How can the need for fast, powerful and accurate linear actuation be met?

There are of course advantages and disadvantages with all technologies and they are sometimes easily overlooked. First of all, it is probably important to define terms. An easy way to think of a linear DC motor is to take a standard DC motor and lay it out flat. For example, it could take the shape of a line of motor stator coils over or along which is passed the permanent magnet sliding actuator. Equally, they can also be manufactured as a rail of magnets with a moving coil. These can be flat or circular shapes. The main advantage of a DC linear motor is speed, as smaller, high-quality units can achieve outstanding acceleration rates. The other advantage offered is operational life. Because there is no gearing and the only friction points are the required linear guides, the lifespan is therefore relatively long. However, linear DC motors also come with a number of downsides. One such is that they offer a very low force. In particular, they offer a very low speed force gradient compared with DC linear actuators. It is of particular importance in this instance to compare the same figure. Manufacturers from different parts of the globe often use a completely different standpoint from which to select catalogue ratings related to force: Peak force; stall force; rated force; holding force; and back driving force to name a few. The most comprehensive test is to use a force gradient, which can either be calculated or informative companies will supply you with the figure representing metres per second per Newton (m/s/N). The gradient of this speed force line represents how much the unit slows down for every Newton of load that is applied and this is a true test of its strength. Another disadvantage of DC linear motors is current draw. Because linear motors are a direct drive solution and there is a higher level of current rise with the required feed force, a geared solution will be proportionally lower. But what of the linear actuators themselves? For the purpsoses of comparison, in this article we are looking at DC motor-driven linear actuators only. These are supplied in many forms, from in-line OEM-style units to off the shelf self-contained units. OEM units are designed specifically for integration into a product being developed from the ground up, where the actuator section of the unit becomes part of the product itself. This is done in the interest of keeping the overall size and weight to a minimum. This could be a system as simple as a DC motor driving a threaded section using an actuation nut or a more robust design of an integrated ball screw, thrust block, DC motor, gearhead and encoder assembly. Self-contained units, on the other hand, are commonly-available linear actuators that typically have a DC motor mounted beside the threaded section driven by a belt and pulley. The threaded or spindle section is contained in a tube. A fixed nut is connected to an overtube that pushes an actuation rod in and out. These are bolt-on units typically used on applications such as hospital beds and low-duty cycle applications. At first glance, these may appear to offer a good solution, however they are bulky and inefficient compared to an OEM-style spindle drive. The low-cost DC motors commonly used in this style of actuator can have quite high radial load applied from the belt and pulley mechanism. This can result in premature failure of the motor bearings. For automation and robotic design engineers, the OEM style is likely to be far more applicable in terms of space, power, efficiency and reliability. Given that, it probably makes most sense to compare the advantages and disadvantages of the OEM style actuators and linear motors. As previously noted, DC motor-driven actuators can produce much higher forces per volume than linear motors, while the nature of the thread or spindle section gives both self-locking and free running options. These units also offer high efficiency, particularly with the recent advances in ceramic and other high-grade running materials. With DC motor-driven actuators, controllability is very easy thanks to the simple mounting of standard encoders on the rear of the motor. Combined with the gearing ratio and the thread pitch, this gives a very high positioning resolution from standard motor position controllers. Finally – and, it might be argued – most importantly, there is the cost advantage offered by such units. However, it is vital to compare like-for-like. One must compare the additional system components required for the same overall result. Linear encoders, linear guide rail, limit switches, etc are typically required alongside linear motors. Much of this additional cost can be avoided with integrated OEM-style units that do not require all of these add-ons. When it comes to disadvantages, however, the main one suffered by DC linear actuators as opposed to linear motors is speed. Top speeds of around 180 to 200mm/s are typical. The second main disadvantage is integration. In order to reduce the overall machine size and by design, the fixation of the spindle nut is part of the application load itself and, as such, a certain level of system design and integration is required from the customer. The latest in linear • Olsen Engineering has launched the Exlar K90, a larger 90mm frame version of the K60 low-cost electric "rolled" linear actuator, which employs satellite roller screw technology. Exlar's K series provides long life and is available as standard in an IP65-rated ingress protected enclosure. Roller screw linear actuators provide an efficient modern alternative to hydraulic or pneumatic units in a much more compact, low maintenance format, without the contamination or noise problems. The K series offers the option of two grades of planetary roller screws "rolled satellite M and X grades" along with an option for an ACME screw. • Festo has launched an intelligent linear actuator for process automation applications.  The DFPI combines the functions of a linear actuator, a positioner and displacement encoder in one, easy-to-install unit. As a result, users can increase operational efficiencies as well as saving time and costs associated with specifying and installing equipment. The DFPI solution is ideal for controlling knife gate valves and shut-off valves with regulated strokes and for use with all linear-actuated process valves. • A new linear motor which brings significant benefits to systems designers and machine builders that is claimed to be the first ever tubular linear motor with an integrated amplifier and controller has been introduced by Dunkermotoren. The ST11 range of linear motors has been extended with an integrated controller option that offers ease of integration with significantly reduced and simplified wiring. It features a range of analogue and digital inputs and outputs, with CAN, Profibus and EtherCAT communications protocols available as standard. Once programmed only power is required to operate in stand-alone mode, with programming completed using Dunkermotoren's own proven and established Drive Assistant.