Hydraulic/Pneumatic Positioner

Transcription

1 Hydraulic/Pneumatic Positioner Purpose of Lab The newest experiment in our lab is the hydraulic/pneumatic positioner. This experiment has all new, industrial-grade components. It is controlled by software running on the computer at each station. Each computer is equipped with a National Instruments data acquisition (DAQ) board. This allows the computer to sense voltage measurements from the outside world and to send voltages out to the outside world. We designed and built this system at Cal Poly through a grant from Parker Hannifin Corporation of Cleveland, Ohio. Parker is the leading supplier of hydraulic and pneumatic actuation systems in the world. Parker s Aerospace Division is in Irvine, California. There they make actuation systems that are used for a variety of aircraft applications. The motion of airplane control surfaces and aircraft landing gear actuation are examples. Parker donated the money to Cal Poly because it feels that fluid power systems are not as prominent in engineering curricula as they should be. Though the experimental set-up is not complete, this experiment is designed to have you look at the system, become familiar with its components, and design the control set-up for the experiment. You will not actually run the experiment. You will come up with a preliminary version of a block diagram for the system. Hydraulic/Pneumatic Positioner The hydraulic/pneumatic positioner is similar in purpose to the Motomatic. Both systems are positioners as opposed to regulators. This means that the control loop expects to see frequent changes in the desired input and reflect those changes by changing the actual position to match those changes. Figure 1 shows the positioning system. The cart on the linear slide in the center of the base is driven by either the pneumatic cylinder on the left or by the hydraulic cylinder on the right. The cylinder that is not being used to drive the cart can be used to load the cart. Only one cylinder at a time can drive the cart. In a typical usage in the lab, we will run only one of the two cylinders. The big differences between the Motomatic and the hydraulic/pneumatic positioner are Motomatic motion is rotational while the hydraulic/pneumatic positioner s motion is linear. The Motomatic is an electro-mechanical system. Its actuation is via an electric motor. The Motomatic control hardware is analog (discrete electrical components) while the hydraulic/pneumatic positioner s control hardware is all digital.

2 Pneumatic Servo Valve Servo Amp Interface Board Hydraulic Servo Valve A B Hydraulic Cylinder Pneumatic Cylinder Linear Potentiometer Figure 1 - Hydraulic/Pneumatic Positioning System Mechanical Components Actuators As mentioned above, the mass cart can be driven by either the hydraulic or the pneumatic cylinder. Note the disproportionate size in the two cylinders. The primary reason for this is that the hydraulic cylinder s pressure source is hydraulic oil at about 1000 psi while the motive fluid for the pneumatic cylinder is air at about 80 psi. Note that the cylinders are double-ended, i.e. a rod comes out of either end of the cylinders. Such cylinders are somewhat abnormal in industrial uses. Usually a cylinder has only one rod coming out of one end. This makes the areas on the piston inside the cylinder different for the rod and blind ends. We chose double-ended cylinders to avoid adding this non-linearity to the control loop. Each cylinder has a fluid line running into each end of the cylinder. Thus the cylinders can be actuated in either direction. Generally these two lines are denoted the A and B ends of the cylinder (Figure 1). Note that on each cylinder there is a cross connection between the A and B lines with a needle valve in the cross-connection line. To drive the cylinder the needle valve needs to be closed so that fluid to, say, the A side of the cylinder does not just run into the B line. When the pneumatic cylinder is being used to actuate the system, the needle valve on the cross-connection of the hydraulic cylinder should be open. When the hydraulic cylinder is being used to drive the system, the needle valve on the pneumatic plumbing should be open. When either needle valve is closed, the system is locked when the power is off. Fluid cannot flow freely between the cylinder ends so the cylinder will not move.

3 Sliding Mass The sliding mass rests on a linear ball slide. The weight of the mass is marked on it. You should note it. You can move the mass back and forth by hand. To do this, unlock the cylinders by opening the needle valve connecting cylinder ports A and B on both cylinders. Push on the mass to slide it back and forth. Note the travel distance of the mass. This distance will be important for designing motion control experiments with this system. Another important parameter is the stiction force on the cart. Have both members of your group estimate this force. Maybe with enough people guessing, we can come up with a ballpark figure for this. Servo Valves Fine motion of the cylinders depends on the careful metering of air or hydraulic oil to the cylinders. This control is supplied by the two servo valves in the system. These are the two black components marked with the logo Dyval. Both of these valves are 4-port valves. Each has a pressure port (P), one port for each end of the cylinder (A and B, Figure 1) and an exhaust or return port (R). Note that the servo valves are electrically actuated. Inside each is a small device called a torque motor. This motor is current controlled. The current can vary between ±50 ma. A positive current will send flow to port A and a negative current will send flow to port B. A large current will result in a large flow to the cylinder, and a small current will result in a small flow. Assume that the flow to the cylinder is a linear function of the current to the servo valve. When the servo valve is actuated, flow will be directed to one side of the cylinder. At the same time, the servo valve internally ports the opposite side of the cylinder to the return/exhaust line (to atmospheric pressure). You will need to determine the conceptual nature of the block diagram for both the servo valve and the cylinder. What are their inputs and outputs and how will they be linked together? As mentioned above, only one servo valve at a time is used to actuate the system. A three-way selector switch (pneumatic/null/hydraulic) will be used in the system to choose between the two valves. This should be noted on your electrical diagram. Data Acquisition/Control Components Data acquisition and control are done by the computer connected to each station. Each computer has a National Instrument s DAQ card in it so that the computer can receive from and send to the outside world analog voltage signals. The computers are equipped with National Instrument s LabVIEW software. With this software, virtual instrument panels can be constructed on the computer screen and different control schemes can be

