Driving mechanism having position encoder for two-dimensional positioning

A driving mechanism comprises a fixed housing, a movable housing on which an object to be driven is mounted and a driving motor which is operative to drive the movable housing to move linearly as well as to rotate relative to the fixed housing. An inductance-type encoder determines both linear and rotary displacement of the movable housing relative to the fixed housing, whereby to provide closed-loop control of the position of the object in both linear and rotary directions.

FIELD OF THE INVENTION

The invention relates to a driving mechanism for positioning objects, and in particular to a driving mechanism with a position encoder to allow controlled positioning by the driving mechanism in two dimensions during operation.

BACKGROUND AND PRIOR ART

Certain driving mechanisms for driving objects such as camera lenses need to be capable of accurately positioning the object in at least two dimensions, such as in the rotary and linear directions, during operation. Usually, the linear and angular positions of rotary-linear driving mechanism can be determined by optical means. For example, U.S. Pat. No. 6,765,195 entitled “Method and Apparatus for Two-Dimensional Absolute Optical Encoding” describes an optical encoder for determining the position of an object in two dimensions. The encoder comprises a scale having a pattern being predetermined to indicate an absolute location on the scale, means for illuminating the scale, means for forming an image of the pattern, detector means for outputting signals and analyzing means for determining the absolute location of the object in two directions. From the scale and pattern, the position of the object is known, and the driving mechanism may thus control the movement of the object in two dimensions.

However, optical encoders are generally bulky and expensive to use. In particular, much space is required for installing the encoder lens assembly as well as an optical scale to detect the position of the object. For a compact apparatus where installation space is at a premium, an optical encoder is not desirable.

Another type of encoder uses an inductance-type sensor which does not need optical means to detect the position of an object. An example of such an encoder is disclosed in U.S. Pat. No. 5,757,182 entitled “Variable-Reluctance-Type Angular Rotation Sensor with Sinusoidally Distributed Winding”. In this set-up, sinusoidally distributed windings are formed on uniformly-distributed slots such that the angular position of the driven object can be determined using variable-reluctance principles. However, such inductance sensors have conventionally not been able to detect the position of an object in two directions, such as both angular displacement as well as linear displacement. Accordingly, the driving mechanisms incorporating inductance-type position encoders are operative to provide controlled driving of an object only in one direction, specifically the rotary direction. It would be desirable to provide an inductance-type encoder that is more compact than an optical encoder and is able to detect the position of an object in two dimensions. Such a driving mechanism may provide controlled driving of an object in two dimensions, especially in applications which require the driving mechanism to move an object linearly as well as to rotate the object.

SUMMARY OF THE INVENTION

It is thus an object of the invention to seek to avoid the disadvantages of an optical encoder by providing an inductance-type encoder which can detect rotary as well as linear displacement of an object by a driving mechanism which drives the object in two directions.

According to a first aspect of the invention, there is provided a driving mechanism comprising: a fixed housing; a movable housing on which an object to be driven is mounted; a driving motor which is operative to drive the movable housing to move linearly as well as to rotate relative to the fixed housing; and an inductance-type encoder operative to determine both linear and rotary displacement of the movable housing relative to the fixed housing, whereby to provide closed-loop control of the position of the object in both linear and rotary directions.

According to a second aspect of the invention, there is provided a method of driving a movable housing on which an object to be driven is mounted, comprising the steps of: driving the movable housing with a driving motor to move linearly as well as to rotate relative to a fixed housing; determining a linear displacement of the movable housing relative to the fixed housing with an inductance-type encoder; and determining a rotary displacement of the movable housing relative to the fixed housing with the inductance-type encoder, whereby to provide closed-loop control of the position of the object in both linear and rotary directions.

It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.

FIG. 1is an isometric view of one end of the driving mechanism10according to the preferred embodiment of the invention which incorporates an inductance-type encoder. The driving mechanism10generally comprises a fixed housing12, a movable housing14for mounting an object to be driven, such as an optical lens system, linear-rotary bearings16and a centrally-located central cylinder18. The driving mechanism10also comprises resolver coils20comprised in the induction-type encoder.

FIG. 2is an isometric view of another end of the driving mechanism10opposite to that shown inFIG. 1. It further illustrates driving motor stator coils22of a driving motor, such as a servo motor mounted on the fixed housing12, for driving the movable housing14to move relative to the central cylinder18. The driving motor is operative to drive the movable housing14to move linearly as well as to rotate relative to the fixed housing12.

FIG. 3is a cross-sectional view of a coil winding structure associated with a slotted resolver23comprised in the inductance-type encoder. The said inductance-type encoder is operative to determine both linear and rotary displacement of the movable housing relative to the fixed housing, whereby to provide closed-loop control of the position of the object in both linear and rotary directions.

The slotted resolver23includes an encoder stator24which has a plurality of poles26extending inwardly from the encoder stator24. A laminated core30, preferably made of iron, is located centrally of the encoder stator24and is rotatable with respect to the poles26. It has an inner hole32at its center for connecting wires or other peripherals to the object to be driven. An air gap34is formed between the laminated core30and poles26, within which the laminated core30is rotatable. The slotted resolver23may be attached to either of the fixed housing12and movable housing14, and the laminated core may be attached to the other of the fixed housing12and movable housing14.

