By Leslie Langnau / Managing Editor
Actuators for robots range from the “tried and true” to newer versions of actuator muscles. Here’s a look at your range of options.
In robot design, electric, hydraulic and pneumatic actuators are the typical choices available when developing the means to convert energy into mechanical work. However, a couple of newer options are coming, including additively made artificial muscles.
The typical choices for electric actuators include various motors, from brushed and brushless DC servomotors to stepmotors and AC servomotors.
Servomotors generally offer more precise control of motion due to their ability to accommodate complex motion patterns and profiles readily. In robotic design, their ability to deliver fast starts, stops and reversals plays well to application needs. The electric motor itself can be either AC or DC, rotary or linear.
Brushed DC servomotors are generally less expensive than brushless servos, but they do require more maintenance because of the brushes needed for motor commutation.
Brushless motors are efficient and reliable, with low noise and low EMI.
One of the simplest and most cost effective ways to deliver linear motion in robots is with pneumatic actuators. Pneumatic actuators are mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear path.
Pneumatic actuators deliver fast and accurate response and can be used to produce linear and rotary motion. One drawback, though, is power. Other options are available for robotic applications that require the delivery of large force.
Some of the available styles of these actuators include diaphragm cylinders, rodless, telescoping and through-rod cylinders. Typically, the actuator body is connected to a support frame and the end of the rod is connected to the robot part to be moved. An on-off control valve directs compressed air into the extended port, while opening the retract port to atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston.
Hydraulic cylinder actuators deliver high force, but they are not necessarily accurate in positioning. When compared with pneumatic, or electric systems, hydraulics can be simpler and more durable, as well as capable of greater power density. Hydraulic cylinders are also available in an array of scales to meet a range of application needs.
A relatively new type of actuator is the air muscle, sometimes referred to as pneumatic air muscles. These actuators generally consist of a rubber bladder covered by a braided fiber mesh. When pressurized with air or gas, the actuator expands. This type of actuator is generally inexpensive. While it is lightweight, it can exhibit a phenomenal strength to weight ratio. Compared to electric and hydraulic actuators, these units are flexible, durable, safe and easy to use.
Along the lines of air muscles, we now have the development of artificial muscles. The Factory Automation and Production Systems (FAPS) Institute at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) is conducting research on artificial muscles, also known as Dielectric Elastomer Actuators–DEAs. To develop them, researchers recently purchased an Aerosol Jet Quad Print Engine from Optomec. This device is a 3D print engine that will be used to develop an automated production environment for the manufacture of artificial muscles that can be used in robots, as well as in various medical applications.
To produce DEAs, you need a 3D printing / additive manufacturing machine that can print ultra thin layers of elastomer film, silicone and electrodes. The Aerosol Jet technology can print a variety of materials and at dimensions below 10-microns, which suits the needs of this artificial muscle application.
Aerosol Jet technology is a non-contact, direct write printing process capable of directly depositing a range of commercial and custom electronic materials, including conductor, insulator and biologic formulations onto almost any substrate. The technology can print a variety of feature sizes, ranging from less than 10 microns to more than one centimeter using aerodynamic focusing nozzles. Due to its ability to handle electronic and biomaterials within the same material deposition system, these systems offer a unique biomedical micro-device development and production solution that bridges these disciplines.
Engineers at FAPS are looking to Aerosol Jet technology to help facilitate the shift of DEAs from fundamental research to their qualification as regular control elements for use in complex and compliant robots and in lightweight biomimetic prostheses. This requires the development of an automated production processes for stacked DEAs that can meet their challenging implementation requirements.
Options in moving robots
The Roh’lix linear actuators from Zero-Max convert rotary motion to linear travel. Suitable for use in gantry robots, these actuators carry loads at speeds up to 70 in./sec. They handle motion thrust loads up to 200 lb. Thrust capacity is adjustable so that the actuator will stop when the thrust setting is exceeded.
