In the second part of our review we will talk about robotic arm kits that feature more slightly more advanced construction and control characteristics, making them equally well suited for research purposes as well as for more complex hobby applications. These kits retain affordability while providing versatility and robustness, allowing for easy customization to integrate with your applications and precise control leading to improved performance. In part 1 of our review we also briefly presented key aspects of robotic arm physical constructions, this time we will start by addressing, in a succinct manner, principles of numerical representation and control of robotic arms.
Forward kinematics (FK) and inverse kinematics (IK) are two methods for positioning of a robotic arm and its end effector, i.e. gripper.
- In FK final position and orientation of a gripper can be established based on known joint angles and length of links or sections of a robotic arm. This is a pretty straightforward method, relying on trigonometry and basic mathematic calculations. In other words we can estabish where the gripper is when the robotic arm is in a certain position determined by its joint angles;
- Conversely, IK is used to establish positions of joints in a robotic arm when the position of the gripper is known. Since link lengths between joints are known, the method is used to calculate joint angles required to reach that desired position of the gripper. This is a much more complex calculation method which can absorb an important amount of resources, especially in microcontrollers where processing power is at a premium. The method is especially difficult since zero, multiple or even infinite configurations of a robotic arm may exist for a single certain position of the end effector, involving multiple exponential equations. In this case a positioning algorithm must take into account many factors such as previous positioning data, physical characteristics of the arm and so forth. Thankfully several tools exist for developing fairly capable IK algorithms suited for microcontrollers employed by the various platforms presented in our roundup.
Some kind of motion planning must also be performed in order to optimize motions of a robot. A simple method would be to consider a desired path or trajectory of a robot and decompose it into simple geometrical forms such as straight lines, circular arcs or angles. Velocities and joint forces must also be considered by taking into account weights of each element of a robot, actuator characteristics and path travelled by gripper to determine response time and optimum behavior. Robotic arms need to be as rigid as possible while maintaining lightweight in order to reduce or cancel parasitic movements, characteristics exhibited by the platforms presented below.
Lynxmotion AL5 Series Robotic Arm Kits
The AL5 series robotic arms from Lynxmotion come in different specs in terms of hardware setup, as well as control electronics and software. The AL5 series arms have a rotating base, a shoulder joint, allowing up-down movement in a single plane, elbow and wrist joints adding up to 4 DOF. There is also an optional rotating wrist joint adding up to 5 DOF and standard parallel or vacuum grippers for a total of 6 DOF. The arms construction is aluminium with different payloads and reach distances, depending on version and they are actuated by Hitec servos. The AL5A robotic arm has a horizontal reach of 14,6 centimeters and a payload of over 110 grams, the AL5B robotic arm can lift up to 142 grams within a reach of 19 centimeters while the top of the range AL5D robotic arm has a reach distance of over 25 centimeters and can lift objects weighting almost 370 grams.
There is a variety of control electronics which can be used to operate the robot. The robotic arm can employ the SSC-32 servo controller module, based on the ATMEGA168-20PU microcontroller, which can control up to 32 servomotors at a time. The robot can be controlled from a computer running the free Lynx dual arm controller software for Windows through the RS232 interface, or the more advanced RIOS software that also features and inverse kinematics engine for more accurate positioning of the arm. There are also stand-alone operation versions employing either the Bot Board II, holding either BasicATOM or BasicStamp2 microcontrollers, with 24-pin or 28-pin form factors, or the BotBoarduino controller based on the Arduino Duemilanove.
Each of the two mainboards has an input port for connecting Sony PS2 game controllers, an onboard user interface comprised of 3 pushbuttons and color LEDs, analog inputs, an I/O bus and a speaker. The RIOS software can also be used to write scripts to be uploaded to the respective microcontrollers.
Depending on versions and software bundled with the kit, prices range from as low as sub-150 US Dollar mark for arm hardware kits with no servos or electronics, to around 300 US Dollars for complete kits with electronics and servo actuators, and up to 340 US Dollars for the AL5A kit or 390 US Dollars for the top of the range AL5D robotic arm when control and scripting software is also bundled with the kit. Options like wrist rotation for grippers are in the range of 30 to 50 US Dollars while a vacuum gripper with wrist rotation feature will not exceed 70 US Dollars, making these a selection of very affordable robotic arm kits.
