Measurement sensing devices
||Sterki, Armin; Sommer, Gerd;
A measurement sensing device has a probe element that is displaced by an object to be measured and is acted on by an electric motor when in its displaced position to control the pressure with which it bears on the object. A control circuit for varying the standstill torque of the motor comprises means for compensating for variations in any biasing force applied to the probe element by its mounting, means for compensating for weight effects acting in the direction of measurement, and means for adjusting the predetermined value of the bearing pressure.
BACKGROUND OF THE INVENTION
The invention relates to a sensing device for length or position measurements, which has a means of presetting the bearing pressure of the sensing element in respect of amount and direction.
A sensing device of this kind is shown for example in the German Utility Model No. 1 920 551, in which both the value of the bearing pressure, that is to say its magnitude, and the direction of the bearing pressure can be adjusted according to a scale by moving a handle.
In addition, a tooth flank sensing device is disclosed in U.S. Pat. No. 3,943,633, in which the pressure and direction of pressure of the sensor can be adjusted, in accordance with a scale, on a ring which is constructed in the form of a sleeve around the cylindrical main body of the sensing device.
Furthermore, U.S. Pat. No. 3,936,946 discloses a sensing device for pitch measurement on gears in which for the purpose of reversing the direction of application of the sensor, control is effected either by a rotatable rotor acting on an eccentric pin or by means of a magnet, whereby the direction of sensing is reversed. In this sensing device the magnitude of the value of the sensing pressure is determined by means of adjustable spring deflection stops.
These devices do not suffice, however, when it is required that the sensing contact pressure should be adjustable very accurately and easily, and above all in a readily accessible manner, in respect of amount and direction and irrespective of the position of the device in space.
SUMMARY OF THE INVENTION
According to the present invention there is provided a sensing device in which the measuring pressure of the sensing element is produced by means of the torque of an electric motor which is arrested in a determined position of the sensing element. This has the consequence that the bearing pressure can be predetermined in respect of magnitude and direction and also weight compensation in the event of a change of position can be achieved, by electronic control of the motor operation, so that the device can be arranged for use in high precision measurements.
As a further advantage a sensing device according to the invention can be arranged to produce only a substantially negligible and constant amount of heat, so that a constant temperature is maintained in the device.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated diagrammatically and by way of non-limitative example in the accompanying drawings, in which:
FIG. 1 is a perspective representation of a sensing device according to the invention, together with a block diagram of a controlling circuit in simplified form,
FIG. 2 shows diagrammatically the force and the corresponding displacement in a view from above, when acting on a left-hand tooth flank or other surface,
FIG. 3 is a similar view to FIG. 2 in the case of a right-hand flank or other surface, and
FIG. 4 is a diagrammatic front view of the device with the corresponding forces for an assumed arbitrary position in space.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The sensing device according to the invention, as shown in perspective in FIG. 1, comprises a base plate 1 on which is fastened an angle plate 2 carrying an electric motor 3 in the form of a permanent magnet direct current motor. Two pairs of leaf springs 4 and 5 are fastened at one end to the base plate 1 and at the other end to a carrier 6, these two pairs of springs 4 and 5 forming a parallelogram mounting for the measurement movement of the carrier 6. On the carrier 6 is mounted a sensing probe or feeler 7 having a ball-shaped sensor 8. In addition, the carrier 6 is provided with a recess 9 engaged by a ball 10 fastened eccentrically on output shaft 11 of the motor 3. To each side of the base plate 1 a bar, 12 and 13 respectively, is fastened on the carrier 6, adjusting screws 14 and 15 respectively being inserted in these bars. The adjusting screw 14 carries an armature 16 of a measurement transducer 17, while the adjusting screw 15 serves as a stop or path limiter for the carrier 6 in that direction. The corresponding adjusting screw (not visible in the drawing) in the bar 13 serves the same purpose for the opposite direction of movement of the probe.
