GB2299670A

GB2299670A - Chart recording apparatus

Info

Publication number: GB2299670A
Application number: GB9507289A
Authority: GB
Inventors: Stuart Rowlands; Roger Bernard John
Original assignee: BRITISH ROTOTHERM Co Ltd
Current assignee: BRITISH ROTOTHERM Co Ltd
Priority date: 1995-04-07
Filing date: 1995-04-07
Publication date: 1996-10-09
Anticipated expiration: 2015-04-07
Also published as: GB2299670B; GB2299670A8; GB9507289D0

Abstract

A chart recording apparatus which is used for providing a permanent trace or record on a recording medium or chart to show the variations of a measured parameter with time comprises a pivotally movable arm which usually carries a pen and a drive arrangement to drive the recorder arm pivotally. The drive arrangement comprises worm gearing having a worm 7 arranged to mesh with a correspondingly threaded or tooth-form curved portion of a worm follower 6 which is pivotally mounted and operatively coupled to the recorder arm 5. Driving the worm effects pivotal movement of the worm follower which causes pivotal movement of the recorder arm. The worm gearing enables a high gear ratio to be achieved between a drive motor 8 and the recorder arm 5 to provide a high resolution and smooth action using only two gears. This is stated substantially to eliminate backlash.

Description

2299670 Chart Recording Apparatu The present invention relates to chart

recording apparatus, and in particular to chart recording apparatus used to provide a permanent trace on a recording medium or chart showing the variations of a measured parameter with time.

chart recorders are known which produce a permanent trace on a rotating recording chart for use in monitoring the history of, for example, process or other parameters. Certain known chart recorders typically comprise pivoted recorder arms which carry plotters or pens over the surface of the chart, the pivotal arms being provided with a drive arrangement actuatable to pivot respective arms in response to an electrical signal proportional to the sensed variation in process or other parameter of which the history is being recorded. The accuracy of the historical record produced on the chart is dependent on the sensitivity and resolution of the recording apparatus, which is in turn dependent on the construction and operation of the drive arrangement. A particular problem with known drive arrangements concerns backlash present in drive gearing which results in a "dead band" noticable in the recorded trace when the recorder arm changes direction.

Chart recording apparatus having an improved drive arrangement has now been devised.

According to the present invention, there is provided chart recording apparatus comprising at least one pivotally movable recorder arm and a drive arrangement to pivotally drive the recorder arm, the drive arrangement comprising worm gearing having an elongate worm element arranged to mesh with a correspondingly threaded or tooth form arcuately extending portion of a worm follower element, the worm follower element being pivotally mounted and operatively coupled to the recorder arm, the worm element being drivable to effect pivotal movement of the worm follower element and thereby the recorder arm.

It is preferred that the recorder arm is provided with, or is arranged to carry, a respective recorder element such as plotter pen or other recording instrument.

The use of the specific worm gearing as def ined enables high gear ratio to be achieved (between a drive motor and the pivotal recorder arm) providing high resolution and smooth action whilst utilising as few gear elements as possible to pivotally move the recorder arm which results in minimal gearing backlash.

Desirably, the threaded or tooth form portion of the worm follower element extends arcuately about (and has a pitch radius which is preferably centred on) the pivotal axis of the worm follower element.

The worm element is preferably arranged to engage the arcuately extending threaded or tooth form portion of the worm follower element substantially tangentially thereto.

Desirably the worm element is pivotally movable out of and into meshed engagement with the worm follower element. it is preferred that biasing means is provided arranged to normally bias the worm element into meshed engagement with the worm follower element. Advantageously, the biasing means comprises resilient biasing means such as spring means, preferably providing a resiliently biased spring arm. The biasing means preferably exerts a biasing force transversely to the worm element in a direction generally radially inwardly toward the pivot axis of the worm follover element.

The provision of biasing means ensures that the worm element is held f irmly in register with the threaded or tooth form arcuate portion of the worm follower element which effectively eliminates gearing backlash. Furthermore, the arrangement is effectively self adjusting as the worm and worm follower thread or tooth forms wear with time.

It is preferred that the worm element is driven to rotate about its longitudinal axis preferably by means of an electric motor. The electric motor is preferably arranged with its drive spindle comprising (or arranged co-axially with) the longitudinal axis of the worm element. The motor may be pivotally mounted to enable pivotal movement of the worm element relative to the worm follower element.

The electric motor is arranged to be actuated in response to a control stimulus or signal. Control meansf such as microprocessor control means or the like, is preferably arranged to provide the control stimulus or signal arranged to actuate the electric motor. Preferably the electric motor comprises a stepper motor; alternatively, the electric motor may comprise a reversible DC motor.

Desirably a feedback arrangement is provided which is preferably closed loop. The feedback arrangement preferably comprises a potentiometer comprising one or more tracks and a relatively movable corresponding wiper or wipers. Desirably the wipers are carried by the pivotally movable recorder arm or worm follower element.

Advantageously, the control means further comprises sensor means such as, for example, microswitch, optical switch or other sensor means arranged to provide control stimulus or signals when a recorder arm reaches or passes in predetermined position.

