Forearm Actuator (FA)
Our quest to design and develop an affordable medium size industrial robotic arm resulted in the following baseline requirements:
The robotic arm must be able to carry a payload of about 100kg.
The robotic arm must have a reach of about 2m.
The robotic arm must have a tool tip positional accuracy of 0.5mm.
The robotic arm must be able to work in harsh environments. This environment stemmed from our vision to supply low cost robotics for widespread industrial use.
A tall order, as similar robotic arms commonly achieve positional end effector accuracy of about 1mm. Achieving 0.5mm proved to be very challenging, but also very rewarding. Rewarding in that a number of innovative technologies developed out of the challenge: innovative technologies with great market potential such as our angular encoder; the MFEA encoder.
To save volume and weight, our first functional robotic arm, called The Wing Manipulator was powered by a VW wiper motors in conjunction with an inexpensive tension based chain and sprocket drivetrain. Such a drivetrain is different from those found in typical industrial robots and it turned out to be greatly advantageous.
Gears are commonly used in robotic drivetrains to reduce motor speed and increase torque while at the same time allowing the robot to make small and precise movements. In typical industrial robotics, gears are housed in gearboxes located around the joints of the robot. These gearboxes are typically sun-and-planetary or strain-wave types which require strict manufacturing precision. This greatly increases manufacturing costs. Also, maintaining these gearboxes are expensive requiring frequent oil changes and difficult repair procedures, not to mention hours of downtime required during such procedures.
Instead of gears, we opted for a tensioned based chain and sprocket system. Our drive-train is comprised of inexpensive of-the-shelve components with a small amount of machined parts. This has greatly decreased manufacturing costs without compromise on performance. Backlash in the drivetrain is reduced to minimal levels by auto-tensioners and the remainder is handled by our Foxhole control loop (software). Our drivetrain system does not require regular oil changes, except for a drop of oil on each sprocket once in a while. Maintenance downtime has been decreased drastically. And other than the proven reliability of chain and sprocket, maintenance on these components is straight forward and cost-effective.
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The roots of Landman Robotics are embedded in the original Caproni Wing Manipulator ADM1 model as illustrated in this picture. This design proved to be lacking in strength, triggering further development and culminating in the ADM3 model of the Forearm Actuator.
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The un-painted Forearm Actuator Chassis is illustrated in this picture. This component is built around a basic steel frame made from 2 x 25 x 50 square tubing. Welded on top and bottom of the basic frame are four expansion sections, made by cutting 2 x 25 x 50 square tubing to 2 x 25 x 20 of suitable length.
Some parts of the Forearm Actuator Chassis are made from 5 x 25 mm flat bar, yielding a type of honeycomb structure. Closing the sides with cover plates attached with multiple bolts yields a very strong torsion box.
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Knuckle Block forming a connection between two Limb Actuators. Four Knuckle Blocks are used to attach the Forearm to the Upper Connecting Arms and the Yoke to the Bottom Connecting Arms.
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Early structural test with ADM2 model of the Forearm Actuator. Here we are lifting 90kg with a safety factor of 1,2 for the Primary Chain. This test represents the worst-case scenario that would occur while lifting the outer wing section of a Caproni glider. The subsequent modification of the Forearm Actuator yielded the ADM3 model with a safety factor of 3 for the Primary Chain under the same conditions.
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Forearm Actuator ADM2 model with improved Secondary Spindle. Stress testing showed bending of the Secondary Spindle, calling for it to be improved by increasing the spindle diameter and the number of sprockets.
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Forearm Actuator ADM3 model with improved Primary and Secondary Spindle. This arrangement was performed satisfactorily during our worse-case fatigue testing procedures.
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The drivetrain safety factor was increased from a factor of 1.2 (ADM2) to a factor of 3 (ADM3).
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Small Form Factor 90° Gearbox prototype 1. We could not find a 90° gearbox small enough to fit in the volume available in the Forearm Actuator, so we made our own. We included bearings throughout the design ensuring high performance and reliability.
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Small form factor 90° Gearbox prototype 1 assembled. Dimensions 60x57x50mm.
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The initial Volks Wagen wiper motor was too slow and was consequently replaced with a salvaged DC motor. Due to spatial constraints, this entailed the installation of a custom-made 90° gearbox. This gearbox will be omitted in the ADM4 model of the Forearm Actuator as we are moving away from gears.
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MFEA Angular Encoder and Actuator Control Unit being integrated with mechanical assembly.
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Final assembly and testing of the ADM3 model of the Forearm Actuator.
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This video demonstrates that a drivetrain with auto-tensioning can effectively produce high Main Spindle torque at low speed and high DC Motor speed at low torque, all with minimal backlash. This arrangement enables this system to make precise movements.
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This video demonstrates that backlash close to the DC Motor is effectively mitigated by combining the Main Spindle angle (as measured on the shaft itself) with our Foxhole Control Algorithm. This system behaves like a stepper motor without overshoot, even though we are using a DC motor (producing a great deal of torque in a small volume).
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This video of the Forearm Actuator ADM3 model illustrates our use of CAN bus and DeviceNet protocol. It also illustrates the difficulty we found calibrating our MFEA Encoder ADM3 model using a crude protractor arrangement. In subsequent MFEA Encoder models this problem is avoided.
Our Forearm Actuator ADM3 model produces about 500 Nm torque at a speed of about 30 RPM. This will lift a payload of 100kg at a reach of 2m at a rate of 1m/s; and do so with a positional accuracy of 0,5mm.