GB2065888A - Position transducers for servo- drives - Google Patents

Abstract

A position transducer in which a sensor moves between two terminal points of a path uses the movement of the sensor to set at least one of the terminal points at the maximum displacement of the sensor in that direction. For example, a slider 28 of a potentiometer is coupled to the motor 32 of the servo-drive via intermediate elements 30. The terminal connectors 34, 36 of the potentiometer are displaceably arranged on the resistor member 26 of the same so that they are pushed away from each other by the slider 28 during its very first displacement, and then remain at these positions. A system of this nature results in an automatic adaptation of the position transducer to the path length of the sensor coupled to the servo-drive. The transducer can be a differential capacitor, a differential inductor or a photodetector variably shielded from a light source. <IMAGE>

Description

SPECIFICATION Improvements in or relating to position transmitters for servo-drives The present invention relates to position transmitters for servo-drives, of the kind comprising a sensor element adapted for coupling to the servo-drive, and arranged to be displaceable by means of the servo-drive between two terminal positions each of which has.an electrical limit value allotted to it, in order to determine an electrical value lying between the limit values and dependent on the momentary position of the servo-drive. Hereinafter, such position transmitters will be referred to as "of the kind described".

A position transmitter of the kind described serves the purpose, in particular, in regulating technology for reporting back the momentary position of an electromotive, hydraulic or otherwise operated servo-drive to a comparator circuit in the regulator or in servo-amplifier postconnected to the regulator. In the heating, ventilation and air conditioning spheres for example, a servo-drive has the task of displacing a setting member to a position proportional to the output signal of the regulator. The setting member may, for example, be a valve, a mixer or an airsetting valve. In order that the full possibility of adjustment of the setting member may be exploited on the one hand, and that stable governing actions may be accomplished on the other hand, it is necessary that the setting path of the servo-drive be adapted to that of the setting member.Since different setting members also have different strokes or angles of rotation, it is necessary to adapt the servo-drive to be coupled to the setting member to the required stroke or pivot angle so that the setting member in question passes through its complete setting path, for example in case of a 100% change of a standardised output signal of the regulator.

To perform an adaptation of this kind, it is known to install a restoring potentiometer as a position transmitter in a servo-drive, which is adapted to the setting path of the setting member by appropriate upward or downward adjustment.

Since analogous servo-drives may be utilised for setting members of different nature, it is necessary because of the different setting paths to produce servo-drives with different position transmitters, or to adapt the position transmitters individually.

The adaptation however requires an intervention in the inside of the servo-drive and may consequently be performed only by a specially trained expert. An adaptation of this kind may also be required if, in the case of a valve, the kvs-value has to be matched in the plant or if the maximum power which is to be adjusted by the setting member has to be limited, for example on the grounds of stability. Keeping servo-drives of identical nature and having different position transmitters in stock, as well as the individual adaptation of the position transmitters to the setting member utilised, are uneconomic.

It is therefore an object of the invention to provide a position transmitter which may be adapted to the required setting displacement or setting angle with minimum difficulty.

Accordingly, the invention consists in a position transmitter for a servo-drive of the kind described, wherein adjusting means adjustable by means of the sensor element, are incorporated to shift and immobilise at least one of the terminal positions on the occasion of the first displacement of the sensor element along its entire path of displacement.

In practice, a position transmitter according to the invention allows of automatic adjustment of at least one of the terminal positions delimiting the effective sensing range of the sensor element. The servo-drive equipped with such position transmitter may be supplied in a basic setting.

After assembling the servo-drive with the servo or setting member, the servo-drive need be driven once only into both of its terminal positions to undertake the automatic adaptation of the position transmitter to the setting path.