A measurement transducer electronic unit 18, which has its output connected to a displacement signal indicator instrument 19, is connected to the measurement transducer 17. In addition, this measurement transducer electronic unit is connected to a compensation circuit 20 for the parallelogram mounting. The compensation circuit 20 is not shown in detail but comprises an adjustable voltage divider and a triggerable signal inverter. The compensation circuit 20 is connected by way of a summator 21 to an amplifier 23, which is combined with a driver acting on the motor 3, the summator 21 also connecting to the amplifier 23 a further amplifier 22 connected by a summator 24 to a potentiometer 25 for setting a weight compensation correction and to a potentiometer 26 for setting a prestressing force value. The compensation circuit 20 and the potentiometer 26 are connected together through a control line 27. The control signal is used to invert the compensating signal of compensating circuit 20. The electronic units 17 to 26 are units which are known per se and are commercially available, or they can be produced in a simple manner with the aid of known electronic components.
On the deflection of the sensor 8 in the use of the apparatus measurement transducer 17 is operated by way of the sensing feeler 7, the carrier 6, the bar 12, the adjusting screw 14, and the measuring armature 16. At the same time the rotor of the motor 3 is also turned through the ball 10 and the shaft 11, the motor 3 however being arrested at a determined position of the sensor.
Thus, on the deflection of the sensor 8 by an object which is to be measured, for example one flank of a gear tooth 28, the measurement transducer 17 supplies a displacement signal which is proportional to the magnitude of the deflection and which is converted in the measurement transducer electronic unit 18 into a direct current displacement signal. The polarity of the direct current signal corresponds to the direction of deflection, and its voltage corresponds to the amount or magnitude of the deflection, as indicated by the displacement signal indicator instrument 19.
Through the increasing magnitude of the deflection the restoring force F of the leaf springs 4 and 5 of the parallelogram mounting also increases (FIGS. 2 and 3). The direct current displacement signal is however now transmitted through the compensation circuit 20 for the parallelogram mounting to the summator 21 in accordance with the amount or magnitude and the sign for the direction of deflection.+-.S (FIGS. 2 and 3). Consequently the amplifier 23 acting at the same time as driver varies the motor current of the motor 3 and consequently the torque of the motor in dependence on the deflection. Since this torque acts through the motor shaft 11 and the ball 10 to exert a force on the carrier 6, the increasing restoring forces of the leaf springs 4 and 5 are compensated and the pressure by which the sensor 8 bears on the object 28 to be measured is kept approximately constant. This pressure F is thus dependent of the magnitude of the deflection or of the magnitude of the error or defect of the object to be measured, that is to say in the given example the left-hand or right-hand flank of the tooth 28. Depending on the type of measurement selected, determined by the setting of potentiometer 26, either the left-hand or the right-hand tooth flank, a corresponding control signal is fed into the parallelogram mounting compensation circuit 20 to change the polarity of the compensating signal depending on the selected measurement. The adjustment of the compensation is dependent on the stiffness of the leaf springs 4 and 5, and is thus an adjustment requiring to be made only once.
The torque of the electric motor 3 is thus adjusted electronically in respect of magnitude and direction, and the variation of contact pressure of the sensor 8, caused by the spring system comprising the springs 4 and 5 in particular of the parallelogram mounting, is electronically compensated in dependence on and to the extent of the deflection to produce the contact pressure that is in dependent of the magnitude of the deflection.
If the movements of the sensing device are out of the horizontal, the weight G of the movable parts produces, with respect to the measurement surface, a force component F.sub.G (FIG. 4) which has to be added or deducted. This force F.sub.G can be compensated by an oppositely directed force F.sub.G ' through the weight compensation potentiometer 25 by presetting a fixed reference value dependent on the orientation of the sensing device. This can be adjusted very simply on the potentiometer 25 for a determined inclined position or even for a vertical position by setting the displacement indication on the instrument 19 to zero by adjustment of the potentiometer 25 while the sensor 8 is freely suspended, so that the sensing device is then in equilibrium without any contact pressure on the probe. With the set value of the prestressing force adjusted to zero, only the set value of the weight compensation then acts through the summators 24 and 21.
In use the set value of the prestressing force is added in the summator 24, with the correct sign, to the set value of the weight compensation and is amplified in the amplifier 22. In the summator 21 the signal is added, with the correct sign, to the value of the parallelogram mounting compensation and the amplifier 23 with the driver is operated and in turn transmits the motor current for producing the torque or prestressing force on the ball 10 and thus on the sensor 8. The bearing pressure of the sensor 8 is thus kept practically constant irrespective of the position of the device in space.
In computer controlled automated measuring systems the potentiometers 25 and 26 can be replaced by digital/analogue converters in a manner known per se. The prestressing and weight compensation values are then preset by the computer.