AN ACTUATOR HAVING MEANS FOR REGISTRATION OF THE ACTUATOR POSITION

Abstract: By the use of actuators, e.g., for servo controlled adjustment of movable parts of hospital beds, it is desirable to make use of a remote indication of the actuator positions or of adjustment to predetermined positions. The position of an actuator is suitably detectable by measurement on an associated potentiometer (4). In accordance with the invention it is also desirable to use only three connector wires between the actuator and an associated control box (6), and when two of these wires have to carry the motor current it is problematic to measure the position of the potentiometer through another pair of wires, because the motor current is variable. According to the invention, the problem is solved in deriving from a point of a voltage divider in the motor circuit an expression of the potentiometer position in combination with an expression of the motor current, while deriving from an outer section of the voltage divider an expression of the motor current alone, whereby it is possible, by means of a calculating unit, to create an expression of the potentiometer position independent of the motor current.

Description: FOR THE PURPOSES OF INFORMATION ONLY Codes used to identify States party to the PCT on the front pages of pamphlets publishing international applications under the PCT. AT Austria GB United Kingdom MR Mauritania AU Australia GE Georgia MW Malawi BB Barbados GN Guinea NE Niger BE Belgium GR Greece NL Netherlands BF Burkina Faso HU Hungary NO Norway BG Bulgaria IE Ireland NZ New Zealand BJ Benin IT Italy PL Poland BR Brazil P Japan PT Portugal BY Belarus KE Kenya RO Romania CA Canada KG Kyrgystan RU Russian Federation CF Central African Republic KP Democratic People's Republic SD Sudan CG Congo of Korea SE Sweden CH Switzerland KR Republic of Korea SI Slovenia CI Cote d'lvoire Z Kazakhstan SK Slovakia CM Cameroon LI Liechtenstein SN Senegal CN China LK Sπ Lanka TD Chad CS Czechoslovakia LU Luxembourg TG Togo CZ Czech Republic LV Latvia TJ Tajikistan DE Germany MC Monaco TT Trinidad and Tobago DK Denmark MD Republic of Moldova UA Ukraine ES Spain MG Madagascar US United States of Amenca FI Finland ML Mali UZ Uzbekistan FR France MN Mongolia VN Viet Nam GA Gabon An actuator having means for registration of the actuator position. The present invention relates to an actuator for servo control of adjustable mechanisms or parts, e.g. parts of hospital beds. Usually, such actuators comprise a reversible DC motor which, via a transmission, drives a spindle or a pivot arm or some other element, whereby the actuator can be mounted in mechanical connection with the adjustable part. The actuator also comprises a control box, which, however, may be in common for more actuators, and which is mounted at a convenient place in wire connection with the actuator. The control box con¬ tains, inter alia, a pole changer for controlling the direction of rotation of the motor, and normally it is connected with a hand operated control unit. In many instances it is desirable that the actuator can signal its position or, respectively, be dictated to a desired position, and there are known different models of solutions in this respect, e.g. based on a spindle revolution counter. The invention focuses on a known and very simple and effective model, viz. the use of a po¬ tentiometer as a position responsive element. When the potentiometer is mechanically coupled to the actuator, its electrical resistance will at any time be indicative of the actuator position. In principle, it is very easy to arrange such a system such that the desired functions are achievable, as it is sufficient to mount between the control box and the actuators the required wires for current supply and signal transfer, respectively. How¬ ever, in a preferred design of the system there are cer¬ tain complications in this respect, as it has been found highly advantageous to make use of only three poled connector plugs, whereby it has to be accepted that at least one of the wires should carry both operation cur¬ rent and counting or measuring signals, this being problematic in practice. The desire of a triple wire connection is dictated by the fact that it is then possible to make use of relatively cheap connector plugs, even for waterproof connections, viz. commercial stereo jacks which, in addition to being cheap, are also easy to mount in or by the enterprise in charge of the mounting work. In parti¬ cular, when the required connector cables are prepared by the producer of the actuators, this will not only facilitate the mounting work, but also incur an improved safety against wrong connections which, per se, can be decisive in connection with actuators for particularly critical functions. These must not act wrongly even by the first operation. For different reasons it is desirable to