It is preferred that the recorder arm is rigidly connected to the worm follower element both preferably being pivotable together about a common pivot axis preferably comprising a central pivot pdst.

In a preferred embodiment a plurality of recorder arms are provided advantageously arranged to be spaced about a common pivot axis, preferably comprising a central pivot post. A respective drive arrangement is preferably provided for each respective recorder arm. Advantageously, the arms are arranged one above the other having respective worm follower element connecting portions nested one inside the other.

The invention will now be further described in a specific embodiment by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view from a first position of chart recording apparatus according to the invention; Figure 2 is a perspective view from a second position of the chart recording apparatus of Figure 1 (showing only one recorder arm and drive arrangement for clarity).

Figure 3 is a schematic plan view of the apparatus as shown in Figure 1; Figure 4 is a schematic side elevation of the apparatus as shown in Figure 2; and Figure 5 is a schematic view of a part of the apparatus of Figures 1 to 4 showing the connection between the recorder arm and worm follower element.

Referring to the drawings, the chassis of the recorder apparatus comprises two flat circular plates (1,2) which are separated and held parallel by three equi-spaced steel pillars (3). The upper plate (1) is cut away to provide clearance for one or more "C" shaped radial arms (5) to pass through. In use, the apparatus is mounted within a conventional chart recorder housing (not shown) such that the upper ends (5a) of radial arms (5) extend over the surface of a rotating circular chart (not shown) The arms are fitted with plotter or pen extensions (not shown) to record a relevant trace or traces on the chart as it rotates with time. Screw holes in the upper plate (1) allow the mechanism to be mounted beneath the writing surface of the recorder at the periphery ofthe chart.

A central rigid vertical post (4) is f ixed to the bottom plate (2), passing through a support in the top plate and extending beyond the writing surface. The centre post carries up to three C-shaped radial arms (5) which are pivotally mounted to rotate about post (4). Spacers set the positions of the radial arms on the shaft and limit vertical play. The dimensions of the radial arms are such that they are "nested" coaxially and turn independently without interference. The upper ends of the radial arms (5a) mate with pen arm extensions (not shown) which are attached by means of plastic clips (also not shown).

A worm follower gear segment (6) is located at, and rigidly connected to, the base of each radial arm (5), being pivotal about the vertical post (4). Each worm follower gear segment (6) has an outer edge (6a) which is concavely indented and carries an M4 thread form. The outer indented concave edge (6a) extends along an arc on a pitched radius about pivot post (4) centred on the axis of the post (4). In a multi-pen mechanism the worm follower gear segments (6) are angularly equispaced about the centre Post (4) and the nesting of the radial arms (5) sets the gear segments at different heights above the bottom plate.

Each worm follower gear segment (6) is driven by an M-4 threaded worm (7) attached coaxially to the shaft of a bidirectional stepper motor (8). (operation with a reversible DC motor is also possible). The worms (7)engage tangentially with arcuately extending edges (6a) of the worm follower gear segments (6) and parallel to the chassis plates (1,2). The motors (8) are held in carriers (9) which are freely pivoted on posts (10) which extend between the top and bottom plates (1,2). This permits the worms (7) to swing towards the central pivot post (4) and matingly engage with the edges (6a) of the worm follower gear segments (6). A spring arm (21) of spring (11) acts to normally bias the worm (7) into permanent engagement with the M4 threaded outer edge (6a) of a respective worm follower gear segment (6). The biasing of spring (11) ensures that the two gear threads are closely meshed at all times and backlash is therefore eliminated. A second benefit of the spring biasing is that the mechanism inherently compensates for wear which might occur during use. In addition to providing the radial force necessary to ensure engagement of the gears, the biasing spring (11) is shaped and connected such that a component of the spring force acts along the axis of the worm (7) and motor shaft to take up axial float, aiding the end-f loat control spring (not shown) fitted within the motor (8). (It is preferable to restrict any axial movement of the motor shaft since it would otherwise cause unwanted rotation of the radial arm).

The biasing spring (11) is fitted over a chassis spacer post (3), one of which is positioned near each motor. The spring is attached to a lowfriction PTFE bearing (12) running on the plain free end of the worm shaft (7) and retained by a circlip.

In use, to move from zero to full7scale deflection on a 250mm diameter chart the pen (and therefore arm (5)) travels over an arc of approximately 450. For stepper motor (8), a moderately-high ratio reduction gearing is required between the motor (8) and arm (5) if the motion is to be smooth with f ine resolution of position. The worm drive provides this with the minimum number of gears (two) allowing the construction of the mechanism to be highly compact. A reduction ratio of approximately 166:1 has been achieved using a worm follower segment gear (6) of diameter 37mm and an M4 threadform with a pitch of 0.7mm. Approximately 1000 motor steps are required to drive the pen over its full 450 arc of travel using a standard stepper motor (8) having a 7.50 step angle.

Each stepper motor (8) is operated in response to signals representative of variations in process or other parameters provided by control apparatus such as a microprocessor or other computer control means. The apparatus is conveniently adapted for use with charts of various sizes by means of alteration of the microprocessor programming or control software.