A transmitter according to the invention also has a series of other advantages. Few types only of servo-drives need be produced at the makers works and kept in stock. The customer himself may assemble servo-drives and setting members without great complexity in accordance with the building block technique. The assembler may install the servo-drive without any complex setting operation. For as long as the working stroke of the setting member extends within that of the servodrive, the latter automatically finds its working stroke. The servo-drive may also be installed reliably by less highly qualified staff. The maintenance engineer need not perform any complementary adjusting jobs on the servo-drive and the regulator. Parallel operation of several servo-drives is assured.The setting path of excessively related valves or other setting members may be restricted to an optional value by terminal stops or terminal switches, since a position transmitter according to the invention is adaptable to any optional setting path.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which show certain embodiments thereof by way of example and in which: Figure 1 shows a diagram for the setting of both terminal positions of a position transmitter, Figure 2 shows a graph depicting one fixed and one adjustable terminal position of a position transmitter, Figure 3 shows an embodiment of the position transmitter as a potentiometer, Figure 4 shows another embodiment of the position transmitter as a potentiometer, Figure 5 shows an embodiment of the position transmitter comprising a light barrier, Figure 6 shows a plan view of an embodiment of the position transmitter comprising a differential condenser, Figure 7 shows a view corresponding to Figure 6 in axial section, Figure 8 shows a side view of a position transmitter comprising induction coils, Figure 9 shows a front view of the device of Figure 8.

Figure 10 shows a plan view of the device of Figures 8 and 9, Figure 11 shows another position transmitter comprising a light barrier in front view, and Figure 12 shows a sideview of the device of Figure 11.

Referring now to the drawings, and firstly to Figure 1, the electrical values determined by the position transmitter as a function of the setting path registered along an ordinate axis 12 are plotted along an abscissa axis 10 in the graph according to the figure. The electrical value of the position transmitter may be an electrical resistance indicated on a potentiometer, a measured voltage or current. The values allotted to the terminal positions of the position transmitter are denoted by 0 and 100% on the abscissa axis 10. The two terminal positions of the position transmitter are denoted by 0 and 1 on the ordinate axis 12. The position 0 may for example correspond to the closed position of a valve or air damper, whereas the position 1 corresponds to the open position.

The dashed straight line 14 interconnects the two points 1 6 and 18 for example correspond to the terminal positions of the position transmitter installed in a servo-drive in the delivery condition.

The positions 0 and I are the terminal positions assumed by the servo-drive when it is combined with a setting member, e.g. a valve, mixer or setting flap, to displace this setting member from the one terminal position to the other and vice versa.

By displacing the servo-drive to the terminal positions 0 and 1, the position transmitter installed in the servo-drive and still remaining to be described in the following is so adjusted that its settings co-ordinated with the values 0 and 100% coincide with the terminal positions of the servodrive or setting element. The terminal position of the position transmitter allotted to the electrical value 0 is also shifted from the point 1 6 to the start of the setting path, 0. The terminal position of the position transmitter allotted to the electrical value 100% is shifted from the point 18 to the end of the setting path, so that the working characteristic of the position transmitter now corresponds to the straight line 20.Due to this adjustment of the position transmitter performed in the directions of the two arrows 22 and 24, the same is then adapted to the setting path between 0 and 1 of the setting member which is to be actuated, as denoted on the ordinate axis 12.

The graph according to Figure 2 differs from that according to Figure 1 in that the start 0 of the setting displacement plotted on the ordinate 12 corresponds to a fixed terminal position of the position of the position transmitter.

For adapting the position transmitter to the complete setting path of the setting member, it is consequently only the terminal position of the position transmitter corresponding to the electrical value 100% which should be shifted from the point 18' corresponding to the delivery condition and in the direction of the arrow 24' until it coincides with the end 1 of the setting path. The working characteristic of the position transmitter is at the same time shifted from the straight line 14' to the straight line 20'.

A first embodiment of a position transmitter is diagrammatically illustrated in Figure 3. A slider or cursor 28 acting as a sensor element is displaceably arranged on a resistor 26 acting as a sensing member. The cursor 28 is connected via intermediate elements 30 to the motor 32 of the servo-drive. The intermediate elements 30 may for example be a reduction gear and/or a lever running on a cam plate.