The resolver coils20are wound around the poles26. At least one resolver coil (such as two resolver coils20and two poles26) is meant for transmitting excitation signals, at least one resolver coil (such as two resolver coils20and two poles26) is meant for picking up Sine signals which are generated depending on a position of the laminated core30, and at least one resolver coil (such as the two remaining resolver coils20and two poles26) is meant for picking up Cosine signals which are generated depending on the position of the laminated core30. The total number of resolver coils20is preferably equal to 3n, where n is an integer greater than or equal to 1. The resolver coils20for transmitting excitation signals receive current from excitation cables36, the resolver coils20for picking up Sine signals transmit current through Sine cables38and the resolver coils20for picking up Cosine signals transmit current through Cosine cables40.

FIG. 4is a cross-sectional view of the driving mechanism10wherein the movable housing14is at a first position relative to the central cylinder18. The servo motor19includes the driving motor stator coils22which are attached to the fixed housing12, and driving motor permanent magnets42which are attached to the movable housing14. Conversely, the driving motor coils may be attached to the movable housing14and the driving motor permanent magnets may be attached to the fixed housing12. The driving motor permanent magnets42are operative to electromagnetically interact with the driving motor stator coils22to drive the movable housing14to move linearly with respect to the fixed housing12, as well as to rotate relative to the fixed housing12.

The movable housing14is slidably supported on the central cylinder18, and the linear-rotary bearings16are located between the movable housing14and the central cylinder18. As the movable housing14moves, rollers44in the linear-rotary bearings16allow the movable housing14together with the linear-rotary bearings16to slide relative to the central cylinder18, as well as relative to the fixed housing12. Accordingly, an object mounted to the movable housing14, such as an optical lens, may be driven to move linearly with respect to the fixed housing12and central cylinder18, and may also rotate relative thereto, whereby to control linear and rotary motions of the object. This is as opposed to the aforesaid prior art driving mechanism including an inductance-type encoder, which only offers controlled rotary motion (but not linear motion) to the movable part of the driving mechanism. Furthermore, the central cylinder18may comprise a hollow center for locating wires and other peripherals for connection to the object.

As described above, the slotted resolver23comprises the resolver coils20and laminated core30. As the movable housing14moves, it will also drive the laminated core30to move by a corresponding extent. Thus, the electrical signals picked up by the Sine coils38and Cosine coils40may be used to determine both the linear and rotary positions of the movable housing14, thereby enabling closed-loop control of the position of the movable housing14.

FIG. 5is a cross-sectional view of the movable housing14of the driving mechanism10at a second position. The movable housing14and linear-rotary bearings16have moved to the second position from the first position by sliding along the central cylinder18. The driving motor permanent magnets42and the laminated core30have moved relative to the driving motor stator coils22and resolver coils20respectively. The movement of the laminated core30allows the extent of linear and rotary motion of the movable housing14to be calculated.

An exemplary method of calculating the extent of movement of the movable housing14from the readings obtained from the Sine coils38and Cosine coils40is set out below.

The Excitation Signal sent through the excitation cables36may be expressed as:E(t)=Aesin ωot, where E(t) is a voltage carried by the Aeis the amplitude of the excitation signal, ωois the frequency of the excitation signal, and t is the time domain.

The Sine and Cosine signals picked up by the respective Sine and Cosine cables38,40may be expressed as:
S1(t)=a(z)sin θ sin ωot
S2(t)=a(z)cos θ sin ωotwhere a(z) is the z-axis position information and θ is the angular position.

The SIN output windings and the COS output windings of the resolver coils20have a phase difference comprising an electrical angle of 90° therebetween. The rotary angle θ of the laminated core30may thus be determined with the above formulae.

To determine the vertical, z position, of the movable housing14, the picked-up signals S1(t) and S2(t) are passed through an all-pass filter with a 90-degree phase shift, which may be implemented using an operational amplifier. Resultant signals S3(t) and S4(t) are obtained:
S1(t)=a(z)sin θ sin ωot
S3(t)=a(z)sin θ cos ωot
S2(t)=a(z)cos θ sin ωot
S4(t)=a(z)cos θ cos ωot

Analogue multiplications and summations are then performed as follows:
yz(t)=S12(t)+S22(t)+S32(t)+S42(t)
yz(t)=2a2(z)

The z-axis position information can finally be decoded with a Digital Signal Processor (DSP) using the formula:

The Excitation, Sine and Cosine signals can be sent to a Resolver-to-Digital (R/D) Converter for calculating the rotational angle information. The above output signal a(z) contains only the z-axis position information using the slotted resolver23, which is independent of the rotational angle information calculated using the same slotted resolver23.

Accordingly, with the above rotary angle θ and z-axis position a(z) obtained, two-dimensional positional information may be obtained for the driving mechanism10, to allow the driving mechanism10to control movement of the object in both the linear and rotary directions using an inductance-type encoder as described above.

The invention described herein is susceptible to variations, modifications and/or addition other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.