Miniaturized linear motion

Transcription

1 Miniaturized linear motion Test and measurement, semiconductor, and precision medical applications are pushing the envelope of modern motion designs. In these and other applications, compact linear motion designs are being leveraged to deliver accurate movement. CONTENTS Rotary actuators Electromagnetic motors paired with rotary-tolinear converters Integrated stepperleadscrew actuator advantages Compact linear motion comes in myriad forms traditional rotary micromotors paired with miniature rotary-to-linear converters, direct-driving linear motors, pneumatic cylinders, and piezomotors that output linear force. These linear motion designs satisfy myriad design requirements, shrinking overall machine footprint (as in pick-and-place robotics and medical devices), reducing power consumption, and boosting precision. Rotary actuators Electromagnetic motors paired with rotary-to-linear converters actuators rotary step or brushless dc motors connected to rotaryto-linear devices are designed to deliver up to several Newtons of force, typically over 2 to 200-cm travel lengths. Only mechanical devices such as leadscrews or belt drives capable of maintaining the required level of accuracy are paired with such motors in exacting miniature applications. Coreless micromotors address some of the limitations of iron-core dc motors. Only the copper coil moves the magnets do not. A tiny ø38-mm version can output 100 W of power. These coreless micromotors are suitable for pick-and-place, avionics, and compact designs necessitating high precision. MOTOR: The proportionality of dc motor output speed to applied voltage, current and torque output linearity, and output predictability by stall torque and noload speed alone all simplify the application of dc-motor-based linear actuation. Myriad options exist: Brushed iron-core motors, brushed ironless motors, brushless micromotors, and permanent-magnet brushless motors used for microstepping. Simplicity and low cost are the key strengths of brushed iron-core motors. Ironcore construction delivers high torque and rigidity, though the heavy armature limits rates of acceleration. What s more, when the motor s brushes pass over each commutator 1 A few notes on miniature slides and guides Miniature linear motors piezomotors Miniature pneumatic linear actuators

2 piezomotors 3 Brushless dc motors are not speed limited by mechanical commutation subcomponents. Actuators based on this motor type incorporate drive electronics, leadscrew, and in some cases, a nut and gearbox. plastic drive nuts, which maintain precision by automatically adjusting for wear, and reduce overall design weight and power consumption. An alternative to actuators employing rotary-to-linear screws is synchronous (toothed) belts. These are used in some miniature linear-motion applications where high speed (to 30 cm/sec) is priority. Advances in material science and belt-tooth geometry analysis have spurred designs capable (with closed-loop control) of generating relatively highsegment, small voltage spikes result. This compounds regular mechanical wear. Motor designs employing an ironless setup (with skewed-wound coils) are stiffer and generally more efficient thanks to a lighter rotor. Faster accelerations are possible, and cogging is eliminated useful features in miniature precision machinery employing linear motion. Brushless motors must overcome starting friction as well as dynamic friction defined as mnm/rpm; viscous ball-bearing friction and Eddy currents cause these losses. However, brushless commutation both necessitates and benefits from electronic control, typically in the form of pulse-width modulation. Closed-loop control with digital actuator advantages Step motors paired with leadscrews are an indispensable miniature linear-actuator combination, largely because actuators with these subcomponents are well-suited for positioning tasks. All step motors turn a discrete amount for each pulse of electrical input, which is a mode of operation widely leveraged in myriad applications. Certain miniature linear actuators take this a step further by integrating the leadscrew into the step motor rotor. In these designs, the rotor s permanent magnets follow the motor s sequentially energized coils to concurrently advance the screw s mechanical nut via the screw s thread along the working linear axis. In other setups, the leadscrew s nut (instead of the screw) is integrated into the rotor to enable linear advancement. Both types eliminate the need for mechanical couplings between motor and leadscrew; zero-backlash gearboxes and quadrature magnetic encoders can then be integrated where required. Miniature actuators integrating step motor and leadscrew are commonly found in laboratory, X-Y table, and optical positioning applications. 2 Hall-effect sensors or even analog sensors for sinusoidal commutation is usually required for turning the motor phases on and off at the correct times. Electronically commutated permanent-magnet step motors are yet another option for small precision installations. 12 pole pairs on the rotor (and stator) is common; one, two, three, and even five-phase motors are possible. These are highly suitable for applications requiring wide speed ranges, holding power even when de-energized, and frequent stopping, starting, and reversing. Microstepping is possible with motors of sufficient angular accuracy and a drive designed to continuously vary winding current (for dividing full steps into smaller discrete steps.) That said, surgical applications and pickand-place robotics that cannot tolerate slight performance variations due to thermal effects might not be suitable uses for these motors. SCREW OR BELT: Conversion of rotary to linear movement with a leadscrew often uses balls that roll through the mechanism s load zone to eliminate sliding friction between nut and screw. For miniature applications, precision-machined steel screws are used for their accuracy. Backlash the play between nut and screw depends on the ball size and groove geometry. Preloading (to negative clearances) can keep backlash below 0.05 mm or better. In other designs where lighter loads are moved, some leadscrews utilize