References 34 and 36 denote terminal position elements having allotted to them the two effective terminal positions on the resistor 26. The two terminal position elements 34 and 36 may also be sliders cr cursors, which are arranged so that they may be displaced away from the opposed terminal position by the cursor 28 acting as a sensor element. During the very first displacement of the cursor 28 through-out the setting path, the terminal position elements 34 and 36 are also displaced into the corresponding terminal positions. The two terminal position elements 34 and 36 then remain in their set positions whereas the cursor 28 is displaced between these two positions by the motor 32 during operation.

If a constant voltage is fed to the terminal position elements 34 and 36 via the co-ordinated connectors 38 and 40, a value corresponding to between 0 and 100% of the voltage applied may be tapped off via the center connector 42 depending on the position of the cursor 28. The voltage tapped off is thus proportional to the position of the cursor 28 within the setting range delimited by the terminal position elements 34 and 36, which matches the setting path of the setting member.

The system described leads to an automatic adaptation of the position transmitter to the setting path of the setting member which is to be actuated, by appropriate displacement of the two terminal position elements 34 and 36 by means of the cursor 28. The partial voltage tapped off at the connector 42 is thus unaffected by the total resistance of the resistor 26. The graph illustrated in Figure 1 applies to the embodiment according to Figure 3.

The position transmitter illustrated in Figure 4 corresponds to that shown in Figure 3 in its operation. In particular, the difference consists in that one only of the terminal position elements 46 is displaceably arranged on a resistor 44. The other terminal position element 48 is firmly connected to the resistor 44. The resistor 44, together with the other terminal position element 48, is displaceably arranged in the direction of the arrow 52 on a stationary support 50. During a displacement of the cursor 54 by a motor 56 in the direction towards the other terminal position element 48, the resistor 44 is displaced in the direction of the arrow 52. During a displacement of the cursor 54 in the opposed direction, the first terminal position element 46 is displaced on the resistor 44 in the direction of the arrow 58.

Compared to the system according to Figure 3, this system has no more than two moving contacts on the resistor 44, namely the cursor 54 receiving drive from the motor 56 via the intermediate elements 60 to act as a sensing member, and the first terminal position element 46. The position transmitter illustrated in Figure 4 is equally effective in accordance with the graph illustrated in Figure 1.

The optical position transmitter illustrated in Figure 5 has a sensor element in the form of a light barrier of which no more than the light receiver 62 comprising a light entry aperture 64 is illustrated. The light barrier 62 is connected to a motor 68 via intermediate elements 66 and is rectilinearly displaceable on a rail 70. Two terminal position elements 72 and 74 are also displaceably arranged on the rail 70. The two terminal position elements 72 and 74 are illustrated in cross-section to show the funnel-like openings 76 and 78, in which openings is held a blade 80 formed as a covering shutter. The funnellike openings 76 and 78 in the two terminal position elements 72 and 74 are positioned at different heights from the guiding rail 70, so that the covering shutter 80 has at least its top edge 82 inclined with respect to the path of the light barrier 62.

During the very first displacement of the light barrier 62, the same pushes the terminal position elements 72 and 74 away from each other in accordance with the setting displacement. This alters the slope of the covering shutter 80 with respect to the path of the light barrier 62. The change of the slope also occurs in such manner that the light entry opening 64 is wholly shut off by the covering shutter 80 when the light barrier 62 is situated at the terminal position allotted to the terminal position element 72. If the light barrier 62 is situated at the other terminal position allotted to the other terminal position element 74, the covering shutter 80 completely uncovers the light entry opening 64.A system of this kind enables the coverage of the light entry opening 64 by the covering shutter 80 to be varied within the range from 0 to 100% between the two terminal position elements 72 and 74, that is to say notwithstanding the mutual spacing between the two terminal position elements 72 and 74. The light source co-ordinated with the light receiver 62, which is not illustrated however, may be arranged to be displaceable together with the light receiver 62. The quantity of light incident via the light entry opening 64 is converted in an electronic evaluator circuit 84 into an electrical signal which corresponds to the electrical values allocated to the values between 0 and 100%. The operation of the position transmitter illustrated in Figure 5 corresponds to the graph according to Figure 1.

