TechArchive

Original article date: April 1999

A breakthrough in moving coil technology has led to the introduction of a new series of electronic actuators from SMC.

Early pneumatic systems such as those used in the 19th century to operate points and signals remotely in underground railway networks were designed to produce a discrete two-position actuation in operation (open-closed on-off).

Many modern industrial applications however require one or more intermediate position to be incorporated and it becomes necessary to introduce a further level of control into the system. A number of different solutions to this problem have been used including a stroke reading system which can be used in conjunction with an integral brake to hold the cylinder at a specified position; electric positioning systems which rely on the conversion of rotary motion into linear motion; and hydraulic positioning. One of the most popular methods of providing intermediate operation however is the use of linear motors.

Put simply linear motors are controlled using a DC power supply which generates a fixed voltage to power the system to a pre-determined position. Initially used in loudspeakers. Linear motors are now being used as moving coil actuators (MCAs) in conjunction with a digital electronic controller. The use of the digital controller means that the power level supplied to the motor can be carefully monitored and consequently the force generated by the actuator can be precisely controlled.

A further advantage of this method of operation is that the MCA can verify its own accuracy as it operates as movements are monitored digitally during operation. As a result of this feature MCAs are increasingly being adopted as a practical alternative to geared linear drive systems and other movement technologies in applications requiring precision and accuracy.

SMC Pneumatics’ SMAC actuators capitalise on the performance of high intensity magnets. The result is a device with an extremely small moving mass (in the region of 200g) and the ability to apply considerable force (around 100N) at accelerations up to 5g. In addition the actuator will operate at precisely controlled speeds ranging from 0.005 to 1000mm/s.

These high programmable forces are generated by the magnet which produces a uniform field across an air gap. The coil which is wound around a centre pole conducts current perpendicular to the flux lines generated and creates a force parallel to the direction of travel.

Available in linear or rotary configurations SMACs will operate independently in two servo axes – Z and theta. Stroke length can be up to 100mm with the stroke being determined by the length of the magnetic field less the width of the coil.

Other features include a 5um resolution linear encoder with a 1um resolution option available Run-out of the actuator rod is less than 0.05mm over stroke with end-of-stroke sensors fitted. For linear and rotary applications a multi-turn geared servomotor with optical encoder is available which is accurate to 0.07deg. A rotary seal can be incorporated with a hollow linear/rotary shaft enabling vacuum to be applied through the rod to the work area.

A dedicated controller developed specifically for the SMAC actuator provides programmable control of position speed acceleration and force. It can operate either as a standalone device in simple applications or to communicate with a higher level control system via RS232C. It can be programmed with a variety of set point windows that allow closed loop monitoring of actuator performance during operations. Hence at any moment in operation the position and force of the actuator can be verified.

Applications for MCAs and the SMAC electronic actuator ion particular include PCB populating medical device assembly switch and relay test calibration and the manufacture of computer peripherals such as disk drives.

• SMC Pneumatics
• 0800 0260004

April 1999