An innovative sled tester

Hydraulic systems & components

1st Quarter 2011 Hydraulic systems & components

UCT’s Mechanical Engineering Department approached Hyflo for the design philosophy on a sled tester. Hyflo was to be responsible for the design and manufacture of the hydraulic and control systems.

The system required a 300 kg sled to be decelerated from speeds of up to 17 m/s to rest in a controlled manner in a short period of time. Hyflo, in conjunction with system partners, Moog, decided that the best system for this task would be a servo-hydraulic system. This would consist of a braking cylinder controlled by a high response servo valve.

In addition to calculations being carried out for the system design, dynamic simulations were carried out using Matlab/Simulink (R14.1) to assist and verify the component and control selections.

Xs = Rotary encoder; Xc = Linear position transducer; Pb = Pressure transducer cylinder rod side; Pa = Pressure transducer cylinder bore side

The selected hydraulic system employed a pressure compensated pump which delivered 40 litres/min of high pressure oil to charge a bank of five accumulators. A separate accumulator was also charged to ensure constant control pressure for the servo valve was maintained during operation. Once the set pressure of 35 MPa was reached, the pump would cut back displacement to zero. At this point, with the hydraulic cylinder extended, the system was ready for operation and in the default position.

To start the cycle the sled, moving at a maximum velocity of 17 m/s, approached the hydraulic cylinder with contact plate. At a predefined point, in position control via the servo valve, the hydraulic cylinder was pre-accelerated using the high flow available from the accumulator bank with a maximum flow of 4900 litres/min entering the rod side of the cylinder and a maximum of 8000 litres leaving the bore side. This predefined point was determined using a system which encompassed a rotary encoder attached to a winch, which is in turn connected to the sled with the winch rope held in constant tension by a high-speed brushless servo motor. This system gave data on the position and velocity of the oncoming sled.

This pre-acceleration of the cylinder allowed the cylinder speed to closely match the oncoming sled speed and thereby minimise the impact forces.

Once the sled and cylinder were connected, the braking sequence began. This took place in a closed loop control comprising the rotary encoder on the sled, a linear position transducer on the cylinder and two pressure transducers on the rod and bore sides of the cylinder. All these transmitted data to a PLC which in turn controlled the position of the cylinder via the servo valve.

The PLC ensured that all parameters were correct and safely decelerated the sled from 17 m/s to 0 m/s with an approximate force of 57 g.

For more information contact Angus Beveridge, Hyflo, +27 (0)21 514 3000, [email protected], www.hyflo.co.za

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An innovative sled tester

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