Pneumatic Positioner

Transcription

1 Pneumatic Positioner Safety Precautions Air can be a very dangerous source of motive power because of its compressibility. In the pneumatic positioning system, air can move the lead mass with surprising suddenness, force, and speed. Therefore, the air to the system should be shut off with the inlet shutoff valve unless you are actually running the system. During the course of this experiment, you may need to get close to the electronic components of the system for inspection and familiarization. When you do this, make absolutely certain that the air to the system is shut off. Since you will be working with a partner in this experiment, take care that your partner is not anywhere near the moving parts of the system when you are operating it. A good practice to adopt to run the system is to let your partner know to stand back before you turn on the air. Your partner should be instructed to remain at a distance from the moving parts of the system during the period that the supply air is on. While you are running the system, keep your hands away from any of the moving parts. Remember that the hydraulic cylinder also moves when you are driving the system pneumatically. Immediately when you are done with your active tests, turn the air supply off. Inform your partner that the air is shut off. If at any time the system starts to malfunction, click the STOP button on the VI and immediately shut the air supply valve. Shut off the power switch on the power strip on the backplane of the experiment station. This cuts power to the servo amp and servo valves. Since we are running the system with air, the needle valve connecting sides A and B of the hydraulic cylinder should be open. Turn this valve completely counterclockwise to open it up. Introduction The Hydraulic/Pneumatic Positioner is operated as either a hydraulic or a pneumatic servo system. The word servo generally implies position control. The system is the conceptual first cousin of the Motomatic system that you have already studied. A user commands an output to go to a certain location. The controller senses this command and orders the actuator to move the plant to this new location. This is the basis for teleoperated systems, where a remote operator causes movements that occur at a distance from the operator s location. We ve talked about airplane pilots operating the (remote) control surfaces on an airliner. Such systems are also found in marine remotely operated vehicles (ROVs) used for off-shore oil drilling and production platforms and deep-sea salvage. Maintenance and inspection of nuclear reactors is done with teleoperated systems.

2 Often, the device to be moved has substantial mass or rotational inertia. Such a system is challenging to control because the mass or inertia seems to have a mind of its own. It takes a large force or torque to start the motion and a large braking force or torque to stop it. For large inertial loads, motion systems often are equipped with hydraulic or pneumatic actuators. The positioning system in the lab is a linear positioning system, unlike the Motomatic. It can be actuated either hydraulically or pneumatically. In this lab we shall use pneumatic actuation. You will see that the pneumatic cylinder can produce a large force, capable of moving and stopping the mass quickly. Positioner Loop Anatomy You are already familiar with the anatomy of a conventional feedback loop, organized into controller, actuator, plant, and sensor. The pneumatic positioner conforms to this configuration. Part of the loop is made up of physical components. Part of the loop resides inside the computer that is used to control the mass position. The physical part of the loop includes: G A - Pneumatic servo valve + pneumatic cylinder G P - Mass on linear slide H - Position sensor (linear potentiometer) The pneumatic servo valve is electrically actuated. It receives a current (-50 ma < i < + 50 ma). The greater the current, the more air the servo valve ports to the cylinder. The greater the air supplied, the greater the velocity with which the cylinder moves. The cylinder, of course, has two ends. It can be driven in either direction, depending on the end to which pressurized air is ported. The end to which air is ported depends on whether the current is positive or negative. Thus the speed and the direction of the cylinder can be adjusted by the current sent to the servo valve. Since the cylinder rod is connected to the sliding mass, the rod motion and the mass motion are identical. It is normal to include the mass of the rod and cylinder piston with the plant mass. The motion of the mass is detected by a linear potentiometer. This device is conceptually like the rotary pots used on the Motomatic, except that the resistor is arranged linearly, not rotationally. Figure 1 is a schematic that shows the wiring arrangement of the linear pot. Note that V x is the pick-off voltage of the pot that contains the information about the x location of the mass slider. The voltage will range from -10 V to +10 V, depending on the position of the mass slider.

3 x - + V x -10 VDC Potentiometer +10 VDC Figure 1 - Linear Potentiometer Wiring The part of the loop inside the computer is the summing junction and the controller. Actually these are part of the software running in the computer. The software is National Instruments s LabVIEW program. It is very similar to Simulink except that in includes modules that allow the block diagram to be connected to physical devices. These modules allow real-world voltages to be converted into numbers (analog-to-digital conversion, ADC) and numbers to be converted into real-world voltages (digital-toanalog conversion, DAC). Thus LabVIEW reads the mass position by measuring the voltage coming from the linear pot. This position is then used by the LabVIEW software to determine the error and calculate a control command from the controller, also within LabVIEW. The digital control command is then converted into a voltage and output back to the real world. This voltage is converted into a current in the servo-amp, which is then sent to the pneumatic servo valve to control air flow to the cylinder. Figure 2 shows the major system components and how they are connected together.

