Servo Unit

Technical Information

Catalogue No: C0833
Category: Flight Control
Object Type: Actuator
Object Name: Servo Unit
Part No: 96G 19031
Serial No: 11384
Manufacturer: Harowe Servo Controls
Division: Unknown
Platform(s): Sentinel Airship 
Year of Manufacture:
Dimensions:
Width (mm):
Height (mm):
Depth (mm):
Weight (g):
Location: Archive Object Store
Inscription(s):

BMH-ISTE-1801-A

Notes:

Harosyn-RCX 21BRCX-500-J12

Part of Sentinel Fly-by-Light system

FARL demonstrated their Fly-by-Light system (in which fibre-optic cables are used instead of wires to pass signals between aircraft electronics boxes) at the 1980 SBAC show held at Farnborough. Following this they were approached by Airship Industries who were attempting to enter the military market, for which an airship is ideally suited for early warning, coastal surveillance and other long duration roles. The original system involved cables and pulleys and gave a heavy pilot workload as a result. Having realised that some form of electronic flight control system was required they ran into another problem that the airship was largely fabricated of non-conducting material and that the electrical cables would give dangerous lightning conductors.

FARL proposed the novel fibre-optic system which also included a highly innovative electrical actuator. The GEC Avionics design team was led by Consultants Staff Ellis and Peter Keay with Paul Buckingham, Trevor Hall, Dave Larner and Phil Lamb of FARL assisting. In January 1983 Marconi Avionics (FARL) signed a contract with Airship Industries Limited to design and build a prototype optically signalled FCS for the SKS600 airship. Delivery of the system was made in September 1983. The maiden flight was made on October 23 1988 fly-by-light by Airship Industries' chief test pilot Dave Burns at the company's US facility in Weeksville, North Carolina.

The major features of the FCS are:
● Fibre optic data transmission between the Flight Control Computer (FCC) and Actuator Drive Unit (ADU) to provide electrical isolation. Single fibres are to be used with data transmitted using Manchester encoding with MIL-STD-1553 format. Two independent supplies are provided.
● A duplicate system with pilot changeover in the event of a failure.
● An electrical actuator incorporating two dc servo motors with integral gear box The Actuator Drive Unit contains all the actuator electronics and fibre optic interface.
● A microprocessor-based Flight Control Computer.

The fibre optic cable used was a single optical fibre produced by GEC Optical Fibres Ltd and packaged by AEI Cables Ltd. Twelve fibre optic links are used within the Airship Flight Control System (FCS). These links handle all the data transmission between the Flight Control Computer (FCC) and Actuator Drive Units (ADU); the FCC being mounted in the gondola and the ADU are mounted within the surfaces on the rear of the Airship.

The duplex actuator is an element in the optically signalled Flight Control System of the Skyship SKS 600 Airship and is required to move the control surfaces (two rudders and two elevators) in response to pilot commands input via the control yokes. It is mounted on the hinge line of the control surfaces. It incorporates two samarium cobalt DC Servo Units mounted on a common shaft, each with a separate power lane. One motor only is used to supply the torque and the second motor switched should there be a failure in the first lane. The motor shaft is part of a simple 3-stage epicyclic gear train designed to give a gear reduction of approximately 90:1 and hence a design running torque of approximately 1066Nm (72lb-ft) i.e. at half the max rated torque of a single motor. Provision is made for two slab tachometers mounted on the motor shaft, to give velocity feedback to each channel as part of the servo loop, if required. A 3-gang plastic-film potentiometer is mounted on the output shaft, two channels to give positional feedback information, one to each lane of the servo loop, and the third giving independent surface positional information.

Click to enlarge Click to enlarge