energize the actuator motor through two wires connecting it with a pole changer in the control box, from which it is then controllable to run one way or the other. Also, a short¬ ing of the motor can be effected in the control box, which is desirable by stillstand and during braking, and even an RPM control can be effected, preferably by pulse width modulation of the motor current. When thus two connection wires are used for the operation of the motor and there are only three wires at disposal, then it requires special precautions to make use of a potentiometer for a position determination of the actuator, not least when also a speed adjustment is involved. The third wire may be connected straightly to the potentiometer, but when the output therefrom is connected to the motor wire system a measurement of the resistance will be strongly dependent of the voltage variations across the motor. According to the invention the input wire to the potentiometer is used for applying a fixed reference value of current or voltage, while the opposite end of the potentiometer is connected to an artificial motor chassis defined by the midpoint of a pair of oppositely directed diodes mounted across the motor. It will then be possible, in the control box, to measure the changes of the setting of the potentiometer, but these measure¬ ments will be influenced by the actual motor current, and according to the invention this problem is overcome by currently effecting a measurement of the motor cur¬ rent, whereby a basis is created for relevant measure¬ ment compensations and thus for isolating the meaurement of the potentiometer setting. However, measuring technical complications may occur as a result of the motor current being pulse width modulated to a higher or lower degree or undergoes changes in this respect. Ideally, the current should be measured in unmodulated condition, and according to the invention this is achieved in effecting the relevant measurements as a series of measurements during very short intervals of time, during which an associated control circuit operates to suppress the current modula¬ tion entirely, i.e. enabling the measurements to be effected at full motor current. In practice, this may be done without affecting the operation, viz. with the use of measuring intervals so brief that the associated peaks of the full current will have no noticeable influ¬ ence on the operation of the motor. Consequently, with the desired use of " three wires it is possible to achieve a current position detection of the actuator by means of the very simple potentio¬ meter component. In the following the invention is described in more detail with reference to the drawing, in which: Fig. 1 is a schematic diagram of a system according to the invention, while Fig. 2 is a corresponding, slightly more detailed diagram. In Fig. 1 is indicated an actuator 2 having a re- versible DC motor M for moving of a non-illustrated actuator spindle or lever, which in its turn is con¬ nected with a potentiometer 4, such that in a manner known per se it is possible to detect the actuator posi¬ tion based on a measurement of the electrical resistance of the potentiometer 4. This is an easy task whenever two separate measuring wires can be drawn from a control box 6 to the potentiometer, in addition to the required two wires 8 and 10 to the motor M. As mentioned, how¬ ever, in connection with the invention it is desirable to make use of only three wires, viz. such that the actuator 2 can be connected with the control box 6 by means of a commercial stereo jack 7. With such a connection, two of the three wires will be dedicated motor wires 8 and 10, leaving but a single wire, 12, for the potentiometer 4. In the control box 6 the motor wires 8,10 are connected to a pole changer 14 which, by contact points 16 and 18, is connected to a voltage supply of 24 V, and which comprises switch-over switches 20 and 22 for changing the direction of the motor current and, respectively, for shorting the motor by any stop thereof. In connection with the contact point 18 is shown inserted a FET transistor 24, by means of which it is possible, through a control wire 26, to effect a pulse width modulation of the motor current for speed adjust¬ ment of the motor M. Symbolically, the potentiometer wire is shown con¬ nected to a unit 28 for detecting the actual position of the potentiometer. However, for the above discussed reasons this is not readily arranged, when there are only the said three connection wires at disposal. As shown in Fig. 2 , the invention provides for the potentiometer wire 12 to be supplied with a constant reference voltage of e.g. +5 V over a resistor Rl, this wire also being connected to an A/D converter 30, which, through a wire 32, is additionally connected to the chassis side of the FET transistor, this chassis connec¬ tion including a small resistor R3. The