Hydraulically controlled Crane

Transcription

1 LiU/IEI Laboratory lesson 4 Hydraulically controlled Crane Laboratory work in the course Fluid Power Systems (TMHP02)

2 LiU/IEI Introduction The goal of this laboratory work is to give the students a feeling of how to control a crane by using a hydraulic system. The application also shows how the control facilities can help the operator to simultaneously control several functions and how the software in the electronic system can be used for design of control facilities and testing of hydraulic and electronic systems. Hardware Hydraulic system Below you will find a short description of the hydraulic system used in the crane. Figure 1 show in principle the hydraulic circuit for control of the two cylinders on the crane boom, which will be studied in this laboratory exercise. The real crane has 6 actuators and 6 control valve sections, one for each actuator. S1 F 1 v 1 S2 F 2 v 2 q p p p q L1 p L1 A p1 q L2 p L2, Ap p2 S p Lmax Figure 1: Load sensing hydraulic system of constant flow type (constant pump flow). Pump The pump is driven by an electric motor and maximum pump flow is about 15 litre/min. The pump pressure is adjusted according to the max load pressure (p Lmax ). In order to satisfy the pump pressure control function it is important that the sum of commanded load flows are less than the available pump flow, otherwise there will be a drop in pump pressure and the load flows will be disturbed. This behaviour requires special control functions to maintain simultaneous control of several loads. Therefore, a flow-meter is available to measure the actual pump flow. Valves The valve block on the crane is from Parker Hannifin. The control valves are L90LS with six valve sections, (one section for each function on the crane). Parts of the valve block used on the crane are shown in Figure 2. Figure 2: Valve block, L90LS (pressure compensated valves for load sensing systems).

3 LiU/IEI The control valves are quite advanced with several features. Some of the features are overpressure protection, anti-cavitation on each valve port, pressure compensation and load sensing lines. According to Figure 1 the pump pressure is controlled by the highest load pressure, which makes the system load sensing. The control valve spool positions can be adjusted both manually and with electric signals via proportional magnets. The flow characteristic of the control valve (L90LS) is illustrated in Figure 3. The area around zero has a dead-band (dödband) because of mechanically over-lapped spool. Inside the dead-band area the load flow is zero. The valve working range (arbetsområde) is the area where the load flow is a linear function of the control signal (X v ). Outside the working range the saturation area (mättnadsområde) is located. In this area the load flow is saturated because of mechanical limitation of spool displacement. Figure 3: Valve characteristics Load flow (q) versus control signal (X v ). Cylinders The crane in the laboratory is equipped with totally six hydraulic cylinders and one small motor. There are two main cylinders to lift the crane boom. The other actuators are for opening of the gripper, rotation of the gripper, control of telescopic boom length and rotation of the crane boom. The two main cylinders are Cylinder 1 and Cylinder 2, see Figure 4, and they are equipped with position transducers. Figure 4: Location of the two main cylinders.

4 LiU/IEI Crane control system The complete system consists of crane structure, hydraulic system and a computer based control system with transducers and software. A preliminary description of the computer and the hydraulic system are presented below. A sketch of the crane system is shown in Figure 5. This figure is good as a reference for further study of the crane system. Figure 5: Schematic sketch of the crane control system. IQAN IQAN is an electronic control system from Parker Hannifin, which consists of different units connected to a data bus of the type CAN. The units used in the crane system are (see Figure 5) a main computer unit/master (MDL) and one expansion unit (XA2) to the main computer. The command signals are created by two three-axis joy-sticks (styrspakar, LM) used to control the motion of the crane. The signals to the control valves (as S1 and S2 in Figure 1) are calculated in the main computer according to command and feedback signals. The expansion unit make it possible to handle several inputs and output signals to the main computer. Programming In the exercise you have to study some of the PC-program used for working with IQAN. The program IQANdesign is used as a base for all of the tasks. From this program you can start simulation tools as well as diagnostic tools. Interface is included in the software of the main display unit (MDL). The two joy-sticks placed on the operator chair are connected to the MDL-unit, which send the signals to the I/O-unit. The MDL-unit is continuously updated to show important information for the operator. User interface In Figure 6 the main display for the user interface is shown. The functions on the display-unit are presented in Table 1. It is notable that all the start up functions is located on the main display. The hydraulic pump is started from the MDL-unit as well as the system key up and

5 LiU/IEI selection of control program. Not that the button A shows previous window and the button B shows next window. Figure 6: Main display for user interface (MDL). Security Button A B C D F1 F2 F3 F4 F5 Function In main window: to windows for flow. Else: previous window. In main window: to windows for flow divid. Else: next window. To page about common geometry. To page about security and operator status. Starting pump, only in the main window. Stopping pump, available in all windows. Crane tip control, ON/OFF. Limitations for moving directions, ON/OFF. Valves ON/OFF. Table 1: Main display functions (MDL). The security system is primary used for setting of the limitations of the boom working area during remote control of the crane. At manual operation of the crane there are no electronic security limitations in action. With remote control of the crane the control system shall be adjusted for only radial and height directions. This function can be eliminated if you want to change the telescopic boom length. The electric motor for the pump can always be stopped but can only be started from the main window on the MDL-unit. Furthermore, the control system can be forced to not accept some commands from the joy-stick and that way close the valve. An important facility is the emergency stop mounted close to the valve block on the crane. Geometry The crane is a down sized lorry crane. This type of cranes has normally a boom consisting of two links where the outer link has a telescopic function. The telescopic boom shall be adjusted for minimum length during the laboratory exercise. This will limit the working area

6 LiU/IEI for the crane as shown in Figure 7, but it will also reduce the degrees of freedom in the equation system for the crane geometry. In turns this makes the equation system solvable. Note that with a constant boom length (no telescopic action), the crane geometry will be similar to an excavator. Make sure that the working area in Figure 7 is limited so the crane tip not comes too close to the crane pillar and the floor, see Figure 8. Figure 7: Working range with retracted telescopic boom (min boom length). Figure 8: Picture of the crane used in this laboratory exercise.

