- The mechanical design

- The actuator system

- The sensor system

And the control system described in section 4 consists at least of :

- The control hardware

- The control software

For each of these parts we will describe the considerations for a robot hand in general and then present the exemplary implementation in the Karlsruhe Dexterous Hand II.

3 Mechanical system

The mechanical system describes how the hand looks like and what kind of components it is made of. It defines the mechanical design, e.g. the number of fingers and the kind of materials used. Additionally actuators, e.g. electric motors, and sensors, e.g. position encoders, are settled.

3.1 Mechanical design

The mechanical design determines the fundamental 'dexterousness' of the hand, i.e. what kind of objects can be grasped and what kind of manipulations can be performed with a grasped object. Three basic aspects have to be settled when designing a robot hand:

• The number of fingers

• The number of joints per finger

• The size and placement of the fingers

To be able to grasp and manipulate an object safely within the workspace of the hand at least 3 fingers are required. To achieve the full 6 degrees of freedom (3 translatory and 3 rotatory DOF) for the manipulation of a grasped object at least 3 independent joints are needed for each finger. This approach was taken for the first Karlsruhe Dexterous Hand [1,2]. However, to be able to regrasp an object without having to release it and then pick it up again, at least 4 fingers are necessary.

To determine the size and the placement of the fingers two different approaches can be taken:

• Anthropomorphic

• Non-anthropomorphic

It then depends on the objects to manipulate and on the type of manipulations desired which one is chosen. An anthropomorphic placement allows to easily transfer e.g. grasp strategies from a human

hand to the robot hand. But the different sizes of each finger and their asymmetric placement makes the construction more expensive and the control system more complicated, because each finger has to be treated separately.

When a non-anthropomorphic approach is taken most often identical fingers are arranged symmetrically. This reduces the costs for the construction and simplifies the control system because there is only one single 'finger module' to be constructed and controlled.

3.2 Actuator system

The actuation of the finger joints also has a great influence in the dexterousness of the hand, because it determines the potential forces, precision and speed of the joint movements. Two different aspects of the mechanical movement have to be considered:

• Movement generation

• Movement forwarding

Several different approaches for these aspects are described in the literature. E.g. the movement can be generated by hydraulic or pneumatic cylinders [3] or, as in most cases, by electric motors.

As the movement generators (motors) are in most cases to big to be integrated in the corresponding finger joint directly, the movement must be forwarded from the generator (most times located in the last link of the robot arm) to the finger joint. Again different methods can be used, like tendons [4,5,6], drive belts [1,2] or flexible shafts. The use of such more or less indirect actuation of the finger joint reduces the robustness and the precision of the system and it complicates the control system because different joints of one finger are often mechanically coupled and must be decoupled in software by the control system. Due to these drawbacks an integration of miniaturized movement generators directly into the finger joints is desirable.