Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P3/00—Measuring linear or angular speed; Measuring differences of linear or angular speeds
  • G01P3/42—Devices characterised by the use of electric or magnetic means
  • G01P3/44—Devices characterised by the use of electric or magnetic means for measuring angular speed
  • G01P3/46—Devices characterised by the use of electric or magnetic means for measuring angular speed by measuring amplitude of generated current or voltage
  • G01P3/465—Devices characterised by the use of electric or magnetic means for measuring angular speed by measuring amplitude of generated current or voltage by using dynamo-electro tachometers or electric generator

Definitions

• the present invention relates to a device for generating a tachometric signal comprising an electrical generator with rotary collector and brushes, providing a measurement voltage proportional to the speed of rotation of the collector.
• Tachogenerators are widely used in electric servo drives to measure the instantaneous speed of rotation of an electric motor or a transmission axis.
• these generators have a structure similar to that of a DC motor in which the stator generates a stationary field created by a permanent magnet or by a winding supplied with constant current, and a rotor carries a winding. connected to a switch.
• the switch comprises a rotary collector with lamellae connected to different parts of the rotor winding, and fixed brushes integral with the stator, between which the tachometric signal appears.
• the brush-collector assembly makes an electrical contact between surfaces which slide over one another which, in the ideal case, would be permanent and continuous contact.
• the invention aims to remedy the aforementioned drawbacks and to provide a device that can be produced in a simple and economical form, making it possible to obtain a tachometric signal free of parasites due to contact interruptions at the generator collector.
• the device, generator comprises a circuit for maintaining the value of the measurement voltage during accidental interruptions thereof, resulting from a fault in electrical contact between brushes and collector, this circuit being arranged so that its bandwidth for the tachometric signal extends, outside the periods of said interruptions, up to an upper limit frequency equal to or greater than 1 kHz.
• the holding circuit comprises a capacitor connected in series with the internal resistance of the generator and the device comprises a measurement circuit with very high input impedance to which the voltage appearing across said capacitor is applied.
• a resistor of very high value can also be connected in parallel to said capacitor.
• FIG. 1 represents the electrical diagram of a known tachometric generator device
• FIGS. 2 A and 2 B illustrate the operation of the device of FIG. 1,
• FIG. 3 is an electrical diagram of a first embodiment of the device according to the invention.
• FIGS. 4 A and 4 B illustrate the operation of the device of FIG. 3,
• FIG. 5 is an electrical diagram of another embodiment of the device according to the invention.
• Figure 6 is an electrical diagram of an example of application of this device.
• FIG. 1 shows a tachometric generator G producing a voltage U proportional to the speed of rotation of the shaft of the generator and supplying a current i. to a resistive external circuit formed essentially by resistors R 1 and R_ constituting an external load for the generator.
• a voltage signal U. is collected at the terminals of the resistor R- and applied to an input of an amplifier A., the output voltage of which is designated by U.
• the voltage U when the current i. undergoes a brief interruption by default of contact at the generator collector, the voltage U also falls at zero during a corresponding time lapse as illustrated in FIG. 2 A at times t., t 2 and t.,.
• FIG. 3 shows a first embodiment of the present device which makes it possible to remedy this defect.
• the tachometric generator G is connected, according to FIG. 3, no longer to a resistive load but to a capacitor C,.
• the internal resistance of the generator is represented by a resistance R. which is essentially formed by the resistance of the winding and the contact resistance between the brushes and the collector. This internal resistance is normally of low value and corresponds substantially to the resistance of the winding.
• the capacitance of the capacitor C is preferably chosen so that the electrical time constant R ⁇ -C, either relatively low, or for example of a few microseconds.
• the amplifier used is an operational amplifier A_ having a very high input impedance.
• the circuit connected to the tachometer generator constitutes a circuit for maintaining the value of the voltage U. during the periods of interruption due to a momentary poor contact at the level of the collector. Its effect is to maintain the voltage U across the terminals of the capacitor C. as well as the output voltage U at values equal to those which they had immediately before an interruption period, as illustrated in FIGS. 4 A and 4 B.
• FIG. 4 A shows for example an interruption occurring at an instant t for a duration t.
• the internal resistance R. suddenly takes an infinite value and no more current i can flow in the charging circuit of the capacitor C ..
• the output voltage U of the operational amplifier will thus correspond throughout the duration of the interruption to the value that it had at the moment t and will not present any discontinuity.
• the bandwidth for the tachometric signal is practically not affected.
• the dimensioning of the capacitance C being such that the upper limit frequency of this bandwidth is at least equal to 1 kHz and in practice generally takes on much higher values.
• capacitors with a capacity of the order of 10 nF are preferably used. It will be noted that under these conditions, the current to be supplied by the generator is obviously extremely low, especially since the input impedance of the operational amplifier is very high.
• FIG. 5 shows another embodiment of the present device, according to which a resistor R g of very high value is connected in parallel to the holding capacitor designated by C-.
• Resistance R- preferably has a value of the order of Megohm.
• FIG. 6 illustrates a case of application of the present device according to which this device is part of a speed regulation loop of an electric motor.
• a motor M for example a DC motor, is supplied via a power amplifier A 5 .
• the axis of the engine is integral with the axis of a generator G so that the signal U supplied by the latter is proportional to the speed of rotation of the engine.
• This signal is applied to the input of an operational amplifier A. which also acts as a control amplifier.
• a capacitor C is connected in a similar manner to the capacitor C of FIG. 3 so as to eliminate the effects of possible interference caused by interruptions of electrical contact at the generator switch.
• a voltage U having a set value is applied via a resistor R 7 to a second input of the amplifier A ..
• a possible difference between the set value U and the value of the voltage U. of the signal The tachometer is detected and amplified by the amplifier A 4 which generates a correction signal applied in turn to the power amplifier A 5 .
• a feedback circuit Z. is dimensioned in the usual manner according to the particular conditions of the application.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Current Or Voltage (AREA)
• Indication And Recording Devices For Special Purposes And Tariff Metering Devices (AREA)
• Control Of Electric Motors In General (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=4183269

Family Applications (1)

Country Status (4)

Citations (2)

Family Cites Families (1)

• 1988
  • 1988-01-27 CH CH27488A patent/CH675774A5/fr not_active IP Right Cessation
• 1989
  • 1989-01-24 JP JP1501240A patent/JPH02503037A/ja active Pending
  • 1989-01-24 EP EP19890901452 patent/EP0352315A1/fr not_active Withdrawn
  • 1989-01-24 WO PCT/CH1989/000013 patent/WO1989007273A1/fr not_active Application Discontinuation

Patent Citations (2)

Non-Patent Citations (1)

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