BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an angle sensor and an angle sensor system for generating an angle detection value having a correspondence with an angle to be detected.

2. Description of the Related Art

In recent years, angle sensors have been widely used in various applications, such as detection of the rotational position of a steering wheel or a power steering motor in an automobile. The angle sensors generate an angle detection value having a correspondence with an angle to be detected. Examples of the angle sensors include a magnetic angle sensor. An angle sensor system using a magnetic angle sensor is typically provided with a magnetic field generator for generating a magnetic field to be detected, the direction of which rotates in response to the rotation or linear movement of an object. Hereinafter, a magnetic field to be detected will be referred to as a detection-target magnetic field. The magnetic field generator is a magnet, for example. An example of angles to be detected by the magnetic angle sensor is an angle corresponding to the rotational position of the magnet. For example, the magnetic angle sensor detects a detection-target magnetic field and generates, as the angle detection value, a value representing the angle that the direction of the detection-target magnetic field at a reference position forms with respect to a reference direction in a reference plane.

A type of magnetic angle sensor is known that includes two detection circuits for generating two detection signals different in phase from each other, and generates an angle detection value by performing arithmetic processing using the two detection signals. In such a magnetic angle sensor, each of the two detection circuits may be subjected not only to a detection-target magnetic field but also to a noise magnetic field other than the detection-target magnetic field. Examples of the noise magnetic field include the earth's magnetic field and a leakage magnetic field from a motor. When subjected to such a noise magnetic field, each of the two detection circuits detects a composite magnetic field of the detection-target magnetic field and the noise magnetic field. When the detection-target magnetic field and the noise magnetic field are in different directions, some error occurs in the angle detection value. The error occurring in the angle detection value will hereinafter be referred to as angle error.

US 2018/0164127 A1 and US 2018/0274896 A1 describe angle sensors that are capable of reducing the angle error caused by the noise magnetic field.

The angle sensor described in US 2018/0164127 A1 includes a plurality of composite magnetic field information generation units and an angle computing unit. The plurality of composite magnetic field information generation units detect the composite magnetic field at a plurality of detection positions different from each other, and generate a plurality of pieces of composite magnetic field information. Each of the plurality of pieces of composite magnetic field information includes information on either the direction or both of the direction and strength of the composite magnetic field. The angle computing unit generates an angle detection value by a least squares method on the basis of the plurality of pieces of composite magnetic field information.

The angle sensor described in US 2018/0274896 A1 includes a plurality of detection units for detecting the composite magnetic field at a plurality of detection positions different from each other, and an angle computing unit for generating an angle detection value. Each of the plurality of detection units includes a first detection signal generation unit for generating a first detection signal representing the strength of a component in a first direction of the composite magnetic field, and a second detection signal generation unit for generating a second detection signal representing the strength of a component in a second direction of the composite magnetic field. The angle computing unit generates the angle detection value by a least squares method on the basis of a plurality of pairs of first and second detection signals generated at the plurality of detection units.

If the strengths and directions of the composite magnetic field at a plurality of detection positions can be represented by linear model functions using unknowns corresponding to the angle detection value and unknowns corresponding to the noise magnetic field as variables, the angle detection value can be generated by using a least squares method, like the technique described in US 2018/0164127 A1 and US 2018/0274896 A1.

For some angle sensor systems, however, the strengths and directions of the composite magnetic field at a plurality of detection positions may not be appropriate to be represented by linear model functions in the above-described manner. An example of such angle sensor systems is an angle sensor system that detects a detection-target magnetic field generated by a rotating magnet and generates an angle detection value having a correspondence with an angle corresponding to the rotational position of the magnet. The angle corresponding to the rotational position of the magnet is the angle to be detected by the angle sensor system. The angle detection value has a correspondence with the angle to be detected. If the strengths and directions of the composite magnetic field at a plurality of detection positions are not appropriate to be represented by linear model functions, the methods described in US 2018/0164127 A1 and US 2018/0274896 A1 could not generate a highly-accurate angle detection value with reduced angle error caused by the noise magnetic field.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an angle sensor and an angle sensor system for detecting a magnetic field to be detected whose direction changes with an angle to be detected, and generating an angle detection value having a correspondence with the angle to be detected, the angle sensor and the angle sensor system being capable of generating the angle detection value with high accuracy and with reduced angle error caused by a noise magnetic field.

