Steering angle detection device

A steering angle detection device comprising:

• first and second rotation angle sensors, each of which includes a sensor element configured to detect a rotation state of a motor connected to a steering system through a power transmitting device, and is configured to calculate a rotation angle of the motor based on a detection signal of the sensor element, to calculate a rotation number indicating a number of rotations of the motor based on the detection signal of the sensor element, and to communicate a sensor signal including information related to the rotation angle and the rotation number, the first and second rotation angle sensors being configured to continue calculation of at least the rotation number during a period in which a start switch of a vehicle is in an off-state;
• and
• first and second control units, each of which includes a microcomputer programmed to execute programmed processes of calculating a steering angle based on midpoint information related to a neutral position of a steering member in addition to the rotation number and the rotation angle acquired from a corresponding one of the first and second sensors, wherein:
• the first rotation angle sensor and the first control unit form a first system including a first battery, and the second rotation angle sensor and the second control unit form a second system including a second battery;
• either one of the first rotation angle sensor and the first control unit has a first memory which holds the midpoint information during the off-state of the start switch, and either one of the second rotation angle sensor and the second control unit has a second memory which holds the midpoint information during the off-state of the start switch;
• the microcomputer of the first control unit executes a process of acquiring the midpoint information and the rotation number from the second system, when the first system has a power supply failure abnormality indicating that power supply form the first battery to the first rotation angle sensor is interrupted and calculation of the rotation number is disabled during the start switch is in the off-state, and the second system has no power supply failure abnormality.