Multiple-Axis Joint with BLDC Servo Motor Makes Robotic Arm Lighter
Refining A Simple Solution For Improved Dynamic Functionality
Freely moving arms are a well-established solution when it comes to handling tools within a three dimensional space, and not just for robots. But most of the solutions available thus far have struggled to overcome the negative interaction between the necessary drive on the rotating axis and the required arm stability. By eliminating the mass from moving parts, the forces of inertia in the arm are reduced. The arm jib is lighter in weight and can work more dynamically, while providing the same level of stability.
We have all seen industrial robot arms: solid constructions for heavy tools and they are pretty hefty themselves. In fact, these designs are simply too heavy for many applications. Engineers at Igus in Cologne wanted to develop a multiple axis joint that would allow small loads to be swiveled or turned around a jib. They needed high levels of flexibility and low weight, with simple upgrade options for additional axes. These requirements have now been elegantly translated into a new concept with a multiple axis joint made from plastic and cable controls. The cables themselves are moved from the arm‘s "shoulder joint" using compact, high performance, brushless dc servo motors. This prevents inertia in the arm, allows dynamic movements and means the drive block can be compact in design.
To avoid having to reinvent the wheel, the team of engineers looked at further refining the trusted human elbow joint. This allowed two degrees of freedom rotation and swivel to be incorporated in a single joint, thereby guaranteeing high levels of precision for the moving parts. The tribo plastic joint is also maintenance free and does not corrode or conduct electricity. The "bones" of the arm to which the joints are attached are also pared down to a lightweight construction. Aluminum, carbon fiber and fiberglass reinforced plastic provide excellent rigidity and good vibration insulation. As in the human arm, the weakest components of this kind of robot arm are not the bones (i.e. the body tube) or the muscles (i.e. the drive motor) but rather the "tendons" which transfer the power. The high-tension "tendons" or control cables are therefore made of a highly wear resistant and tensile load resistant fiber made from highly crystalline, UHMW-PE (ultra-high molecular weight polyethylene). With tensile strength of 3000 to 4000 N/mm2, it only breaks under its intrinsic weight at a length of 400 km. This design allows a load / intrinsic weight ratio of 1:1 to be achieved, or even less if the structure is optimized. As well as the traditional robot arm functions, the joint can be used for new functions such as special fittings for cameras, sensors or tools where lightweight construction is key. In the future, it may even be used on prosthetics. It is the separation of the tool, the moving parts, the control module and the actual drive that make it ideal for mobile use. As the components that have been "eliminated" from the moving parts represent a sort of counterweight to the mass of the arm, the overall structure is very light. A magnetic angle position sensor is built into every joint to improve precision. Combined with the highly dynamic drives, this produces a precision-controlled robot module.
Dynamic, Powerful and Compact
Compact drive solutions are required in order to operate the largest possible number of cable controls within a confined space. Where high levels of dynamism, real power yield and precise, controllable use are required, electronically commutated brushless dc servo motors like those from FAULHABER are the best option. The small compact drives have a low moving mass and an outstanding performance to volume ratio, making them ideal for dynamic use. The excellent heat deflection allows considerable overload when operated for short periods rather than permanently, which is an additional advantage, especially when it comes to fast swivel movements. The operating voltage of 24 V DC is perfectly tailored to battery power, which is crucial for mobile applications. With a 32 mm diameter, a high permanent output with 80% efficiency can be achieved in 4-quadrant mode. The 97 mNm torque of the four-pole motors increases the diameter-compliant planetary gearheads to the values required to operate the arm. Depending on the area of application for the swivel movement, the planetary gearheads allow transformation ratios of 1:4 to 1600 or 6 to 20 Nm continuous torque at up to 96% efficiency. The stainless steel casing is as weatherproof as the plastic joints and cable controls. The fact that brushless drives have no wearing components apart from the rotor bearing guarantees a long service life over many tens of thousands of hours. The precision gearheads also boast a long no maintenance service life. The motor can be operated in a temperature range from – 40 to +100°C, the gearheads from –25 to +90°C. This means they can both cope with environmental conditions from the Arctic to the tropics and thus a wide range of industrial operating conditions. Modern servomotors are the tool of choice for various applications where movement is required in the construction process. High precision levels and a long service life combined with easy integration into the control system mean they can be used both for mobile applications and in compact engineering.