An angle sensor of the present invention is configured to generate an angle detection value having a correspondence with an angle to be detected. The angle sensor includes a plurality of magnetic sensors and a processor. The plurality of magnetic sensors detect a composite magnetic field at a plurality of detection positions different from each other and generate a plurality of detection values that represent directions of the composite magnetic field, the composite magnetic field being a composite of a magnetic field to be detected and a noise magnetic field other than the magnetic field to be detected. The processor generates the angle detection value.

At the plurality of detection positions, directions of the magnetic field to be detected change with the angle to be detected. Manners in which either the directions or strengths, or both of the directions and strengths of the magnetic field to be detected change with the angle to be detected vary among the plurality of detection positions.

The processor assumes a group of unknowns, a group of provisional unknowns, and a group of estimated unknowns. The group of unknowns is a set of a first unknown, a second unknown, and a third unknown. The first unknown is a value corresponding to the angle detection value. The second unknown is a value corresponding to a direction of the noise magnetic field. The third unknown is a value corresponding to a strength of the noise magnetic field. The group of provisional unknowns is a set of provisional values of the first to third unknowns. The group of estimated unknowns is a set of estimated values of the first to third unknowns.

The processor executes a first process and a second process so that the number of executions of the first process is two or more, the first process being a process for determining the group of estimated unknowns on the basis of the plurality of detection values and the group of provisional unknowns, the second process being a process for assuming the group of estimated unknowns determined by the first process as a new group of provisional unknowns for the first process to be executed next time. The processor further executes a third process for assuming an estimated value of the first unknown in the group of estimated unknowns determined by the last executed first process as the angle detection value.

In the angle sensor of the present invention, the processor may further assume approximate functions, the approximate functions being first derivatives of model functions representing the plurality of detection values with the group of unknowns as variables. In such a case, the first process may determine the group of estimated unknowns on the basis of the plurality of detection values, the group of provisional unknowns, and the approximate functions.

The processor may further assume a plurality of temporary detection values, a plurality of discrepancy values, a group of change amounts, and a group of approximate discrepancy values. The plurality of temporary detection values are values in the group of provisional unknowns in the model functions. The plurality of discrepancy values are differences between corresponding ones of the plurality of detection values and the plurality of temporary detection values. The group of change amounts is a set of change amounts of the estimated values of the first to third unknowns in the group of estimated unknowns with respect to the provisional values of the first to third unknowns in the group of provisional unknowns. The group of approximate discrepancy values is a set of a plurality of approximate discrepancy values corresponding to the plurality of discrepancy values, and is determined from the approximate functions and the group of change amounts. The first process may determine the group of change amounts so that the group of approximate discrepancy values becomes a set of approximate values to the plurality of discrepancy values, and may further determine the group of estimated unknowns on the basis of the group of change amounts thus determined. The first process may determine the group of change amounts so that a sum of squares of differences between corresponding ones of the plurality of discrepancy values and the plurality of approximate discrepancy values is minimized. The third process may be executed if the change amount of the estimated value of the first unknown in the group of change amounts is smaller than or equal to a predetermined value.

The model functions may be nonlinear functions including a plurality of periodic functions with the first unknown as a variable.

In the angle sensor of the present invention, each of the plurality of magnetic sensors may include a first signal generator for generating a first signal, a second signal generator for generating a second signal, and a computing circuit for generating a corresponding one of the plurality of detection values on the basis of the first and second signals. The first signal has a correspondence with the cosine of an angle that the direction of the composite magnetic field forms with respect to a reference direction. The second signal has a correspondence with the sine of the angle that the direction of the composite magnetic field forms with respect to the reference direction. In such a case, the corresponding one of the plurality of detection values may represent the angle that the direction of the composite magnetic field forms with respect to the reference direction. Each of the first and second signal generators may include at least one magnetic detection element.

An angle sensor system of the present invention includes the angle sensor of the present invention and a magnetic field generator for generating the magnetic field to be detected. In the angle sensor system of the present invention, a relative position of the magnetic field generator with respect to the plurality of detection positions may be variable. The angle to be detected may have a correspondence with the relative position of the magnetic field generator.

According to the angle sensor and the angle sensor system of the present invention, the first to third processes are executed so that the number of executions of the first process is two or more. This makes it possible to generate a highly-accurate angle detection value with reduced angle error caused by a noise magnetic field.

Other and further objects, features and advantages of the present invention will appear more fully from the following description.