A capacitive position transmitter is illustrated in Figures 6 and 7. This comprises two first capacitor electrodes 86 and 88, of which the electrode 86 is situated on a plate 90 and the other electrode 88 is situated on a ring 92 arranged concentrically with respect to the plate 90. The plate 90 and the ring 92 are rotatably arranged around a spindle 94.

The radial width of the electrode element 86 diminishes clockwise in the peripheral direction.

The width of the other electrode element 88 diminishes counterclockwise in the peripheral direction.

Opposite to the plate 90 and the ring 92, a sector-shaped second electrode 96 is rotatably arranged on the spindle 94. A first stop 98 is situated on the plate 90, and a second stop 100 is situated on the ring 92.

The second electrode 96 acting as a sensor element is connected to the motor 104 via intermediate elements 102. During the very first displacement of the second electrode 96, the two elements 86 and 88 of the first electrode are turned with respect to each other by means of the stops 98 and 100 which co-operate with the edges 106 and 108 of the second electrode 96, until they have assumed positions corresponding to the setting path. During operation, the second electrode 96 acting as a sensor element contradirectionally sweeps over the two parts 86 and 88 of the first electrode. The two stops 98 and 100 also act as terminal position elements.

The capacitive position transmitter illustrated in Figures 6 and 7 corresponds to a differential condenser in principle. The evaluation of the prevailing position of the second electrode 96 acting as a sensor element is performed by means of an electronic evaluator circuit 11 0, which is connected to the electrodes 86, 88 and 96. This position transmitter illustrated in Figures 6 and 7 is also active in accordance with the graph illustrated in Figure 1. Instead of constructing the capacitive position transmitter illustrated in Figures 6 and 7 in circular form, the electrodes may be formed and arranged so that they describe a rectilinear displacement with respect to each other.

An inductive position transmitter is illustrated in Figures 8, 9 and 10. This device comprises two coils 114 and 116 situated on a support 112.

These coils form the sensor element together with the support, and are arranged to be displaceable rectilinearly in the directions of the double-headed arrow 118. The coils 114 and 116 each have an iron core 120 with an air gap 122. A metal tongue 124 and 126, respectively, projects into the air gap of each coil. The metal tongues 124 and 126 are held stationary but may alternatively be displaced rectilinearly. During a displacement of the sensor element comprising the elements 11 2, 114 and 11 6 in the direction of the doubleheaded arrow 118, one of the two metal tongues 124 and 126 is entrained in each case in one direction of displacement, in the arrowed directions 128 and 130 respectively. For entrainment purposes, the support 112 has projections 132 and 134 respectively at its two extremities.The projection 1 32 acts on a projection 136 of the metal tongue 124 during the very first displacement of the sensor element. The projection 134 of the sensor element acts in the opposed direction on a projection 138 of the second metal tongue 126 during said very first displacement.

The depth of penetration of the two metal tongues 124 and 126 into the air gap 122 of the two coils 114 and 116 is altered contradirectionally by displacement in the direction of the arrow 1 8 of the sensor element comprising the parts 112, 114 and 116. The output signal of the inductive position transmitter illustrated in Figures 8, 9 and 10 may be evaluated in a differential oscillator circuit, so that a signal of a value between 0 and 100% is delivered as a function of the momentary position of the sensor element 112, 114, 116. The projection 136 and 138 of the two metal tongues 124 and 126, respectively, thus forms one of the two terminal position elements in each case.This position transmitter also operates in accordance with the graph illustrated in Figure 1.

