Solenoid Power Manager

Many motion control applications, such as valves, clutches, brakes and positioners, employ solenoids for actuation. Most of these devices are of the closing-the-air-gap variety, in which the actuator is designed so that the force-to-power ratio increases by a factor of 3 to 4 as the solenoid's plunger moves to its energized position. A typical solenoid can be modeled as an RL circuit, where the inductance is determined by the arrangement of the windings and the material used in the unit, in addition to the plunger position. The resistance is derived from the resistivity of the armature wire, which naturally heats up during operation. Excess heat, as occurs when the actuator is energized for long or frequent periods, can damage actuators and consume unnecessary power.

While force characteristics can vary substantially throughout the solenoid's stroke, an optimum configuration uses a strong, timed DC pulse through the armature to close the air gap, then holds the plunger in its energized position at reduced power, a technique often referred to as "pulse and hold." Acro Associates, Concord, CA, refers to their 900R Solenoid Controller as providing "adaptive pulse and hold." An onboard microprocessor and integrated current sensor, along with proprietary signal processing algorithms, analyze inrush current at the rate of 3000 times per second to keep the strong initial DC pulse at its minimum necessary length. This pulse width modulation is common in driving power switches in motor control devices.

According to Acro Associates, their controller makes a solenoid 3 to 10 times more power efficient than with other control techniques, a critical factor in battery-powered applications. The 900R provides up to 5A of drive current, and the user interface consists of an opto-isolated differential input port that can be directly wired to relays, transistor logic, digital I/O boards and PLCs, for high-performance applications. The current sensor and microprocessor that analyze load current also protect the device against short circuits and over/under-current conditions, and there are safeguards against power reversals on the supply pins.

-- RM

For more information: Circle 555 - Acro Associates Inc., or connect directly to their website via the Online Reader Service Program at http://www.1rs.com/008df-555