7 LiU/IEI Home tasks The system used for the laboratory exercise is a complete hydraulic system, but in miniature. The most important parts of a hydraulic system are there together with a number of transducers. Preparation task 1 Draw a schematic hydraulic circuit for the crane application with help of this PM and the formula book. The following components shall be included in the circuit. 1. Electric motor 2. Load sensing pump with LS-controller (hydraulic pilot control) 3. Select one or more crane control functions and draw the valve sections for these. Use 4/3 direction control valves with electro-hydraulic pilot control (pressure reducing valves). A complete description of the real valve L90 on the crane is not required Main pressure relief valves. 4. Cylinders according to the valve sections in point The flow meter in the system 6. Pressure transducers and or position transducers to the functions shown in point 3. Extra task The control valve (4/3) has a pilot circuit to adjust the spool position in the main valve electrically. Add to the hydraulic circuit a pressure reducing valve to feed the pilot valves (also pressure reducing valves) for the main control valve spool, so it is clear how the pilot circuit is used to control the main spool. Preparation task 2 The control of the crane tip motion requires that the cylinder motion must be co-ordinated. This co-ordination means that the flow to each cylinder is determined in every time point according to the desired velocity from the control levers. If several functions are operated simultaneously there is a risk that the operator asks for a load flow, which is higher than the maximum flow from the pump and some load will stop. Think about some functions (hydraulic or electric), which help the operator to control the crane without any unplanned load stops.

8 LiU/IEI Achievement Task 1 Have a look at the crane system, both hardware and user interface. Try to find the locations of the components 1-6, from the Preparation task 1. Start up and stop the system from the control panel on the crane and test the emergency stop. Observe that there is no security system in action with manual control of the crane. Task 2 Start up the system and try to control the crane by using the manual levers on the valve block. Control one function each time. In which order are the 6 valve section placed Task 3 Control several functions simultaneously. Try to follow a straight line for the crane tip. Try to move slow and fast. Do any functions want to stop? If so, explain which function stops and why? Stop the system when you have finished the test. Task 4 Make an error search and download a program from a PC to the main system computer. To be able to use the control levers on the chair a program, a so called application-file, (*.ida), must be loaded into the MDL-unit. This has to be done ones or if any changes have been made. Open the program IQANdesign and look after the file minikran.ida and open it. An important step in development of applications is to avoid faults. IQANdesign has several options to check that no faults appear. By using application check, which can be reached from the menu Application (Ctrl+F9), it s possible to check if there are any serious faults. If a fault has been detected, the application can not be sent to the control system, without a correction of the fault. By a Double-click on a fault message it is possible to navigate to the position of the fault in the program. 4.1 A lazy designer has forgotten some details in the interface correct these misstakes. Select some suitable input channels and make a new application check. Another important feature is the possibility to simulate the complete system. To make this work the application must be free from faults. Start the simulation in the menu Simulate (F9) or with the play-button. Now you can see a new window in the program IQANsimulate.

9 LiU/IEI If the simulation is running it s possible to see what happens when the lever signals are changed. Use the simulation interface to activate the crane, which make it possible to control the output signals. Check that no outputs are active, when the levers (joystics) are in neutral positions. 4.3 The same lazy designer has made more misstakes, so the program generate an output signal even when the outputs from the joystics are zero. Follow the calculations in the simulation model by using the reference lines between the channels. When the fault has been rezogniced the simulation must be stopped before the fault can be corrected in IQANdesign. Correct the fault and verify the functionality by simulating the application again. Observe that it s possible to follow the calculations in IQANdesign when the simulation is running. 4.4 When all faults are detected the application can be downloaded to the main computer. Do this by selecting Send application in the menu Communication (F11). Check that the USB-cable from the system is connected to the PC. Don t start the electric motor yet. Check one more time that there are no signals on the outputs to the control valves. Use, alternatively, the menu Measure (F7) and check that the output signals are zero, when the joystics are in neutral positions.

10 LiU/IEI Task 5 Start up the electronic system via the interface and try to control the crane by using the control levers on the chair. Activate the valve pilot circuit by switching to Remote control and start the pump. Which lever and direction are now controlling the crane? Task 6 Control several functions simultaneously. Try to follow a straight line for the crane tip. Try to move slow and fast. Since it s relatively easy to control several functions simultaneously with high velocity the load flow consumption tends to be higher than available pump flow. Do any functions want to stop now? Task 7-10 The task is now to correct and understand what happens in the system. For this purpose you have to use the tool IQANrun, which is started from the menu Tools in IQANdesign. Start the programme and follow the instructions there to continue with the exercises.