The mechanism design includes microswitches (13) and feedback potentiometers (14),which may be used to establish the relative and absolute position of each pen according to the method of control used. The mechanism may be controlled in one of three ways depending on whether stepper motors (8) or DC motors are used, as follows:

(a) Open-loop with stepper motor. With the stepper motor (8) under microprocessor control a "home" datum position is established by driving the pen towards the centre (or edge) of the chart where its position is sensed by a microswitch or optical switch (13). The pen is positioned by counting the number of motor steps relative to this datum point.

(b) Closed-loop with reversible DC motor. A potentiometer (14) having three independent sections deposited on a circular insulating substrate may be mounted on the bottom chassis plate concentric with the main vertical post (4). The resistive and collector tracks of these potentiometers are open and are electrically connected by means of wipers (22) carried by - but insulated from - the radial arms (5). Thus, the setting of the potentiometer is proportional to the absolute angular position of the radial arm and hence to the position of the pen on the chart.

Using a conventional DC servo motor and error amplifier operating with feedback from potentiometer (14) a closedloop servo positioning system may be constructed in which the position of the pen arm is directly proportional to the amplitude of an analogue control signal. The absence of backlash in the worm reduction gearing improves the positional accuracy and prevents the servo loop from "hunting".

(c) Open-loop with stepper motor and analogue feedback. When operated in open-loop mode with a stepper motor as described in (a) above, a potentiometer (14) as described in (b) may also be included. A feedback signal from the potentiometer (14) may be fed to an analogue-to-digital converter (ADC) and the digital data so obtained may be compared with a theoretical calculated value to determine the positioning error. A further microswitch (15) interfaced to the control apparatus and actuated by a sprung overtravel arm (16) pivoting on the centre shaft detects excessive upscale travel by any of the radial arms (5).

Claims

CLAIX8:

1. Chart recording apparatus comprising at least one pivotally movable recorder arm and a drive arrangement to pivotally drive the recorder arm, the drive arrangement comprising worm gearing having an elongate worm element arranged to mesh with a correspondingly threaded or tooth form arcuately extending portion of a worm follower element, the worm follower element being pivotally mounted and operatively coupled to the recorder arm, the worm element being drivable to effect pivotal movement of the worm follower element and thereby the recorder arm.

2. Chart recording apparatus according to claim 1, wherein the threaded or tooth form portion of the worm follower element extends arcuately about the pivotal axis of the worm follower element.

Chart recording apparatus according to claim 2, wherein the threaded or tooth form portion of the worm follower element has a pitch radius which is centred on the pivotal axis of the worm follower element.

4. Chart recording apparatus according to any preceding claim, wherein the worm element is arranged to engage the arcuately extending threaded or tooth form portion of the worm follower element substantially tangentially thereto.

5. Chart recording apparatus to any preceding claim, wherein the worm element is pivotally movable out of and into meshed engagement with the worm follower element.

6. Chart recording apparatus according to any preceding claim, wherein biasing means is provided to normally bias the worm element into meshed engagement with the form follower element.

-g- Chart recording apparatus according to claim 6, wherein the biasing means exerts a biasing force transversely to the worm element.

8. Chart recording apparatus according to claim 6 or claim 7, wherein biasing means exerts a biasing force in the axial direction of the worm element.

9. Chart recording apparatus according to any preceding claim, wherein the worm element is driven to rotate about its longitudinal axis by means of an electric motor.

10. Chart recording apparatus according to claim 9, wherein the electric motor is pivotally mounted to enable pivotal movement of the worm element relative to the worm follower element.

11. Chart recording apparatus according to claim 9 or claim 10, wherein the electric motor is arranged to be actuated in response to a control stimulus or signal.

12. Chart recording apparatus according to claim 11, wherein control means is arranged to provide the control stimulus or signal arranged to actuate the electric motor.

13. Chart recording apparatus according to claim 12, wherein the control means comprises data processing means such as a microprocessor or computer.

14. Chart recording apparatus according to any of claims 9 to 13, wherein the electric motor comprises a stepper motor.

15. Chart recording apparatus according to any preceding claim, further comprising means for determining the position of the recorder arm.

16. Chart recording apparatus according to claim 15, wherein a positional feedback arrangement is provided to determine the position of the recorder arm at positions in its arc or sweep.

17. Chart recording apparatus according to claim 16, wherein the feedback arrangement comprises a potentiometer arrangement.

18. Chart recording apparatus according to any of claims 12 to 16, wherein the control means further comprises sensor means arranged to provide control stimulus or signals when a recorder arm reaches or passes a predetermined limit position.

19. Chart recording apparatus according to any preceding claim, wherein the recorder arm is rigidly connected to the worm follower element.

20. Chart recording apparatus according to any preceding claim, wherein a plurality of recorder arms are provided advantageously arranged to be pivotable about a common pivot axis.

21. Chart recording apparatus according to claim 20, wherein the recorder arms are arranged one above the other having respective worm follower element connecting portions which are nested one inside the other.

22. Chart recording apparatus substantially as described herein, with reference to the accompanying drawings.