Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D3/00—Control of position or direction
  • G05D3/10—Control of position or direction without using feedback
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
  • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
  • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
  • G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
  • G01D5/16—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying resistance
  • G01D5/165—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying resistance by relative movement of a point of contact or actuation and a resistive track

Definitions

• 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.
• 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.
• 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.
• a po ⁇ tentiometer as a position responsive element.
• the potentiometer When the potentiometer is mechanically coupled to the actuator, its electrical resistance will at any time be indicative of the actuator position.
• the actuator motor 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.
• connection wires 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.
• 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.
• 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.
• Fig. 1 is a schematic diagram of a system according to the invention.
• Fig. 2 is a corresponding, slightly more detailed diagram.
• Fig. 1 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.
• motor wires 8 and 10 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.
• 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.
• a FET transistor 24 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.
• the potentiometer wire is shown con ⁇ nected to a unit 28 for detecting the actual position of the potentiometer.
• this is not readily arranged, when there are only the said three connection wires at disposal.
• 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.
• the voltage of the voltage divider to the A/D converter will vary dependent of the load of the motor.
• 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.
• the measure ⁇ ments should be made with full current on the motor M or, respectively, with the motor fully stopped.
• 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.
• 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.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Automation & Control Theory (AREA)
• Control Of Electric Motors In General (AREA)

Applications Claiming Priority (3)

Publications (1)

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• 1994
  • 1994-08-17 DK DK95094A patent/DK95094A/da not_active Application Discontinuation
• 1995
  • 1995-08-17 WO PCT/DK1995/000336 patent/WO1996005542A1/en not_active Ceased
  • 1995-08-17 AU AU31619/95A patent/AU3161995A/en not_active Abandoned
  • 1995-08-17 EP EP95927663A patent/EP0776495A1/en not_active Withdrawn

Non-Patent Citations (1)

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