4 +/- 50 ma Air to A Servo Valve Air to B Parker Servo Amp +/- 15 V SC V Interface Board Potentiometer +10 V +/- 10 V PCI-MIO-16E-4 Computer Figure 2 - Major Components of Pneumatic Positioner Purpose of Lab The lab has two main purposes: 1. Familiarize yourself with a computer-controlled motion control system. 2. Characterize as much of the system as possible. Speculate on the transfer function for the part of the system not modeled, based upon a step response test of the closed-loop system. Procedure System Familiarization Study the system and become familiar with its major components and how they are connected together. (You might want to use the lab handout Hydraulic/Pneumatic Positioner for this.) For each component, make an I/O chart telling what signals are inputs and which are outputs. Give the minimum and maximum value of each signal in or out of the device. Do this for the computer, the interface board, the servo amp, the potentiometer, and the servo valve. Each device s I/O chart should have the format:

5 Device: Tag I/O Units Minumum Maximum A B C The tags should line up between devices. For example there is a +10 volt signal out of the interface board, supplying input voltage to the linear potentiometer. The tag for this output on the interface board sheet should be the same letter as the tag for this input on the potentiometer sheet. Running the System Follow the safety precautions stated at the first of this handout when running the system. To run the live part of the experiment, you will use two LabVIEW virtual instruments (VIs). One VI is called OpenLoopController.vi, and the other is called PIDController.vi. These can be accessed by starting LabVIEW and then opening each VI in turn. The VIs can be run by pressing the start button (a little rightward-pointing arrow on the toolbar). They can be stopped by pressing the STOP button on each VI. Do not run the VIs simultaneously. The system will not run right if you do. One VI will be telling the system to do one thing, while the other will be telling it to do something else. This causes unnecessary wear and tear on the servo valves. First run the system using OpenLoopController.vi. Make sure the air is turned on before running the system. Start the VI by clicking the start button on the toolbar. With this VI, you can control the output voltage to the Parker Servo Amp manually with a slider control on the screen. Practice driving the mass left and right using this control. Please try to avoid running the mass onto the stop at either end of its travel. After getting familiar with system operation using this VI, stop the VI using its STOP button. Close the VI. Now open PIDController.VI. Run the system using this VI by pressing the VI s start button on the toolbar. The system should center itself at its 0 position. You will perform a step response test of this system. Move the position slider to 1.5 inches. Now click the toggle switch to the left of the position slider in the VI. The system should move to 1.5 inches. Click the toggle switch back to off. The system should return to 0. Once you have gotten familiar with how to run the system, stop PIDController.vi using the VI s STOP button. Turn the air to the system off. Now explore the internal functioning of the VIs. Each VI has a wiring diagram, similar to a Simulink model. This diagram can be accessed from the VI panel by typing CTRL+E. Start with OpenLoopController.vi. Print this diagram out so that you can write on it as you are trying to figure it out. The input channel named x is the voltage input

6 from the potentiometer. The output channel named u is the voltage output to the servo amp. Now open PIDController.vi and access its diagram. It is a bit more complicated. It contains the subvi Simple PID. This subvi and its diagram can be access by double clicking on it and then typing CTRL+E. You may want to print these diagrams out for study. You probably will not understand all of the details of this VI and subvi. But I would like for you to be exposed to it, just so you can get some idea of its general configuration. Some of you may be interested in becoming more involved with LabVIEW and projects involving its use. Characterization of Potentiometer Figure 1 shows the wiring of the potentiometer. The pot is supplied with +/- 10 V at either end of the pot s resistance element. A mechanical wiper arm slides along this resistance element and picks off a voltage between -10 V and +10 V, depending upon its position. If it is midway between the ends of the pot, the pick-off voltage will read 0 V. You will characterize the pot using resistances instead of voltages. Make sure that the air to the system is turned off and that the power to the servo amp and thus to the pot is off (use the power strip on/off switch to shut off this power). Connect an ohmmeter to the pot. Put the negative end on the -10 V lead to the pot. But the positive end of the ohmmeter on the pick-off voltage lead. Now slide the mass from its leftmost position to its rightmost position in steps. For each position, read the resistance. This will give you a table of resistance between -10 V lead and pick-off lead. You should be able to convert this into a pot transfer function where x is input in inches and voltage is output. Characterization of Servo Valve This is a live, air-on test so follow the safety precautions given at the first of this handout. Hook the ammeter (see below) into the circuit before turning on the air. Before turning the air on, open the needle valve connecting sides A and B of the pneumatic cylinder. This will allow the air to bypass the cylinder and flow straight from the pressure source to atmosphere (through a muffler on the back of the lab bench). This is a steady state test. What you are interested in is the relationship between current to the servo valve and flow produced by it. To read current to the valve, you will have to install an ammeter in the electrical lead from the servo valve. Remember that an ammeter is installed in series to read current. So you will have to disconnect the lead from the servo amp and install the ammeter in this circuit. To do this, get assistance from the lab instructor.