Trossen Robotics PhantomX Pincher Robot Arm
The PhantomX Pincher is a 5 DOF robotic arm which can be employed either in stand-alone fashion or as an addition to the TurtleBot ROS platform. The complete arm has a weight of 550 grams and has a horizontal reach of 28 centimeters. Vertically fully extended it has a reaching distance of 21 centimeters and comes equipped with a parallel gripper. The robot has a sturdy ABS plastic structure and is also equipped with bearings for its rotating base and various mounting brackets for accessories, sensors, etc. The gripper has a 500 gram holding strength while the wrist has a lifting strength of 250 grams. The robotic arm itself can hold up to 100 grams, 40 grams when fully extended.
The robot is controlled by an ArbotiX controller board holding an ATMEGA644p AVR microcontroller, which is designed for controlling the advanced AX-12A Dynamixel actuators. The Arduino compatible board supports XBee wireless radio modules, has 32 I/O pins from which 8 can be configured as analog inputs. There is no bundled or optional software for this robotic arm kit, however Vanadium Labs have a software suite for it. The PyPose for ArbotiX software, developed under Python, captures predefined positions of the robot from which sequences that can be played back are created. the software includes several other features such as exporting memorized parameters into files manageable through the Biloloid library, a servo terminal and the NUKE, short for Nearly Universal Kinematics Engine, a very powerful program that simplifies to a great extent implementing IK into programming of robots employing the ArbotiX controller.
The robot requires a 12 VDC 2 A power supply, included with the kit. Also included is a FTDI programming cable and interestingly enough, a bottle of Turbo-Lock thread locker to prevent parasitic movements and vibrations of the arm. This may be a good addition for any robot employing screws and nuts for fixtures. The kit can be bought at a price of 400 US Dollars.
Trossen Robotics PhantomX Reactor Robot Arm
The PhantomX Reactor can be regraded as a heavy duty research-grade version of the Pincher. It features an extremely sturdy ABS plastic construction with dual-servo design for shoulder and elbow joints, featuring 7 AX-12A Dynamixel actuators in 5 DOF for and 8 of them for wrist rotating versions. The robotic arm has a weight of about 1,4 kilograms and has a horizontal reach of 38 centimeters in standard for or 43 centimeters with wrist rotation option. The vertical reach can be over 55 centimeters. The gripper has a 500 grams holding strength and the robotic arm itself can hold up to 200 grams when fully extended. The maximum payload is of 600 grams when the robot is partially extended at 10 centimeters.
Just like the Pincher, the Reactor is controlled by the ArbotiX controller module from Vanadium Labs, with similar features and programming and control options. It can operate under autonomous control, in tethered mode, controlled by a computer through USB or radio and can also be driven directly from a remote control.
The robot requires a 12 VDC 5 A power supply included with the kit and can be programmed with the aid of the included FTDI cable. The standard 5 DOF kit can be bought at a price of around 550 US Dollars while the wrist rotation 6 DOF kit is available at about 600 US Dollars.
RobotShop M100RAK Modular Robotic Arm
The M100RAK kit is a versatile 4 DOF robotic arm supporting a wide range of applications. The standard robotic arm has a reach of over 53 centimeters and can hold a payload of up to 500 grams when fully extended, however aluminium tubes of various lengths can be used for different confingurations, thanks to its modular joint design. The arm is actuated by Hitec HS-785HB servos each joint allowing for 252 degrees of motion. Its construction is of aluminium and its joints feature ABEC-5 bearings for smoothness and strength. The standard kit has a weight of 1,2 kilograms.
The robotic arm kit includes all arm hardware and the 4 actuators. A variety of end effectors like grippers, cameras or other devices can be used, as well as different control electronics. The robot requires input voltages between 4,8 to 6 VDC with power supplies capable of 10 A spike current as required by the servos. The kit is available for a price of 600 US dollars.
Resources
- AL5A, AL5B, AL5D robotic arms, Lynxmotion
- RIOS, Lynx Dual arm controller software, Lynxmotion
- SSC-32, Bot Board II, BotBoarduino controllers, Lynxmotion
- BasicStamp 2 microcontroller, Parallax
- BasicATOM microcontroller, Basic Micro
- Arduino Duemilanove
- Reactor and Pincher robotic arms, Trossen Robotics
- PyPose with NUKE for ArbotiX, Vanadium Labs
- Dynamixel AX-12A specs, Trossen Robotics
- Hitec servo motors
- M100RAK modular robotic arm, RobotShop