With a special “overload” feature, Roh’lix linear actuators halt motion when hitting an obstruction or exceeding a pre-set level of thrust force.
The linear actuators consist of three precision ball bearings at each end of a two-piece aluminum carrier block. Mounted at an angle to the drive axis, the six bearings convert drive shaft rotation into proportional linear travel. Designed for maintenance free operation, the actuators provide a minimum of 90% mechanical efficiency and an operating life expectancy is 2 million to over 100 million inches of linear travel. Five standard Roh’lix sizes are available for 3/8 to 2-in. diameter shafts, and metric sizes from 8 to 50-mm diameter shafts.
Small ball-screw actuators have been used in the development of a robotic hand that NASA experimented with for space exploration. THK supplied the actuators. The robotic hand traveled onboard the cargo transporter KOUNOTOR13 (HTV3) to the International Space Station (ISS). A mission at the Japanese Experiment Module KIBO used an Astrobot called REXJ to study the viability of Astronauto Support Robots, or Astrobots. This mission was conducted using a standard payload unit attached to the exposed facility of the module. An extendable robot arm and tether systems moved cargo out of the pressurized and unpressurized logistics carrier and into the ISS. At the module, experiments were conducted to determine the extension, position-ing and manipulation capability of the robot arm.
A system that closely emulates human motion is the Robolink System from igus. The joints in this system are controlled by cable tension, in a similar way to the human mechanics of bones and tendons. All data cables are routed safely through the jointed arms, which are effectively the robot’s skeleton. These cables convey images, acoustics and forces, which are the artificial senses of humanoid robots.
The robot unit was primarily designed as a lightweight engineering solution for handling and automation. The development objective was to keep the moving mass as low as possible, so that the actuators can be separated from functioning tools, such as grippers, hands, suction cups, and so on. Particular attention was given to enabling a quick assembly and user-friendly design, as well as using tribo-optimized plastics to provide both freedom from lubrication and a low weight.
Robolink is comprised of a drive-and-control unit, joints in different lengths and arms in different sizes, including a duct for additional control cables. At the end of this jointed system, igus offers the option to connect to different types of tools.
Since the system is modular, it can be constructed with all kinds of humanoid robot configurations. This ranges from jointed arms and moving ‘digger’ arms to four-legged ‘creatures.’ The joints can be easily combined as required.
The drive-and-control unit was purposely designed as a black box. Robot developers have the option to work with pneumatics, electro technology or hydraulics within it. The jointed arms are made from carbon-fiber reinforced plastic and other lightweight materials.
The bionic core of the robot’s skeletal parts is the injection molded plastic joints. They are controlled with cable pulls that transfer tensile forces, just as tendons function in humans. The cable sheath is held and the inner cable moved. This way, the gripper, shovel, hook—or whichever tool the developer chooses—is moved and operated.
The cable pulls are routed through from one joint to the next—just as joints are connected in humans. Only four cables are required for each plastic joint to be able to rotate and swivel freely.
The cables themselves are made from technical synthetic fibers. These are extremely strong, hardly stretch at all, are resistant to chemicals, and are lubrication free and very wear resistant. When compared to steel, their lighter weight also makes them more energy efficient.
PHD, Inc., a manufacturer of actuators for industrial automation, offers a series of parallel pneumatic grippers. Series GRH Parallel Pneumatic Grippers are available in four sizes that provide long jaw travel, while accommodating long tooling lengths. Their design incorporates an extended support guide system with wide slot jaws to minimize tooling deflection, support large moment capacities and provide side load stability. A dual bore provides higher total grip force, and low breakaway pressure allows for gripping of a wide variety of part sizes, including delicate parts.
Series GRH Grippers feature a hard-coated aluminum body for corrosion-resistance, making them suitable for harsh environments. They have total jaw travels up to 125 mm (4.921 in.) that allow for gripping of larger and multiple sized parts, as well as encapsulated tooling. Large moment capacities provide for a wide variety of applications, and their low profile reduces moments for robots.