3 Permanent magnets for positioning PMDC motors are useful for positioning and other precision applications; stepping often offers sufficient trackability, and inherent detent toque is useful where power is at a premium. precision linear motion: Glass fiber or aramid reinforcements extend life and ensure stable belt length for precision; modified trapezoid, curvilinear, and customized tooth profiles allow for no-slip positive pulley engagement. SUPPORTS AND ACTUATOR STRUCTURES: Rod and rodless screw-based actuators exist. Applications positioning small or pivoting loads often use rod-type electric cylinders, especially if the load is otherwise supported. Here, an electric motor rotates the screw and advances a car; where needed, intermediary gearboxes match motor output to required linear speed and thrust. Positioning actuators (often for conveyed items) can move and stop loads in various positions with accuracy to ±0.010 mm. For fragile loads, controls also ensure gentle decelerations. Finally, these actuators can guide loads in isolated environments, as the rod can extend into a space while the cylinder main and motor remain outside. One potential concern when utilizing leadscrews in precise miniature applications is that imbalance of the rotating component can transversely excite natural system frequencies. Therefore, turning shaft supports often consist of bidirectional bearings at each end where possible. Setups with one free (unsupported) end are less stiff and typically less accurate. In contrast, rodless actuators provide support and positioning from one compact design. Here, a belt 3 or a screw s linear bearings support a moving carriage. Capacity depends on motor torque capacity, screw diameter, and the application s inherent amount of pitch or roll. motors -motor designs abound. In one miniature iteration, three coils position a shaft packed with permanent magnets. Driven by a servocontroller, the shaft is positioned with almost zero residual static force a suitable mode of operation for micro-positioning. As in larger designs, feedback is provided by a linear encoder or Hall-effect sensors. The latter reduces design cost as well as size, with some housings 10 cm 3 or smaller. The elimination of static force (associated with traditional rotary-motor types) and linearity between current input and output force make these motors suitable for micropositioning. The main advantage of linear motors is their direct driving of attached loads, which eliminates the lost slides and guides Common miniature slides and guides are designed to deliver 3 to 1,000 cm travel lengths, depending on actuation method. To sufficiently support the load over such travel lengths, the primary linear motion device can require supplemental load support in the form of slides and guides. Here, mechanical subcomponents are precision machined; aluminum alloy structures boost rigidity and reduce weight. Some miniature linear stages have one guide and one bearing instead of two; elsewhere, miniature slides may incorporate friction-free surfaces in lieu of rolling elements. Where accuracy trumps the aim to reduce friction, miniature guides utilizing rollers are often preloaded. Positive clearance is eliminated for zero or even negative values with rollers that are larger than the guide s roller channels. The rigidity and accuracy of such setups is indispensible in applications such as wafer welding and probing (during final testing.)

4 Piezomotors are suitable for nano-positioning applications. motion and accuracy degradation of intermediary mechanical devices. Typically, linear motors for compact designs move loads of 10 N or more over 4 to 1,000 cm travel lengths. Accelerations commonly reach 7 g with sufficiently stiff framing with smooth motion even at low speeds. In some designs, one unit can support multiple cars to simultaneously position multiple loads. Top speeds exceed 900 cm/ sec; thrust depends on motor size. piezomotors Focusing, adjustment, and inspection in medical and electronics machinery are applications for which piezomotors are suitable, thanks to this type of motor s ability to deliver submicrometer positioning even down to a few nanometers in some designs. For example, in a measuring application, piezomotors might slowly move objects past machine vision set up to record their geometry. Delivered travel lengths are typically 2 to 40 cm. Four linear piezomotor types exist: Single-element, flexure-guided, standing-wave piezomotors, as well as For micron-positioning accuracy, some tubular linear motors use permanent magnet thrust rod paired with a primary mover. The high cost of the magnets is less of an issue in smaller designs. This electrically commutated linear dc servomotor delivers precise linear motion. 4 piezo stepmotors. The former sometimes come in stackedcrystal designs that generate motion in a way that resembles the operation mode of a traditional solenoid with stroke direction in line with crystal polarization useful for dispensing applications. In contrast, standing-wave

5 piezomotors are suitable for longer strokes: Piezo crystals vibrate ultrasonically to advance a screw that translates attached loads. Stepmotor designs activate a series of crystals through various positions to walk loads forward; speeds to 100 cm/sec make this design useful in tiny robotics, unmanned drones, and fine-tuned positioning applications. Visit for more information on miniature piezomotor technologies. linear actuators s are manufactured in several designs. Prime fluid-power generation is based on the conversion of compressed air into mechanical power for the linear translation of loads. Actuators leveraging this means of operation excel in positioning of light loads over multiple stops, particularly when equipped with digital sensors and valves or sensors that linearly vary output signal with piston movement. Traditional miniature servopneumatic actuators deliver accuracies to 0.20 mm or better sufficient for numerous positioning applications. They move loads to 500 cm/sec without inducing shock in the load or machine. Challenges include stick-slip behavior and degraded speed consistency when carried loads vary. Even so, 20-g acceleration exceeds that of certain electromechanical linear designs, which is useful for particularly quick or short linear strokes. An alternative design is miniature pneumatic motors. Capable of 30 mn-m or more at 5 to 60,000 rpm (depending on the miniature gearbox fitted to the design) these are driven by air pumped through a rotary series of vanes. Penton Media Inc