Desarrollo cerebral en la adolescencia

A single DoF setup with a stylus like end effector, grasped between the thumb and index finger and operated from wrist and forearm movements has the most similarities with the intended layout for the 5 DoF haptic interface. This layout deviates from the two setups as described above as the first one is based on whole-hand grasping and the second one on a squeezing motion between thumb and index finger. This means that a new device has to be designed and built. The same design will be used for the master and slave device. This design can be based on a rotational or translational movement of the end effector.

In case of a translation, the user feels the inertia of the end effector and the friction of the linear (ball) guides directly. In a rotational setup, the friction force has its origin near the rotation axis and scales downwards with an increasing distance between rotation axis and end effector tip. The apparent moving mass at the tip of the end effector is less, as the contribution of the mass of the different components to the total inertia decreases quadratically with a decreasing distance towards the axis of rotation. Furthermore, it

is to be expected that the friction of the translational setup will increase on long term use, as the unshielded ball races are sensitive to contamination. A work around is to apply air bearings, but this makes the setup more expensive. Therefore, it is preferable to build a device with a rotational layout. Furthermore, three of the four main DoFs during vitreo-retinal eye surgery are rotations as well. A motion range of ±45◦ _equals the ϕ and ψ rotation of an instrument during surgery.

Inertia and friction are just two of the design specifications for haptic devices. The following is a listing of the most important specifications as found in [26, 36, 60, 85]. The specifications are valid in combination with impedance control.

• low inertia/moving mass

A high inertia makes it more difficult for the operator to perform a specific task and can result in fatigue. System performance decreases as function of inertia, due to lower eigenfrequencies. This makes it, for example, more difficult to represent the high frequency content of the force feedback. As result of this, the force feedback will be less realistic.

• high stiffness

The device stiffness must be higher than the simulated contact stiffness. Furthermore, it results in higher eigenfrequencies and a higher control bandwidth. Most ideal, but most likely not feasible, is a bandwidth that equals the 1 kHz perceptual bandwidth for vibrotactile stimuli.

• low friction

Friction results in a lower force resolution as it directly interferes with the feedback of the forces as measured at the slave. A lower friction allows a more precise manipulation of the instrument. The friction level should be ≤1% of the continuous force.

• backdrivability

A device without force measurement at the end effector must be backdriveable. This means that the drive train must employ relatively low gear ratios.

• zero backlash

Backlash can result in control instabilities. Furthermore, it has a negative effect on performance as the slave does not instantaneously follow when the end effector is moved in the opposite direction. Direct drive or a preloaded transmission avoids backlash.

• weight compensation

A statically balanced system avoids operator fatigue and reduces the required motor power. Balancing can be done with counterweights, but the additional

mass results in a lower eigenfrequency. An alternative is to use spring based compensation.

• uniform workspace

With an optimal system design, the actuator and sensor capabilities will be consistent throughout the workspace. This minimizes the requirements for the actuators and encoders/sensors. Singularities must be avoided as they create directions in the workspace in which the end effector cannot be moved by the operator. This reduces the manipulability of the system and the dexterity of the user.

• degrees of freedom

Six DoFs is ideal for a general purpose hand controller, but a large number of DoFs has a negative effect on the moving mass, stiffness and friction.

• output force/torque

An operator can generate large static forces and torques. These must be matched by the haptic device to simulate stiff and hard contacts. The output force and torque influences the peak acceleration.

Design specifications such as a high stiffness, low friction and zero backlash are together with a statically determined system design [75] of importance for a reproducible and predictable system behavior.

The requirements for an admittance controlled device are less strict as friction and inertia can be compensated via the force/torque measurement and the control loop. Nevertheless, a good mechanical design reduces the necessary control effort. This means that the design of the 1 DoF setup can be based on the requirements as stated above, as long as it is equipped with a torque or force sensor. The combination of a position measurement system and a force/torque measurement enables the implementation of four channel haptic control algorithms.

The maximum force that can be applied with the index finger is 50 N [13]. Experiments showed that in the case of grasping, discomfort is encountered after 10 minutes at a level of 25% (=12.5 N) of the maximum force. These forces are much higher than the maximum and continuous forces of the commercially available haptic devices in table 2.3. Therefore, the forces are reduced to respectively 30 N and 10 N.

To take full advantage of human haptic capabilities, a resolution of 0.01 N is recommended. Hand motions must be detected with a resolution of at least 50 µm, as this is the accuracy a surgeon can position his hand with [74]. Passive elements, like dampers, are left out as they interfere with the force feedback. With a modular design of the master and slave, it is possible to add them later if it turns out to be necessary for stability reasons.