New electric cylinders that utilize patented tubular linear motor technology are electrically driven, drop-in alternatives to their pneumatic counterparts, offering similar dimensions and mechanical interfaces. The cylinders offer fully programmable, closed-loop position, acceleration and speed control, and are designed for extremely fast and accurate positioning applications.
The heavily patented linear motors are based on an innovative tubular design that uses high flux annular magnets on the actuator rod, surrounded by a series of specialized windings on a long stator coil. The approach effectively turns conventional linear motor design inside out because, in most linear motors, the fixed stator contains the permanent magnets and the moving element contains the coil windings.
The design offers a number of significant advantages including low moving mass and a complete absence of flexible cabling to the moving parts, both of which improve reliability and dynamic performance.
"The advantage of this technology is its scalability and its modular and stackable design," says Nuzha Yakoob, a product manager for Festo. "The linear motors are also flux-free because there is no external magnetic field which is usually an issue with linear motors."
Linear motor technology offers a distinct advantage when very high dynamic and precise motion is required. Yakoob says Festo's newly patented design offers higher performance, in terms of force density, compared to other manufacturers of tubular linear motors.
The linear motor employs a stack of annular magnets to form a tubular core which is embedded in a coil arrangement which is fixed and the magnetic stack moves. Depending on the position of the tubular magnetic core in relation to the outer coils measured by the read head of the measuring system, current injected through the coils using sinusoidal commutation generates a force and creates linear motion.
Yakoob says the coil and magnet arrangement was designed with a very compact cross section that allows the motor to be used in existing pneumatic actuator profiles offered by Festo. The coils are produced in a flat flexible membrane and then rolled around the tube. The assembly is then coated in resin to form a seal. The linear motors have no external magnetic fields, as the tube itself creates a magnetic return path. This allows the motor to be used with other instruments and devices that might be sensitive to external magnetic interference and also suitable for applications where there is iron ferrite dust and metal particles.
Linear motors typically show a reduction in force when approaching the end of stroke as the magnetic field exposed to the coil arrangement reduces toward the end of stroke. Although incomparable to the forces produced by a pneumatic cylinder, Yakoob says the DNCE and DFME linear motors maintain a consistent force along the full stroke with negligible drop at the ends.
The SFC-LACI drive and position controller is a compact package of both the power and control electronics usable with both the DNCE-LAS and DFME-LAS linear motors. The drive is configurable with 31 user-defined motion profiles. These pre-configured motion profiles can be linked to form a sequence which significantly reduces PLC programming, thereby reducing engineering time and cost.
A complete single-axis package comprises the motor, controller and connecting cables. The controller is configurable and is parameterized using the company's standard configuration software platform called the Festo Configuration Tool (FCT). The controller could be interfaced to a PLC using discrete I/O or popular industry-standard fieldbus interfaces such as Profibus, DeviceNet or CANopen.
Festo's new DNCE-LAS tubular linear motors have the same form factor as the DNC series of piston rod pneumatic cylinders. The DNCE and DFME come in two sizes (32 and 40 mm profiles) and stroke lengths (100, 200, 320 and 400 mm). The largest DNCE-LAS model can accelerate at up to 125 m/sec2, handle a velocity of 3 m/sec and is capable of a thrust force of 202N peak and 55N continuous.
DNCE-LAS cylinders are designed for loads up to about 1 kg that do not exert twisting forces on the actuator and applications such as small parts unloading, sorting and packaging machines. For applications requiring higher load capacities and resistance to twisting forces, DFME-LAS cylinders are equipped with two ball race guide assemblies to provide extra rigidity. The DFME provides a different mechanical configuration. If you were to split open the DFME and DNCE-LAS, the core or magnetic architecture is the same. The difference is the type of guiding system. The built-in guide in the DFME is more of a mechanical guiding system and is targeting applications where the single rod would not be suitable.
The new linear motors have the same mechanical profile and footprint as the company's pneumatic cylinders, and share the same mechanical components and accessories. However, usage depends on the specific application requirements. Linear motors are suitable for applications where dynamics and precision are required, along with positioning flexibility in comparison to discrete pneumatic component that offer high forces but lack flexibility.
"This is Festo's first line of linear motors," says Yakoob. "Considering all of the positioning technologies from basic pneumatics to electromechanical axes, servo pneumatics and now linear motor technology, we can now provide solutions to a wide range of positioning applications."
Targeted applications are machines using discrete pneumatics and small cylinders that want to introduce increased flexibility. "If an engineer wants higher accuracy and the requirements for feed forces are within the capabilities of the linear motor, now we can take out that cylinder and drop it into the same footprint as a linear motor system that can provide that flexibility. Now you can do multiple positions and control dynamics, so there is better control of the system and more flexibility in terms of the machine," Yakoob says.
In addition to the DNCE-LAS and DFME-LAS, Festo has also recently released an ELGL-LAS flatbed linear motor package. This linear motor axis employs a magnetically preloaded air cushion bearing to provide precise positioning and excellent linearity. As with the tubular linear motors, the unit does not generate strong external magnetic fields, and comes in three different sizes with an option for multiple sliders. Strokes are available up to 1,750 mm offering peak forces of 450N with a repeatability of ±10 microns. The ELGL-LAS can be used in applications with larger loads up to 40 kg, where the tubular linear motors are targeted for applications with smaller loads.
Festo is also in the process of developing a hybrid linear motor which brings together the best of pneumatics (force density) and linear motor technology (accuracy and dynamics) packaged in a single compact actuator.
Click here for technical information on Festo's new tubular linear motor actuators.