Khepera IV Gripper Module
The Khepera IV gripper arm is a small and compact arm with two degrees of freedom. By placing the robotic gripper arm on the Khepera IV you will be able to manipulate objects in the robot's direct environment.
The Gripper is made of two fingers which are moved by the same DC motor. These two fingers are parallel in every position to improve gripping ability.
The Gripper was designed to carry objects with different size and weight. There is no minimum size as the gripper can apply a continuous torque even if the object is very thin. The maximum size and weight are respectively 50mm and 50g.
The arm is moved by a DC motor coupled with a position sensor (potentiometer) which provides absolute positioning. The regulation is made by the microcontroller in the base board. The maximum positions (limits of movement) of the arm are limited by the software, the two limits positions (High and Ground) are saved in the EEPROM of the microcontroller as soon as a search limits function is made. These two limits can be modified by the user if a new search limits is done. This is useful if an extension is plugged above the gripper, to avoid the Gripper hitting the extension during each movement.
The gripper is controlled in position and torque by the microcontroller in the head board. The user can set every position between the closed and the max opening position. The user can also read the position of the gripper to measure the size of the object gripped. There are two possibilities: The simplest solution is to linearize the position of the Gripper; the second possibility is to take care of the entire mechanical characteristic to get a better conversion.
Once the fingers are blocked by an object, the microcontroller switches to the torque control mode and limits the motor current to a given value. Configuring a small torque can avoid damaging a fragile object. Conversely, it’s necessary to configure a high torque if you want to carry a heavy object.
Battery: The Khepera IV Gripper has an integrated battery to avoid draining the Khepera's battery, keeping the autonomy of the robot at the same level as usual. The Li-Po Battery of the gripper has a 1000mAh capacity under 7.4V to ensure autonomy up to 4 hours. The battery status is managed by the microcontroller in the base board (same as arm management). The user can view the voltage and the remaining capacity.
IR sensors and optical barrier: The Khepera IV Gripper is equipped with two IR sensors (one on each finger) to create an optical barrier capable to detect if an object is between the two fingers. Each IR sensor is, in fact, composed of an emitter and a receiver. Along the optical barrier, the user can read the value of the reflected light on the object, which gives the opportunity to detect the colour of the grabbed object. This measure must be done only when the gripper is closed to ensure that the distance (object <=> sensor) is always the same. Besides this, you can use these sensors value to measure the distance between the object and the fingers. The second option is only viable if the colour is known, and useful if you want to place the object in the centre of the gripper before grabbing it.
The Khepera IV's embedded Linux system is used for interfacing with the gripper. A library including all the available functions is provided with the libkorebot version 1.14 or greater.