Another optical position transmitter is illustrated in Figures 11 and 12. This has a covering shutter 140 the top edge 142 of which slopes with respect to the rectilinear direction of displacement of an optical sensor element 144. By contrast to the embodiment illustrated in Figure 5, the covering shutter 140 is fixedly arranged and its slope is not adjustable. The sensor element 144 is displaceable rectilinearly on a guiding rail 146.The sensor element 144 has a U-shaped carriage 148, its branches 1 50, 1 52 having fitted therein a light source 1 54 and a light receiver 1 56. The light source 1 54 and the light receiver 1 56 are arranged to be rotatable around a spindle 158 within the U-shaped support 148, and are connected by a rod 1 60. The rod 160 simultaneously acts as a guiding element which slides along the top edge 142 of the covering shutter 140, when the sensor element 144 is being displaced on the guiding rail 146.During the very first displacement, the light barrier comprising the elements 1 54 and 1 56 is turned around the spindle 158, so that an opening 162 of the light receiver 1 66 or of the light source 1 54 is displaced. The light entry opening 1 62 is situated outside the pivot spindle 1 58, so that the position of the light entry opening 1 62 is altered with respect to the top edge 142 of the covering shutter 140. Due to this arrangement, the adjustment already hereinbefore described in the other embodiments, may be performed during the very first displacement of the sensor element 144 in the direction of the arrow 1 64. The evaluation of the position scanned by means of the light barrier 154, 1 56 is performed in analogous manner to that of the evaluation described with reference to Figure 5. In the position transmitter illustrated in Figures 11 and 12, only the one terminal position is established, whereas the other terminal position starts from a fixed datum point.

The operation corresponds to the graph illustrated in Figure 2.

Claims (13)

1. A position transmitter for a servo-drive of the kind described, wherein adjusting means adjustable by means of the sensor element, are incorporated to shift and immobilise at least one of the terminal positions on the occasion of the first displacement of the sensor element along its entire path of displacement.

2. A position transmitter as claimed in claim 1, wherein the adjusting means comprise at least one terminal position element co-ordinated with the terminal position in question, which are displaceable with respect to the other terminal position by the sensor element.

3. A position transmitter as claimed in claim 2, wherein the two electrical limit values are established by means of two terminal position elements on a sensing member.

4. A position transmitter as claimed in claim 3, wherein the sensing member is arranged to be stationary and the terminal position elements are arranged to be displaceable with respect to the sensor member

5. A position transmitter as claimed in claim 3, wherein the sensor member is firmly joined to one of the terminal position elements and is arranged to be displaceable together herewith.

6. A position transmitter as claimed in claim 4 or 5, wherein the sensing member is an electrical resistor element and the sensor element is a slider displaceable on the resistor element, the terminal connectors of the resistor element forming the terminal position elements.

7. A position transmitter as claimed in claim 3, wherein the scanning member is a covering shutter secured by the terminal position elements, and which is situated between the emitter and receiver of a light barrier forming the sensor element.

8. A position transmitter as claimed in claim 7, wherein the covering shutter is a blade arranged to slope with respect to the rectilinear trajectory of the sensor element, the slope of which diminishes in proportion with the increase of the mutual spacing of the terminal position elements.

9. A position transmitter as claimed in claim 3, wherein the scanning member comprises two first capacitor electrodes which are displaceable and each of which has a stop allotted to it as a terminal position element, and the sensor element has at least one second capacitor electrode opposed to the first capacitor electrodes, in the manner of a differential device which co-operates with one of the stop elements in each direction of displacement to move each of the two first capacitor electrodes into a position corresponding to the terminal position.

10. A position transmitter as claimed in claim 2, wherein at least one induction coil is provided, whose inductance is variable by displacement of the sensor element.

11. A position transmitter as claimed in claim 10, wherein the sensor element comprises two induction coils whose inductances may be varied contradirectionally by a displacement with respect to a metal element in each case, a terminal position element being connected to one of the metal elements in each case.

12. A position transmitter as claimed in claim 1, wherein a covering shutter is situated at a slope with respect to the rectilinear trajectory between the emitter and receiver of a light barrier forming the sensor element, the light barrier being adjustably rotatable on a carriage around a spindle positioned outside its centre and having an entraining element acting on the covering shutter in order to be turned into one of the terminal positions on the occasion of the first displacement.

13. A position transmitter substantially as hereinbefore described with reference to Figure 3, to Figure 4, to Figure 5, to Figures 6 and 7, to Figures 8, 9 and 10, or to Figures 11 and 12 of the accompanying drawings.