Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-02-09
2001-10-16
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S660000, C324S661000, C324S658000, C324S683000
Reexamination Certificate
active
06304091
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to sensing devices, and more particularly to absolute position sensing devices with high sensing resolution.
Position sensors are used in a variety of devices to allow electrical systems to sense motion or position of moving objects and components. For example, one use of position sensors is in computer interface devices, which allow a user to provide input to a computer system to manipulate computer-generated objects and environments and to instruct the computer to perform tasks. Computer interface devices use position sensors to detect a user's input to the computer system. For interface devices such as joysticks and mice, position sensors determine the location of a user manipulandum within a workspace in particular degrees of freedom. The position of the manipulandum is used by the computer to control a cursor or otherwise manipulate a computer environment. Force feedback interface devices also use input from position sensors in the determination of an output force, which is then output on the manipulandum by motors or other actuators in the interface device. Position sensors are also used in a variety of other applications, such as sensing the position of a rotary knob. Traditional position sensors used in computer interface devices and other devices include analog potentiometers and digital encoders.
Sensors have a sensing resolution, which determines the amount of motion or displacement detectable by the sensor; for example, a larger sensing resolution allows the sensor to detect smaller increments of movement. One problem with existing position sensors is that resolution of the sensors is limited by cost considerations. For example, digital encoders can provide precise position readings but have a resolution limited by the spacing of encoder divisions (e.g., slots or marks) whose movement is detected by the sensor. The higher the resolution, the more closely spaced the encoder divisions must be, requiring high precision and greater cost to produce the sensor. This can present a problem for interface devices that require very high precision, such as force feedback interface devices, yet are sold as low-cost, high-volume consumer products.
Another type of position sensor is a capacitive sensor. These types of sensors typically detect changes in position by measuring capacitance between two relative-moving pieces or components, i.e. the charge of the capacitor is measured. These types of conventional capacitive sensors rely on charge measurement electronics which are cost prohibitive for high volume consumer products and typically suffer from poor signal to noise performance, and thus may not be suitable for many applications. Other types of capacitive sensors measure a frequency of an output signal from the sensor, and determine a position of moving components based on the sensed frequency. However, these types of sensors also suffer from high-cost components that read the output signal frequency.
SUMMARY OF THE INVENTION
The present invention provides a high-resolution absolute position sensor. A variable capacitor provides changes in phase of an AC signal, allowing low cost and high resolution position sensing.
More particularly, a capacitive position sensor and method of the present invention includes a stator and a vane and a dielectric provided between the vane and stator. The vane moves parallel to the stator such that the vane overlaps at least a portion of the stator, creating a capacitance that varies as the vane moves. An input driver signal having an input frequency is input to a circuit including the capacitor, and the circuit outputs a signal having a phase shift with reference to the driver signal and based on the relative positions of the vane and stator. The phase-shifted signal is used to derive the absolute position of the vane with respect to the stator. The phase shift is based on a degree of overlap of said stator by said vane, thereby indicating a position of said vane relative to said stator. Preferably, the stator and vane are approximately planar in shape, and the stator can be provided on a printed circuit board for low cost production. Linear or rotary motion of the vane can be sensed.
A circuit is included in (or coupled to) the sensor that receives the phase shifted signal and outputs a pulse width modulated signal which is linearly proportional to the position of the vane with respect to the stator. For example, the circuit may include an XOR gate. A filter can also be provided to receive the pulse width modulated signal and produce a filtered output signal. The filtered signal can be measured with a microprocessor to resolve a position of the moving member.
In a preferred embodiment, two stators are provided, where the second stator is positioned adjacent to the first stator approximately in the same plane, and where the vane overlaps both of the stators during its movement. A circuit is connected to each stator to provide two phase-shifted signals; the second signal increases in amplitude while the first signal decreases in amplitude, and the signals are subtracted to provide a difference signal free of common mode effects.
The sensor can be used, for example, to detect the position of a user manipulandum in a computer input device that is coupled to the vane and moved by a user, such as a mouse or joystick. A mechanical linkage can be positioned between the user manipulandum and the vane; the linkage can include multiple members rotatably coupled to each other.
In another embodiment, four stators are provided in a stator array and a circular vane is moved parallel to the stators to provide four output signals which are subtracted to provide a two-degree-of-freedom position. In yet another embodiment, a stator is comprised of first, second and third interdigitated stator sections positioned adjacent to each other in a range of movement of the vane. A greater sensing resolution can be provided in the same movement range of the vane. In another embodiment, multiple variable capacitors are provided in a multi-pole circuit, which increases the dynamic range of the sensor.
A method for sensing the position of a moving member provides an input signal, shifts a phase of the frequency of the input signal by inputting the input signal to an RC circuit, and provides the phase-shifted signal to a phase detector circuit that outputs a pulse width modulated signal. The modulated signal is linearly proportional to the position a plate of a capacitor in the RC circuit with respect to a different plate of the capacitor.
The apparatus of the present invention provides a high resolution absolute position sensor. Since phase shift is detected rather than charge or frequency, the capacitive sensor includes components that are easy to manufacture and thus are low cost. The sensor produces a signal having low interference or noise and little error. The high resolution of the sensor allows the sensor to be cost-effectively used in interface devices that require high sensing precision, such as force feedback devices.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following specification of the invention and a study of the several figures of the drawing.
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Baxter, L.,Capacitive Sensors, Design and App
Hasser Christopher J.
Schena Bruce M.
Shahoian Erik J.
Deb Anjan K.
Immersion Corporation
Metjahic Safet
Riegel James R.
Tucker Guy V.
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