7 The flow output from the servo valve will be read from the flow meter mounted on the backplane of the lab bench. Use OpenLoopController.vi to drive the servo valve. Start at 0 output voltage and increase the output voltage in steps to its maximum. For each voltage out, record V u, i, and Q. Once you have made your table, shut off the air supply to the experiment. Remove the ammeter from the circuit. Note that you probably will not use this characterization in your system model. This test models air flow to the cylinder without load. When a load is included in the system (by closing the cross-port needle valve), the flow through the valve must work against an inertial and frictional load, which changes the Q/i transfer function. We may use this characterization on our inverted pendulum experiment. Step Response Tests This is a live, air-on test, so follow the safety precautions given at the first of this handout. Hook the oscilloscope up to the experiment before turning on the air. Here we will run three separate step response tests to compare responses with different controller gains. The controller gains are K p, K i, and K d. They are set by three knobs on the panel of PIDController.vi. Be very careful when entering these values. A mis-entered value could cause quick, dangerous motion. Also be ready to click the VI s STOP button if the system motion begins to oscillate too wildly. To run a step response test, you will have to connect an oscilloscope to the potentiometer s pick-off voltage. The BNC ground should be connected to the signal common. Get help from the lab instructor before running the system. You will perform step tests with three different sets of controller gains. They are: a. K p = 80, K i = 0, K d = 0 b. K p = 100, K i = 0, K d = 0 c. K p = 100, K i = 0, K d = 20 For each test, start with the toggle switch in the OFF position. Set the desired position to 1.5 inches. Click the toggle switch to ON. With the oscilloscope triggering set correctly, you should get a plot of the potentiomenter output as a function of time. Plot out the response curve for each of the three tests. When you have collected your data, turn off the air supply to the system. Turn off power to the power strip on the backplane of the lab bench. System Modeling

8 From the tests run and information collected, you should have data to construct a partial model of your system. What is missing is the actuator model and the plant model. That is, what is unknown in the experiment is G A G P. But you also have results from a closedloop step response test of the system. Your task is to try to come up with a transfer function for G A G P through a process of reasoned trial and error. Create a model that has the known loop transfer functions in it (controller, potentiometer). Look at the results of the closed-loop response test (use case a. for this). Try to decide what type of transfer function could be used for G A G P so that the simulation of the closed-loop system will produce a good match with the step test of the real system. This may take several trials and some tuning. See how good a match you can get. Deliverables 1. A conceptual block diagram of the system. Boxes should contain names of components (not their transfer functions). Signal lines should include units (voltage, inches, etc.) and the max and min values expected for the signal. 2. A symbolic block diagram of the system. Same as above but with variables replacing component names in the blocks. 3. A numerical block diagram of the system. Include units on the signal lines and above or below the blocks. This can be done in Simulink. 4. A characterization graph for the potentiometer. Use an ohmmeter to characterize the pot. 5. A characterization of the servo valve using as input current in and as output air flow rate. 6. A step response plot (from the oscilloscope) for the closed-loop system at K p = 80, K i = 0, K d = 0. Your step response plot should show what happens when you move the system from 0.0 inches to 1.5 inches. 7. Matlab output of a step response from your Simulink model. This plot should also include the step response from the oscilloscope (case a.), so that you can compare your model s response with the real system s response. Because of the unknown nature of part of the system, you may have several iterations of model tuning shown here. Every time you modify your model to get a closer match with the real system s response, plot the model s step response. On the final plot, make sure to identify the variant of the model that produced each response. You can do this by labeling the responses with numbers or letters and then providing a legend or key that describes each model variant. 8. Report your suggested transfer function for G A G P. Report what is good and what is bad about your transfer function.

9 9. Matlab plot showing the response of the actual system at a. K p = 80, K i = 0, K d = 0; b. K p = 100, K i = 0, K d = 0; and c. K p = 100, K i = 0, K d = 20. Write a paragraph or two that explains the important parts of what happens to the response in going from a to b and then to c.