3.2 Hydraulic part
3.2.2 Complete hydraulic layout
Now the basic layout has been chosen, the individual parts have to be sized. The demands specified in the previous chapter have to be taken into account. Figure 3.4 shows the com- plete layout of the hydraulic system. The torque transducer and flywheel will be mounted between drive motor F10-39 and brake pump Fll-58 as described in the previous subsection.
Specification of the chosen parts:
Bosch
(power supply unit)
Figure 3.4: Complete hydraulic layout
14
Hydraulic motor
/
pump unit combination: The performance of the drive motor (F10- 39) depends on the motor itself and the pump unit that supplies the oil flow. Appendix C shows available hydraulic motors and pumps in the fluid power laboratory. The motors and pumps are fixed displacement machines. The maximum power of the Bora engine that has to be simulated is 75 [KW]. Pump B and C supply the power for the hydraulic motor. The pumps are similar and generate 31.5 [KW] (90 [Llmin] at 210 [Barj). A valve combines both flows. The maximum pressure remains 210 [Bar], the flow is doubied. The resuiting pump power (63 [Kwjj stiii doesn't correspond to the engine power (75 [KW]) but is high enough to test the system. Now an appropriate motor has to be selected. The mechanical working conditions of the hydraulic motor have to correspond to the engine map of the Bora engine. Figure 3.5 shows the working conditions of the available hydraulic motors supplied by the combined flow of pumps B+
C. The engine map of the Bora is plotted as well. The last part of the hydraulic motor number code corresponds to the displacement in [cm3/rev]. Operating motor models Fll-10, Fll-19 and Fll-39 in the light gray area (high motor speeds) is only allowed for a short time. It is clear that the mechanical work area of motor F10-39 corresponds the best to the engine map of the Bora. That's why this motor has been selected. The TI-ratio of the hydraulic motor (28603 [rad/s2]) is much more than the TI-ratio of the Bora engine (1550 [rad/s2]). The combined oil flow supplied by pump B and C can be adjusted between 0 and 180 [Llmin] each by changing the swash plate angle. The maximum pressure in the supply line is controiled by a relief valve. The pressure can be adjusted between 0 and 210 [Bar]. By controlling the flow and maximum pressure, the pump characteristic can be adjusted (figure B.2).1
Motorspeed [rpm]
Figure 3.5: Engine map of the Bora engine and the working conditions of the available hydraulic motors supplied by pumps B+C
Filter: After the pumps, a filter is mounted for filtering out the oil contamination.
Pressure pulsation damper: Due to the finite number of pistons in the pumps of the power supply unit, the oil flow is not constant. This causes a pressure ripple. The ripple is damped out by the pressure pulsation damper.
Hydraulic brake: The hydraulic brake consists of a hydraulic pump (Fll-58) and a variable resistance (Fig 3.4). In fact, the brake pump is the same kind machine as the motor
used. The only difference will be the direction of the energy flow. The machine is now limited to 4500 [rpm], about 150 [rpm] lower than the maximum motor speed.
The Bow t o the brake is supplied by both the hydraulic motor (discharge side) and pump D of the power supply unit (max 100 [Llmin] at 10 [Bar]). Because the flow from the discharge side of the motor is smaller than the desired flow for feeding the brake pump, pump D is necessary. A pressure relief valve sets the feeding pressure at a constant value of 10 [Bar] to prevent cavitation.
The mechanical energy transmitted to the brake pump is converted into oil flow resulting in a pressure due to the resistance. The pressure causes brake torque and can be adjusted by the variable resistance. Actually, the resistance consists of three different sized valves mounted in parallel. By opening or closing the valves, the resistance can be adjusted so the load characteristic changes (figure B.2).
P r o p o r t i o n a l directional valve: The proportional valve is the most important part of the rig. The valve is responsible for generating the desired dynamic torque. The chosen valve (Bosch PL-NG 6) has the largest bandwith of the available valves in the laboratory.
The data sheet can be found in appendix D. The valve is a small system consisting of the actual valve (mechanical part) and an electronic part (actuation and position control).
The valve has four connections ports: P, T , A and B (see appendix). For normal use, the first two ports have to be connected to the P u m p and Tank. A and B have to be connected to the supply and discharge of the controlled part (cylinder or motor).
Because the valve can reverse the connection from port P and T to ports A and B, the cylinder and motor can move in two directions. In neutral position, the valve is closed.
In normal position, port P is connected to port A and port T is connected to port B.
In reverse position, port P is connected to port B and port T is connected to port A.
Unfortunately, the size of the valve is small, the nominal flow through is only 4 [Llmin]
at a pressure diEerence of 35 [Bar] per port. Because no return flow is necessary (only a flow from the high-pressure pipe to the reservoir), a external connection can be made between port T-A and P-B. Now the flow through the valve is doubled. Figure 3.4 shows the valve with all external connections used. The distance between the valve and motor will increase due to the external connections.
In this report, the spool position is indicated by dimensionless variable u
[-I.
If the spool is in neutral position, u = 0. If the valve is completely opened in normal position/
reverse position, u = +1 respectively u = -1. u d represents the desired spool position, u, represents the actual spool position.The electronic part of the valve consist of a internal amplifier, and controller. The desired spool position is presented to the electronic part as a voltage between -10 and +10 [V] (corresponds to u between -1 and +I). The electronics activate and solenoid
that puts the spool in the right position. An internal sensor measures the actual position of the spool continuously. The position is fed back to the internal control system.
Needle valve: An extra valve is mounted close to the proportional valve, between the pul- sation damper and the valve connection. The valve improves the dynamic behaviour of the system. The partially closed valve (resistance) is a barrier for the pressure drop.
?Vhen the prcpcrticnal valve cpens, only in the pipe between neelde vahe and the mctor the presswe decrezses. When the needle v&ve ~~ou!d he e ~ i t t e d , the pressEre in the entire supply pipe drops. In that case, the pressure drop would not be that high. More about the influence of the needle valve in section 3.2.3 and 4.2.2.
Hydraulic medium: The hydraulic fluid in the system is Mobil DTE-25 hydraulic oil. The fluid properties for modeling the system (next section) are the density p [ k g / m 3 , the
Figure 3.6: Oilproperties: bulk modulus, density and dynamic viscosity The bulk modulus
modulus represents
of the oil is comparable with the E-modulus of steel. The bulk the oil stiffness: