Servo drives key on ejection seat test rig

The HPRU performs a number of critical functions. It extends and retracts, allowing the pilot freedom of movement during normal flight, and restrains the pilot when gravity limits are exceeded, such as when the aircraft is in a dive or climb. It also restrains the pilot when high decelerations occur to prevent impact with the console, for example during sudden stops, crash during taxiing, etc, and restrains the pilot manually on demand by means of an HPRU Lock override lever.

Once the ejection sequence is initiated the HPRU retracts and restrains the pilot. This ensures the correct posture protecting the human body when it is exposed to the significant ejection acceleration forces. It also ensures the pilot’s body and limbs are clear of the aircraft interior during ejection which could be impacted causing injuries. The HPRU then releases the seat from the pilot and parachute after the ejection.

Needless to say the HPRU is a critical component in the Ejector Seat system. Each ejection seat has a specific maintenance schedule which must be adhered to in order to guarantee the safe operation of the seat, and now with the new HPRU Test Machine (HTM) the time to complete the HPRU maintenance activities will be significantly reduced. Since the first Martin Baker ejection in 1946 the company estimates a total of 7,429 lives have been saved by their ejection seats and this improvement in the testing of those seats will contribute towards assuring continued pilot safety.

The legacy Acceleration Test Rig has been in service for 30 years. It comprises a large flywheel driven by an electric motor connected to a sliding stage via a connecting rod (con-rod), this mechanical configuration produces a simple harmonic motion test profile. To ensure test repeatability the legacy Acceleration Test Rig is calibrated periodically. Conversely the new HTM employs servomotors, drives, motion controllers, rotary and linear slides to provide a configuration that can produce a variety of motion profiles allowing one machine to test many products and uses a control system that reduces human error. Martin-Baker selected Optimal Industrial Automation to provide expertise in developing control algorithms and graphical user interfaces for this machine. Optimal Industrial Automation successfully delivered the qualification programme and was then selected to provide production units which will be used by customers around the world.

Optimal Industrial Automation’s scope of work also included the assembly of HTM prototypes, supporting the product development and management of acceptance test phases to achieve compliance with UK Regulations and European Directives.

The challenge for Optimal Industrial Automation was to replicate the same simple harmonic motion profile as the legacy Acceleration Test Rig using hardware and software within the new HTM to ensure that the same test methodology could be maintained. Optimal developed a control code with built-in test removing the need for calibration, saving the end user time and money usually required in managing and controlling calibration assets.

The servo driven linear slide on the HTM is programmed to operate with a precise motion profile, with built-in over-torque devices preventing any damage to the HPRU under test and the machine itself. The HTM also features a tilting function which replicates the angle of flight of the aircraft, such as. a dive or climb, and executes specific acceptance tests.

Martin Gadsby, director of Optimal comments: “We have been able to use our expertise in automation integration and motion control and our experience in managing military contracts, to deliver a turnkey solution for this crucial piece of test equipment. We have had to implement a number of new systems for this project to ensure we complied with all of the requirements involved in dealing with military aircraft components. That has included a new audit process as well as maintaining additional records with regard to materials and personnel.”

The HMI software developed by Optimal has been carefully designed to use as little text as possible to help customers whose native language is not English. In this way it has reduced any opportunity for incorrect interpretation of the test results. Martin-Baker supplies ejection seats for 93 air forces worldwide and the test rig could potentially be required by all of them, hence the universality of the user interface is important.

The HTM has also been subjected to a rigorous test regime, including high and low temperature testing, humidity testing, EMC and Low Voltage, noise, vibration and endurance tests, endorsing the product for longevity for the military bases around the world.

The HTM has several benefits over the original. Foremost is the speed with which the tests can be completed and the short time required to change the rig to test a different model of HPRU unit. To date, Martin Baker has produced around 75,000 ejection seats, covering approximately 20 types of seat design which means the new test rig represents a significant reduction in inventory for some operators.