A robot system includes a robot including a robot arm, and a first hand and a second hand which are connected to the robot arm and which are provided to independently rotate about an axis on the robot arm; and a controller configured to control an operation of the robot. When the robot arm and the first hand are operated so that the first hand reaches a predetermined target position, teaching values for the first hand in the target position is generated. When the first hand and the second hand are rotated based on the teaching values for the first hand, a relative error in rotation amount around the axis between the first hand and the second hand is acquired and stored in a memory. Teaching values for the second hand is generated from the teaching values for the first hand based on the acquired relative error.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A robot system, comprising: a robot including a robot arm, and a first hand and a second hand which are connected to the robot arm and which are provided to independently rotate about an axis on the robot arm; and a controller configured to control an operation of the robot, wherein the controller includes: a memory unit; a first generation unit configured to, when the robot arm and the first hand are operated so that the first hand reaches a predetermined target position, generate teaching values for the first hand in the target position; an error acquiring unit configured to, when the first hand and the second hand are rotated based on the teaching values for the first hand, acquire a relative error in rotation amount around the axis between the first hand and the second hand and store the acquired relative error in the memory unit; and a second generation unit configured to generate teaching values for the second hand from the teaching values for the first hand based on the acquired relative error. 2. The robot system of claim 1, wherein the first hand includes a sensor, the second hand includes an indicator portion which is detectable by the sensor of the first hand, and the controller is configured to allow the error acquiring unit to acquire the relative error, by rotating the second hand and the first hand based on a rotation amount value around the axis, which is included in the teaching values for the first hand, and then rotating the first hand with respect to the second hand until the indicator portion is detected by the sensor. 3. The robot system of claim 2, wherein the controller is configured to allow the error acquiring unit to acquire the relative error, by operating the robot arm so as to take a first posture differing from a second posture of the robot arm, the second posture being a posture taken by the robot arm when the first hand reaches the target position, and then rotating the second hand and the first hand based on the teaching values for the first hand. 4. The robot system of claim 3, wherein the robot arm includes a pivot joint, and a first arm and a second arm which are serially connected to each other through the pivot joint, and the first posture is a posture in which the first arm and the second arm are folded by the pivot joint so as to overlap with each other. 5. The robot system of claim 4, wherein the first hand and the second hand are provided at a free end side of the second arm which is a tip side of the robot arm, and the rotation amount around the axis is defined by rotation angles of the first hand and the second hand with respect to an extended direction of the second arm. 6. The robot system of claim 4, wherein the controller is configured to allow the error acquiring unit to acquire the relative error, by rotating the first hand and the second hand clockwise or counterclockwise in accordance with a rotation direction of the first hand when the first hand has reached the target position. 7. The robot system of claim 1, wherein the target position includes a plurality of target positions, the first generation unit is configured to individually generate teaching values for the first hand with respect to the respective target positions, the error acquiring unit is configured to individually acquire relative errors corresponding to the respective teaching values for the first hand and to store the relative errors in the memory unit, and the second generation unit is configured to individually generate teaching values for the second hand based on the respective relative errors. 8. The robot system of claim 2, further comprising: a detected jig provided to be mounted to the second hand and provided with the indicator portion. 9. The robot system of claim 2, further comprising: a detecting jig provided to be mounted to the first hand and provided with the sensor. 10. The robot system of claim 8, further comprising: a detecting jig provided to be mounted to the first hand and provided with the sensor. 11. The robot system of claim 1, wherein the controller further includes a state monitoring unit configured to allow the error acquiring unit to acquire the relative error at a predetermined timing. 12. The robot system of claim 2, wherein the controller further includes a state monitoring unit configured to allow the error acquiring unit to acquire the relative error at a predetermined timing. 13. The robot system of claim 3, wherein the controller further includes a state monitoring unit configured to allow the error acquiring unit to acquire the relative error at a predetermined timing. 14. The robot system of claim 4, wherein the controller further includes a state monitoring unit configured to allow the error acquiring unit to acquire the relative error at a predetermined timing. 15. The robot system of claim 5, wherein the controller further includes a state monitoring unit configured to allow the error acquiring unit to acquire the relative error at a predetermined timing. 16. The robot system of claim 1, wherein the controller further includes an operation control unit configured to control operations of the robot arm, the first hand and the second hand, based on prior teaching information previously stored in the memory unit, the teaching values for the first hand and the teaching values for the second hand. 17. A robot teaching method for teaching a robot including a robot arm, and a first hand and a second hand which are connected to the robot arm and which are provided to independently rotate about an axis on the robot arm, the method comprising: generating teaching values for the first hand in a predetermined target position by operating the robot arm and the first hand so that the first hand reaches the predetermined target position, rotating, based on the teaching values for the first hand, the first hand and the second hand, acquiring a relative error in rotation amount around the axis between the first hand and the second hand, and storing the acquired relative error, and generating teaching values for the second hand from the teaching values for the first hand based on the acquired relative error. 18. The method of claim 17, wherein the second hand includes an indicator portion which is detectable by a sensor provided in the first hand, and said acquiring the relative error includes rotating the first hand with respect to the second hand until the indicator portion is detected by the sensor. 19. The method of claim 17, wherein before said rotating, the robot arm is operated so as to take a first posture differing from a second posture of the robot arm, the second posture being a posture taken by the robot arm when the first hand has reached the target position and then the second hand and the first hand are rotated based on the teaching values for the first hand. 20. A control device for controlling an operation of a robot including a robot arm, and a first hand and a second hand which are connected to the robot arm and which are provided to independently rotate about an axis on the robot arm, the control device comprising: a storage means; and a control means, wherein the control means includes: a first generation unit configured to, when the robot arm and the first hand are operated so that the first hand reaches a predetermined target position, generate teaching values for the first hand in the target position; an error acquiring unit configured to, when the first hand and the second hand are rotated based on the teaching values for the first hand, acquire a relative error in rotation amount around the axis between the first hand and the second hand and store the acquired relative error in the storage means; and a second generation unit configured to generate teaching values for the second hand from the teaching values for the first hand based on the acquired relative error.