The UCT 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 thereof.

The system required a 300kg sled to be decelerated from speeds of up to 17m/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/Simulink (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 35MPa 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 1.1 below).

To start the cycle, the sled moving at a maximum velocity of 17m/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/min leaving the bore side. This predefined point is determined using a system which encompasses a rotary encoder which 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 gives data on the position and velocity of the oncoming sled.

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

Once the sled and cylinder have connected, the braking sequence will then begin. This will take place in 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 and thereby safely decelerates the sled from the 17m/s to 0m/s with an approximate force of 57g.


Figure 1.1

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