On Manipulator Calibration

   Now I'm trying to make NAO catch sth like a flower  for 2010 RoboCup singapore.That's a challenge because of his poor

kinematics accuracy..

This is some note for study on robot calibration.

    Robot Calibration is one of the key technologies of off-line programming.Many reason can cause errors like joint angular sensors accuracy,elasticity compensate.When you send an angle to a motor,the angle your motor can reach  is not always equal to what you sent.So we can focus on the joint angular error compansation.

   Joint angular sensors are widely used in the industry,from highly effective robots in product lines and heavy construction machines in building sites to small knobs on home appliances. They measure joint angles and convert them into electrical signals, from which the control system can derive and calculate the relative positions of the connected links. They greatly vary with reference to working principle, complexity,accuracy, cost, reliability, and lifetime. Two of the most popular joint angular sensors are magnetic rotary encoders[Y. K 97] and optical rotary encoders [T. K 04].Magnetic encoders normally incorporate two parts, one on each link of the joint. One part is a multipole magnet, and the other part is a Hall-effect [Hall effect]sensor, and both of these parts must closely be coupled together (within a distance of few millimeters).Magnetic encoders usually provide low-resolution angular results (8–12 bits); they are of low cost and general purpose, are contactless and very reliable, have long lifetimes,and have been used in different industrial applications (automobiles,factory automation, white goods, control panels, etc.) for a great number of years. The disadvantages of these sensors are
that they require special magnet coupling alignment and directcurrent magnetic shielding, and these requirements can result in expensive mechanical coupling and packaging design.

    Optical encoders also incorporate two parts, and they are also installed one on each link. One part has a disk with a precision-cut window edge, and the other one has a light source and a photodetector to sense the relative angle of the disk.Optical encoders can usually provide much higher resolution angular results than is possible for magnetic encoders (up to 30 bits). They can also be very accurate but are also very expensive and have been used in all cases involving a requirement for high-accuracy angular measurements (industrial robot, semiconductor factory, scientific experiment, aerospace industry, etc.). Optical encoders also have a contactless sensing mechanism, but due to their high accuracy and, hence, tight matching, they require expensive high-precision mechanical coupling, packaging, and installation, normally incorporating a conventional contacting mechanism, and they can be more sensitive to environmental influence (shock, vibration, stress, etc.).

    Calibration on robot include the following steps:
  1 Establish an accurate modle of the kinematic
  2 Measure the specific robot with known precision measuring devices
  3 Modify the robot kinematic model.

Mesuring method is an important element whitch restrict its development.Since 1980s,many method has been introduced into this area(camera system,hand /eye calibration,tempratrue model,friction model,etc.).
  

 

[Y. K 97] Y. Kikuchi, F. Nakamura, H. Wakiwaka, and H. Yamada, “Index phase output characteristics of magnetic rotary

encoder using a magnetoresistive element,” IEEE Trans. Magn., vol. 33, no. 5, pp. 3370–3372,Sep. 1997.
[T. K 04] T. Kojima, Y. Kikuchi, S. Seki, and H. Wakiwaka, “Study on high accuracy optical encoder with 30 bits,” in Proc.

8th Workshop IEEE Adv.Motion Control, Mar. 2004, pp. 493–498.
[Hall Effect] [http://en.wikipedia.org/wiki/Hall_effect_sensor][http://zh.wikipedia.org/zh-cn/%E9%9C%8D

%E5%B0%94%E6%95%88%E5%BA%94%E4%BC%A0%E6%84%9F%E5%99%A8]