Hydraulic system for UCT sled tester

Fourth Quarter 2005 Hydraulic systems & components

The University of Cape Town, 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 system.

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 comprise of a braking cylinder controlled by a high response servo valve. In addition to calculations being carried out for system design, dynamic simulations were carried out using Matlab/Simuling (R14,1) to assist and verify the component and control selections.

The selected hydraulic system employs a pressure compensated pump which delivers 40 l/min of high pressure oil to charge a bank of five accumulators. A separate accumulator is also charged to ensure constant control pressure for the servo valve is maintained during operation. Once the set pressure of 35 Mpa is reached, the pump will cut back displacement to zero. At this point, with the hydraulic cylinder extended, the system is ready for operation and in the 'default' position (See Figure 1).

To start the cycle, the sled moving at a maximum velocity of 17 m/s approaches the hydraulic cylinder with contact plate. At a predefined point, in position control via the servo valve, the hydraulic cylinder is pre-accelerated using the high flow available from the accumulator bank with a maximum flow of 4900 l/min entering the rod side of the cylinder and a maximum of 8000 l leaving the bore side. This predefined point is determined using a rotary encoder that is 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 produces position and velocity information on the oncoming sled. The pre-acceleration of the cylinder allows the cylinder speed to closely match the oncoming sled speed and thereby minimise the impact.

Once the sled and cylinder have connected, the braking sequence begins. This makes use of a closed loop control comprising of 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, which all transmit their data to a PLC which in turn controls the position of the cylinder via the servo valve.

The PLC ensures that all parameters are correct safely decelerates the sled from the 17 m/s to 0 m/s with an approximate deceleration of 57 g.

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