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The Split Field Series Actuator

This type of actuator has two differentially wound series field windings, each producing a flux in the opposite direction. Only one winding can be energized at any one time, and the direction of rotation depends on which winding is energized.

Limit switches which are operated by the mechanical load are normally fitted in series with the field windings, these stop the motor automatically when the load reaches the limits of its travel.

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Figure 7.8 The Split Field Series Actuator

The Split Field Series Actuator Operation

With an OPEN selection, a supply is fed to the armature via the limit switch, the open field and brake coils. Energizing the brake coils releases the brake (if fitted, allowing the motor to operate.

On completion of the actuator travel the limit switches are tripped as follows:

•Open Limit Switch. This breaks the supply to the motor on completion of travel and makes the circuit to the ‘open’ position indicator.

•Close Limit Switch. This sets up the ‘close’ circuit ready for completion when a selection of ‘close’ is made on the control switch.

Note: The brake solenoid operates immediately the supply is broken thus preventing over-runs or creep.

A slipping clutch may also be fitted between the armature shaft and gearing to prevent damage which could be caused by mechanical overload.

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Motor Actuators

There are two types of motor actuators in use:

•Rotary actuators

•Linear actuators

Rotary Actuators

Rotary actuators are operated by small reversible motors which rotate an output shaft through a gearbox.

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Motors DC - Electrics DC

They are used for the operation of fuel valves and air/oil shut-off valves.

Control is by means of an ON/OFF or OPEN/SHUT selector switch. Two limit microswitches control the extent and direction of travel and also operate the visual indicators in the cockpit. One limit switch is always closed, allowing current from the selector switch to the actuator. The limit switches change over at the end of travel.

	‘DOLL’S EYE’	BRAKE COILS
	POSITION
	INDICATOR
	OPEN	OPEN	CLOSE
		MOTOR
	REDUCTION
	GEAR		CLUTCH
	OPEN LIMIT	OUTPUT	CLOSE LIMIT
	SWITCH		SWITCH
		DRIVE
	OPEN	SELECTOR SWITCH
		CLOSE
	28 V DC

Figure 7.9 Rotary actuator

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Linear Actuators

Linear actuators have small reversible motors which are coupled through a reduction gear to a screw jack which extends or retracts a ram or plunger.

They are used for any operation which requires a push/pull action, e.g. flaps, undercarriage, trim tabs, and also as inching controls for oil cooler shutters.

Operation is by means of selector switches when used for full up/down operation, but for small movements, such as those required with trimming controls, a spring-loaded self-centring ‘OFF’ switch is used, movement of the switch one way or the other away from centre supplying power to the actuator motor, which will then operate in the selected sense.

Two limit switches control the extent of travel and direction, and also operate visual indicators. The respective switch opens to stop the motor at full travel.

With an inching actuator, both limit switches will be closed at any time the actuator is not at a full travel position, this will facilitate motor reversal by means of the inching control switch.

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Figure 7.10 A linear actuator.

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Motors DC - Electrics DC 7

Actuator Brakes

Many actuators are fitted with electromagnetic brakes to prevent over-travel when the motor is switched off.

The design of brake systems vary with the type and size of the actuator, but in all cases the brakes are spring-loaded to the ‘on’ condition when the motor is de-energized, and the operating solenoids are connected in series with the armature so that the brakes are withdrawn immediately power is applied.

Actuator Clutches

Friction clutches are incorporated in the transmission systems of actuators to protect them against the effects of mechanical over-loading.

Visual Indicators Used with Linear Actuators

Press-to-test lights or magnetic indicators are used where no intermediate stopping positions between actuator limits are required.

Position indicators with a graduated scale are fitted in situations where movement either side of a datum, or between open or closed, is to be shown.

Visual Indicators Used with Rotary Actuators

These indicate to the pilot the position of the actuated equipment which would typically be fuel or oil valves. These are only ever in the ‘OPEN’ or ‘SHUT’ position.

In both cases an indication of either Loss of Power supply, or that the actuator is travelling between selected positions, will be required.

Indicator Lights

Indicator lights are usually of the ‘press-to-test’ type. Application of finger pressure on the front glass of the lamp unit enables the filament to be tested without operating the control switches of the actuator.

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