 of the first actuator.

The second actuator 20 has a screw or bolt 25 which is threaded through a lower part 26 of the second actuator.

The end of the bolt is opposed to an upper surface 15 on a lower part 16 of the first actuator.

When the rod 31 is in the lowermost first position, the end of the bolt 25 comes into contact with the first actuator surface 15, and the stronger second spring 24 then makes the first actuator 10 depress the button 4.

The rod is able to move some short distance farther below this level, in which case the flange would no longer contact the second actuator convex surface, but this will have no effect on the switch mechanism.

If the rod 31 is then moved towards the second position, that is upwards as drawn, for example upon the increase of a pressure behind a bellows, the flange will rotate both actuators 10,20 clockwise. The flange 34 will first pivot the second actuator 20, causing the bolt 25 to rise off the first actuator surface 15. The first spring 14, however, provides enough bias to keep the button 4 depressed.

If the rod 31 is moved further upwards, then the flange 34 will come into contact with the first actuator convex surface 13, as shown in Figure 5. Further movement of the rod will then and start to pivot both the first and second actuators clockwise until, as shown in Figure 6, the button is released and the switch operates to the second state, at which point the rod is in the second position.

Further upwards movement of the rod will have no further effect on the state of the switch.

Referring now to Figure 8 to 11, for ease of comparison, Figure 8 shows the switch mechanism 1 in the same position as Figure 7. When the rod begins to move back, it will first pass through the second position, as shown in Figure 9. The switch remains in the second state as the first and second actuators pivot anticlockwise.

The first actuator button pad will come into contact with the switch button 4, as shown in Figure 10, but the first actuator spring 14 is not strong enough to urge the button to the first state, and so operate the switch 2. The switch therefore remains in the first state up to the point where the end of the bolt 25 begins to press downwards on the first actuator surface 15, whereupon the bias from the stronger second actuator spring 24 causes the switch button 4 to be depressed to the first state, as shown in Figure. 11. The rod is then back in the first position.

As may be appreciated from the above, between first and second states there is a gap 29 between the bolt 25 and the first actuator surface 15, and the size of the gap determines an amount of lost motion during the downward travel of the rod, during which toggling of the switch 2 is deferred until the gap 29 is closed.

Adjusting the size of the gap 29 therefore adjusts the differential range of the switching mechanism. Referring back to Figure 1 and 2, the second actuator spring 24 forms a flexible linkage between a rotatable differential adjustment knob 28 which, when rotated, acts to turn the bolt 25 to raise or lower the end of the bolt.

An indicator arm 51 extends from the side of a threaded sleeve 26 on a shaft 27 driven by the knob, so that the differential range adjustment setting may be displayed.

Although changing the size of the gap 29 has a slight effect on the compression of the second spring 24 when the gap is closed, this need not significantly affect the movement of the rod 31, and hence the temperature or pressure at which the first and second positions are reached by the rod. This is because it will normally be the case that the spring constant of the second spring is much less stiff than the external stimulus driving the rod.

A similar means of adjustment is provided to adjust equally both the first and second positions. A set point adjustment knob 38 rotates the engagement member 33, which is free over some range to slide longitudinally over the end of a shaft 17 that may be rotatably driven by the set point knob 38. An indicator arm 41 extends from the side of a threaded sleeve 36 on the shaft 27, so that the set point adjustment setting may be displayed.

The engagement member 33 is shown partially cut away in Figure 1, to reveal a central threaded bore 42 into which is screwed a similarly threaded end 43 of the rod 31.

When the engagement member is rotated by the set point adjustment knob 38, the engagement member will travel in one direction or the other along the end of the rod, so setting the relative position of the flange 34 with respect to the rod 31. This has the effect of adjusting equally both the first and second positions of the rod, at which the switch is operated.

The adjustments in effect adjust points on the travel of the rod at which the switch operates.

The adjustable switching mechanism may, of course, be used with push button switches other than the microswitch illustrated. If the push button is relatively stiffly sprung, then the adjustable switch mechanism may be provided with heavier first and second springs.

The movable member may be provided with a bias spring which could, for example, be provided in the gap visible between the threaded end 43 of the rod and the end of the rotatable shaft 17. It would also be possible to use the principle of operation described above with a movable member which did not move linearly, for example a movable member with had a rotational component of motion.

Most components of the switch, in particular the first and second actuators and the adjustment means therefor, may be formed relatively inexpensively in moulded plastics.

The switch described in detail above is relatively simple in construction, having just a few main moving parts, and yet allows substantially independent adjustment of the differential range of the switch, and also of the set point adjustment relative to an external stimulus moving the rod.

Claims

1. An adjustable switching mechanism comprising: an electrical switch; a first actuator which when moved operates the switch between a first state and a second state; a movable member that may be moved between a first position and a second position so as to operate the switch and which when moved from the first position to the second position engages with the first actuator to operate the switch to the second state when the member is at the second position, and which when moved from the second position partly towards the first position disengages with the first actuator, the first actuator then leaving the switch in the second state; a second actuator biased to engage with the movable member and arranged when the member is moved from the second position to the first position to move and engage with the first actuator to operate the switch to the first state when the member is at the first position, the second actuator having differential adjustment means to adjust the setting of the first position.

2. An adjustable switching mechanism as claimed in Claim 1, in which the adjustment of the differential adjustment means comprises an adjustment of a gap between the first and second actuators.

3. An adjustable switching mechanism as claimed in Claim 2, in which the gap is adjusted by a screw passing through the second actuator, the screw being turned by a linkage comprising a spring.

4. An adjustable switching mechanism as claimed in Claim 3, in which the second actuator is biassed by the spring.

5. An adjustable switching mechanism as claimed in any preceding claim, in which the first and second actuators are pivotable about a common axis.

6. An adjustable switching mechanism as claimed in any preceding claim, in which the movable member has set point adjustment means to equally adjust the setting of both the first position and second position.

7. An adjustable switching mechanism as claimed in any preceding claim, in which the movable member has a shoulder for engaging with the first and second actuators.

8. An adjustable switching mechanism as claimed in Claim 7, in which the set point adjustment means moves the shoulder with respect to the movable member to adjust the setting of both the first and second positions.

9. An adjustable switching mechanism as claimed in any preceding claim, in which the movable member is a pushrod.

10. An adjustable switching mechanism as claimed in any preceding claim, in which the first actuator is biased to engage with the switch, the biasing being sufficient to retain the switch in the first state, but insufficient to operate the switch from the second state to the first state.

11. An adjustable switching mechanism as claimed in Claim 10, in which the first actuator is biased with a spring.

12. An adjustable switching mechanism substantially as herein described with reference to and as shown in the accompanying drawings.