Projects

WMRA Robotic Gripper

Abstract

This paper presents a low-cost and self-contained robotic gripper that can be integrated into various robotic systems. This gripper can be used to accomplish a wide array of object manipulation tasks, made easy by its design and features a wide array of sensors that can be used to help accomplish these tasks. Furthermore the gripper has been made to be compatible with many robotic systems.

Motivation

Many existing grippers used in robotics today have varying capabilities which only allow them to perform certain tasks or grab certain objects for manipulation. These grippers may lack additional features (i.e. cameras or sensors) that allow them to better perform tasks. Additionally, many of these robotic grippers will only interface with the system they were designed for. To get these features that we needed for our projects, along with the flexibility to migrate the gripper from system to system, a new gripper had to be made.

Design Features

The gripper features a small 24v DC motor with an encoder that can be used to read the position and speed of the gripper. It also has a slip clutch to prevent over extending of the gripper. The unique cupped design of the gripper, emulates a human hand, allowing for more accurate grasping. Furthermore, the gripper contains a camera, and a distance sensor for finding the objects' position. Finally, the gripper has a current sensor that can be used to shut off the gripper if the force on the gripper exceeds a certain threshold for a given object.

Packaging

In order for this gripper to be able to interface with other robotic systems without modification to the design of the gripper it was necessary to bundle all of the required components into a single control box. This control box consists of the microcontroller, motor controller, voltage regulators, and current sensing equipment required for the system to function. Additionally, the microcontroller allows for interfacing with the gripper via Ethernet, Wi-Fi, Bluetooth, or USB. The Gripper assembly is powered via an external Power Supply (battery or DC adapter). In order to interface with the various added components in this gripper, a full software library was written in C++ that allows users to control the gripper, grab sensor information, and view the camera, without any additional programming. This library is cross-platform and can be used on mobile devices.

Operation

The gripper control overview showing the flow of control and sensory information when using a TCP/IP based interface within the gripper system, including the transmission with the users.

The gripper system has several different interfaces through which it can be controlled. The gripper can either be controlled internally, in which the control programs are stored and run on the gripper's controller automatically, or remotely, in which the user is controlling the gripper from another device. The gripper system supports remote operation via Ethernet, Wi-Fi, and Bluetooth, with expansion to other communication formats available via the gripper controller's onboard USB ports. Included with the gripper system is a custom Gripper Operation Program that grants the user easy operation of the gripper from a computer. The Gripper Operation Program allows the user to control the openness / closeness of the gripper via a simple slider bar, as well as the speed at which the gripper opens / closes. Additionally, the Operation Program allows the user to see all of the sensor information coming from the gripper, including the distance sensor, pressure sensors, and camera. The Operation Program also allows the user to set pressure cutoff limits. These cutoff limits force the gripper to stop closing / opening when a pressure threshold has been reached. This feature prevents the gripper from exerting too much / too little force on an object. The Operation Program is available for Windows, Mac OS X, and Linux.

Results

While the all-in-one control system for the WMRA Gripper was still being developed, the Gripper itself has been used in the Wheelchair Mounted Robotic Arm (WMRA) as the system's primary gripper. Using this Gripper, the WMRA has successfully manipulated objects of different classifications including bottles, cups, markers, etc. With the addition of the current feedback and shutoff, the Gripper should gain the ability to grasp objects that can be crushed by the gripper. Finally, the high resolution and high frame rate of the camera would better allow for the usage of computer vision algorithms to perform object detection and classification of the target objects, and more accurate grasping.

Conclusion

This gripper is an effective, fully featured, and cost effective device that can be used in a wide array of robotic manipulation tasks. Additionally, the gripper can be added to existing robotic systems with little (if any) modification to the existing system or the gripper.