Motors & Drives
positioning accuracies. In contrast to actuators based on dc servo or stepper motors, the Nexact units use ‘piezo walk’ drive technology to provide linear motion. In operation, piezoceramic bending elements act on a ceramic runner, which is connected to the moving limb of the actuator. For long-distance motion, the stepping mode is used. Innovations in moving coil linear actuators have
come recently from SMAC and BEI Kimco. For example, SMAC’s CAL36 series actuators have a 36 mm diameter body and stroke lengths of 15, 25 and 50mm. Basing the design on a round-centred coil and guide results in no internal moment, and low internal friction enables very light forces to be applied to the workpiece. Due to the innovative design and construction, the units can achieve up to 50 ‘g’ acceleration and can offer a long life expectancy due to over-guiding and internal permanent lubrication. Te CAL36 series of electric cylinders also benefits
from independent control of the position, speed and force. Further to this, SMAC’s patented ‘softlanding’ capability is incorporated to enable the actuator to land on parts gently and then apply the required force - which can be useful when handling fragile components and materials. Te actuator also has the ability to do work and provide feedback simultaneously. Most recently, SMAC introduced a vacuum-
through-shaft version of its linear actuator that the company says eliminates dust problems that are common in pick-and-place applications. As a result, the new LAR31 series is claimed to boost reliability significantly in areas such as assembly of printed circuit boards (Fig. 1).
BEI Kimco, meanwhile, recently unveiled the small
but powerful LA100-90-001 cylindrical voice coil linear actuator. With a stroke of 50 mm, this offers a continuous stall force of 1.29kN and a peak force rating of 2.45kN. It is described as a two-terminal, hysteresis-free, cog-free, direct-drive linear servo with infinite position sensitivity and a linear force constant characteristic that simplifies control. Unloaded
ICs feature built-in translator
featuring built-in overcurrent protection and targeted at the office automation market. The A4982/A4984/A4985 and A4988 are complete microstepping motor drivers, each using a simple ‘step and direction’ interface and featuring a built-in translator for easy
A
llegro Microsystems Europe is offering four new DMOS microstepping motor driver ICs
acceleration capability is 12.8 ‘g’ continuous and 24 ‘g’ for ten seconds. It also features a 71mm central bore for linear bearing installation, forced ventilation or other requirements such as cable runs, optical paths or sensor installation. In some cases the engineer will wish to specify a linear motor that can be incorporated within an overall design, but sometimes it is preferable to specify an all-in-one unit that includes guidance bearings and other functions, as this avoids the need to specify the components separately then run the risk of incompatibilities. In late 2009 Yaskawa launched a linear actuator that consists of a linear motor, profile rails, linear guides, sliders, encoder and plug-in power electronics (Fig. 2). Furthermore, this unit is said to be maintenance-free and, thanks to the application of a special surface coating, it may be cleaned using aggressive detergents and pressure washers. Standard versions are available in four installation sizes with instantaneous peak thrusts of up to 5400N and stroke lengths of up to 6m. If required, Yaskawa can also supply customised linear actuators, gantries and multi- axis systems. Other features claimed for the Yaskawa product
include excellent positioning accuracy, dynamic movements even at high speeds, short stabilisation times that help to increase machine throughput, and high efficiency to minimise energy consumption. From the above it can be seen that the latest
innovations in linear motors and linear actuators have moved the technology forward significantly, making electromechanical devices a realistic alternative to hydraulics, pneumatics and actuators based on conventional rotary motors. With such a wide range of alternatives available,
however, care is still required to ensure that the right product is selected to suit the application requirements, as the breadth of performance characteristics available from the different products is inevitably extensive to enable this family of technologies to compete with such diverse alternatives as servo-hydraulic rams and high-speed pneumatics. ●
“In any
application for linear
motion there are design
considerations relating to speed,
acceleration and
positioning accuracy.”
operation. The devices require few external components and occupy a small footprint in a compact, thermally efficient package. These devices are designed to operate bipolar stepper motors in full-, half-, quarter-, eighth- and sixteenth-step modes, with an output drive capacity of up to 35V and ±2A. They include a fixed off-time current regulator that can operate in slow or mixed decay modes. A built-in translator is said to be the key
to the easy implementation of the devices. Simply inputting one pulse on the ‘step’ input drives the motor one microstep; there are no phase sequence tables, high-frequency
control lines, or complex interfaces to program. According to Allegro Microsystems, the interface is an excellent fit for applications where a complex microprocessor is unavailable or already overburdened. The chopping control circuitry
automatically selects the current decay mode (slow or mixed). When a signal occurs at the ‘step’ input pin, the device determines if that step results in a higher or lower current in each of the motor phases. ●
For more information, visit
www.allegromicro.com
www.engineerlive.com 19
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