Robotic machinery provided with an automatic tool exchanging apparatus, for example, is detachably fastened with an automatically exchangeable tool at an end of the manipulating hand, and an acceleration sensor is mounted on the same end of the hand for detecting a motion of the hand. With the advancement of intelligent apparatuses in other fields, acceleration sensors of the same kind are also utilized for detecting the motion of subjects being controlled.
The acceleration sensors are provided with vibration units that vibrate integrally with the motion of the subjects being controlled or detected. As methods of detecting vibration of the vibration units, the acceleration sensors of electromagnetic induction type, electrostatic capacitance type, or of a type utilizing a piezoelectric element, etc. have been generally known to date.
The electromagnetic induction type comprises a magnetic body connected to a vibration unit, and a coil positioned in the magnetic flux that varies by a displacement of the magnetic body, so as to measure acceleration through a voltage output generated across the coil. The electrostatic capacitance type comprises a pair of electrodes arranged to form a capacitive space between a vibration unit side and an absolute stationary side, so as to detect acceleration through a variation of capacitance due to a change of the space gap between the electrodes, when the vibration unit side electrode displaces with respect to the absolute stationary side. The sensor type which uses a piezoelectric element, includes the piezoelectric element mounted on a part of the vibration unit side, which is subject to distortion caused by vibration of a load, and detects acceleration through variation of an output of the piezoelectric element due to the distortion of the vibration unit.
Any acceleration sensor of the above types is considered to have a function suitable for a system that detects acceleration in a moving unit of single-axis. With respect to the acceleration sensors of the electrostatic capacitance type and the piezoelectric element type, a reduction in overall sensor size has been achieved as a result of advances in semiconductor and thin film manufacturing technologies. This allows for production of the electrodes and even the piezoelectric elements in thin-form, including a casing of the acceleration sensor as well as a mass point (mass unit) on the vibration unit provided in it. Also, in a field of the acceleration sensors of electromotive type which utilize the electromagnetic induction method, advances in technology have enabled thin-film permanent magnets of reduced size to be produced by an epitaxial growth process in combination with a flat-shape coil, as disclosed in Japanese Patent Laid-Open Publication No. H05-142246.
On the other hand, the acceleration sensors are required to be capable of detecting a displacement in three dimensions: not only one axis, but along three axes, X, Y and Z of the orthogonal spatial coordinates, as an indispensable condition for controlling robotic hands and the like. Detection of a three-dimensional displacement of this nature can be attained by combining three acceleration sensors of single-axis, one for each direction of the X-, Y- and Z-axes, for instance. Such combination had been the mainstream technology in the past. Especially for those of the electromagnetic induction type, it is structurally quite impracticable to design an acceleration sensor of dual-axis or triple-axis types, since each of the acceleration sensors contains coils. Hence, acceleration sensors of the foregoing single-axis structure are utilized by combining them to sense displacement in three dimensions.
If at least three sets of the ordinary acceleration sensor of single-axis type are combined, however, the overall size of the structure simply becomes a size that is nearly three times as large as the size of a single acceleration sensor of single-axis type. Therefore, if the subject being detected is small in size and light in weight, an increase in both weight and size (i.e. volume) due to the addition of the three acceleration sensors, can result in a considerable and undesirable effect. Therefore, simply combining three individual acceleration sensors may not be adaptable to apparatus in the fields where reduction in size is of primary importance.
On the contrary, there still remains a possibility of adapting acceleration sensors of the electrostatic capacitance type and the piezoelectric element type to the three-axis application, since they can be reduced in size by taking advantage of the semiconductor and thin film manufacturing technology, as described above. In the case of the electrostatic capacitance type, for example, it has been known that there is a structure adaptable for the three axes, X, Y and Z. In that known structure, each of the electrodes on the vibration unit side and the stationary side are split into four regions within a plane that includes the electrodes, and a variation of capacitance corresponding to each of the axes is then detected by using a combination of individual displacements of the electrodes in these split regions. Similarly, the piezoelectric element type can also be expected to be produced at a reduced size to a certain extent, due to advances in semiconductor and thin film manufacturing technology.
An acceleration sensor of the electrostatic capacitance type detects a displacement of the subject being controlled by utilizing a variation of capacitance due to a change in distance of the space gap formed between a pair of the electrodes: one on the vibration unit side and, and one on the absolute stationary side. It is therefore desirable to narrow the gap between the electrodes in order to detect the change in capacitance. Accordingly, the maximum magnitude of displacement of the electrode on the vibration unit side is restricted within this space gap. For the above reason, a range of the measurable acceleration (hereinafter referred to as "dynamic range") for the acceleration sensor of electrostatic capacitance type is relatively small. Since the maximum amplitude for the vibration unit also needs to be designed small, a range of the detectable acceleration is limited.
In the case of the piezoelectric element type, it is unavoidable that an adverse effect to the detecting characteristic is experienced due to a pyroelectric effect of the piezoelectric element caused by changes of the external temperature. In order to stabilize the detecting characteristic, it is therefore necessary to make a temperature compensation, which complicates the apparatus and its control, due to the inclusion of additional controls for the temperature compensation. It also has a relatively small dynamic range, as in the case of the electrostatic capacitance type, since a movable range of the vibration unit is limited to a magnitude which will not break the piezoelectric element fixed to a cantilever.
As described, the acceleration sensors utilizing the electrostatic capacitance type and the piezoelectric element type include shortcomings which restrict their general versatility due to a limitation of the adaptable fields, since their dynamic ranges are small, although a reduction in the size of these sensors is possible.
On the other hand, electromagnetic induction type-acceleration sensors can be adapted for wide range of acceleration-sensing applications by providing a sufficient magnitude of displacement for the magnetic body to move about with respect to the coil. This increases the dynamic range so that it is substantially greater than that of the electrostatic capacitance type-acceleration sensors and the piezoelectric element type-acceleration sensors. Since this does not restrict the magnitude of vibrating amplification, these sensors are adaptable for positional detection of a minute displacement of an element, e.g. in a compact precision machine, as well as a large robotic hand, for example, thereby providing for a broad versatility.
The majority of electromagnetic induction type-acceleration sensors are adaptable only for the single-axis use. Effective means for adapting these sensors for triple-axis (three dimensional) use have not yet been proposed or established. One of the reasons for this limitation is that the disposition of at least three coils for the three axes prevents the necessary size reduction. While providing multiple coils for the three axes increases the size and bulk of the acceleration sensors, it is an indispensable condition to dispose the coils in a manner that magnetic flux through them varies with motion of the magnetic body.
Although electromagnetic induction-type acceleration sensors having wide dynamic ranges can be used effectively in a wide variety of the fields and can be used to detect almost all subjects, they are not adaptable for the triple-axis use as means for making three-dimensional measurements using convention technology.
An object of the present invention is to provide an acceleration sensor which is capable of sensing in three-dimensions and which has a wide dynamic range that is capable of detecting a three-dimensional movement and a magnitude of displacement of a subject being controlled with high accuracy and high sensitivity, and which is also of reduced size.