wire 32 includes an amplifier 34. The actuator or potentiometer position is measured as the voltage between Rl and the potentiometer R2, this voltage divider being connected to "chassis" through the relevant diode Dl or D2 and the associated relay switch 20 or 22 in the pole changer 14, all according to the rotation direction of the motor as well as the FET transistor 24 and the shunt resistance R3. Since the relay, the FET transistor and the shunt are run through by the motor current, the voltage of the voltage divider to the A/D converter will vary dependent of the load of the motor. By the addition of the resist¬ ance R3 it will be possible to measure the motor current and therewith also to produce a subtraction of the asso¬ ciated voltage contribution, such that this contribution may be compensated for by the final measurement. The amplifier 34 serves the purpose of making it possible to achieve a suitably high measuring voltage for a desir¬ able, very low resistance value of the shunt R3, prefer¬ ably less than 1/10 Ohm. For achieving a fixed measuring basis the measure¬ ments should be made with full current on the motor M or, respectively, with the motor fully stopped. On the other hand, the measuring time can be so brief that for the operation of the motor it will be without any real effect that the pulse width modulation is suppressed during the measuring intervals, hence the possibility of effecting the measurements also when the motor is driven in dampened mode. By way of example, a series of measurements may be made with intervals of the magnitude 1 msec, producing a measuring result as an average value based on the knowledge of the fixed impedances in the system and the measured motor current, i.e. based on a table read into the measuring unit. The diodes Dl and D2 at the motor M act limiting on the voltage interval in which the potentiometer ope¬ rates, but if the applied voltage, as mentioned, is 5 V, then the operative interval may be some 0.9-4 V, which will normally be sufficient. The voltage characteristic of the potentiometer R2 will be marked non-linear, and to the extent this weakens the desired measuring accuracy it will be possible to use, instead of the voltage feeding through Rl, a constant current generator for feeding the poten¬ tiometer through the wire 12, whereby the voltage on the A/D converter may be directly proportional with the positioning of the actuator.

Claims: C L A I S : 1. An actuator for servo adjustment of movable parts and of the type including a reversible DC motor, which drives an adjustment element and is wire connected with a control unit comprising a pole changer for con¬ trolling the motor, the actuator comprising a potentio¬ meter, which is mechanically connected with the adjust¬ ment element and is likewise wire connected with the control unit for a remote indication of the actuator position or, respectively, for a remotely controlled adjustment of the actuator to predetermined positions, characterized in that with the use of a triple wire connection between the actuator and the control box, two wires are used for the current feeding of the motor, while the third wire is connected to a first pole of the potentiometer, the opposite pole thereof being connected to "artificial chassis" at the midpoint of a pair of oppositely directed diodes connected across the motor, whereas the control unit is supplied with a fixed refe¬ rence voltage on the connection to the said first pole of the potentiometer, while one of the motor wires is connected to its associated supply pole through a vol¬ tage divider having a tap, from which a measuring voltage referring to the potentiometer position and the actual motor current is supplied to a measuring unit; an outer part of the voltage divider consists of a low " resistance connected to a unit for measuring the motor current only, and in a calculator unit the voltage- con¬ tribution originating from the motor current is sub¬ tracted from the said measuring voltage for isolation of the voltage contribution from the potentiometer. 2. An actuator according to claim 1, characterized by the c h a n g e that instead of a reference voltage supplied to said first pole of the potentiometer, a constant current generator is arranged to feed the po- tentio eter with a constant current. 3. An actuator according to claim 1 or 2 and of the type comprising a variator for pulse width modulated adjustment of the motor current, characterized by con¬ trol means for causing the measurements of the potentio¬ meter position to be effected in separated brief inter¬ vals of time, in which the pulse width modulation is suspended, i.e. briefly by full motor current.

Inventors: JOERGENSEN PREBEN, JOERGENSEN, PREBEN
Applicants: JOERGENSEN PREBEN, LINAK AS, JOERGENSEN, PREBEN, LINAK A/S
Application year: 1995
Main classification: G05D3
IPC codes: G01D5, G05D3