4 programmed to control the experiment. All computer control software works as follows. 1. Read data from sensors 2. Calculate control response 3. Send control response out to actuator 4. Unless otherwise directed, go back to 1. Thus the computer works in a very fast loop that repeats continually until interrupted. Instrumentation Linear Potentiometer In front of and below the hydraulic cylinder is another cylinder-like component. This is the linear potentiometer for the system. Like most potentiometers, it is supplied with a positive and negative voltage at either end of a resistor (see the pot schematic on the Motomatic s θ pot). A third lead from the pot picks up a voltage between the supply voltages to the pot. The value of this voltage depends on the position of the pot slider. The linear pot is supplied by voltage from the servo-amp. The DAQ card expects to receive voltages between ±10 VDC. As part of the experiment, you should work out the transfer function for the pot. Think about what the input and output are. To characterize the pot, work out a way to use resistances and an ohm meter to get a transfer function. Accelerometer Attached to the mass is an accelerometer. The output from this is a charge that then is fed into an amplifier to produce a voltage. Assume the charge/voltage relationship of the amplifier is linear. Acceleration will not be used in the control loop. Instead, acceleration can be integrated up to velocity to form an inner loop in the control loop. This is often done. We did not do this with the Motomatic, but you can look at the Motomatic s schematic on the control box and see how this is configured. The voltage from the accelerometer amp will be sent to the DAQ card. The integration to velocity from acceleration will be done in LabVIEW. This is one of the reasons for using digital control. Capabilities can be programmed into the control algorithm of the experiment. Rotometers The experiment is equipped with hydraulic and pneumatic rotometers. These measure the

5 flow rate of the fluid through the servo valves. These are not electronic components, unfortunately. They were very expensive. So if they are used in component characterization tests, data from them will have to be manually taken and entered into the computer. Data Acquisition Card The DAQ card is a National Instrument s PCI-MIO-16E-4 general purpose card. It is mounted in the computer, so you will not see it during this experiment. A cable from this card can be seen coming through the back panel of the lab bench. It is connected to the SC-7075 interface board. So inputs and outputs from the experiment are connected to the SC-7075 and from there are connected to the DAQ card in the computer. The board can be used in a number of ways. We are using the analog-to-digital (ADC) input channels into the board to sense the motion of the mechanical slider. We are using the digital-to-analog (DAC) output channels from the board to send a voltage control signal out to the experiment. The board has 8 ADC channels and 2 DAC channels. You will need to specify in your lab results which channels are used and what they are used for. Analog input or output refers to the voltages that the DAQ card receives or sends out. The word analog contrasts with the word digital. A digital signal is either high or low, off or on. An analog signal can vary continuously over a range. Instead of sensing whether something is off or on (input) or turning something off or on (output), the voltage level of the signal indicates where something is (i.e., its position), its speed, its level (tank), or its temperature. As output, the signal s strength reflects the vigor with which you want to push something. The push could be mechanical, electrical, fluid, etc. The DAQ card also has digital I/O capabilities, but they are not used in this experiment. The DAQ card can work with several voltage ranges: ±5 VDC, ± 10 VDC, 0-5 VDC, or 0-10 VDC. In general, the larger the range used, the better the resolution on the input or output. As part of the experiment, you should specify what signal levels each of the signals connected to the DAQ card will have. You may have to add resistors to get these voltages. The DAQ board has 8 differential analog inputs (Analog Inputs: ±CH0-CH7) and 2 analog outputs (Analog Outputs: CH0 & CH1). These can be seen on the SC-7075 interface board. Interface Board

6 The SC-7075 is a National Instrument s interface board. It is the green board to the right on the lab bench backplane. It is used to connect the DAQ board in the computer to the outside world. The analog input channels are located at various points on the board. Three different terminal types can be used. We plan to use BNC cables. The two analog output channels are located in the upper center of the board. They have BNC connectors also. You will notice that the interface board has provisions for an external 5 VDC power supply (silver box on upper left hand side of board). We shall not use this and will get power from the computer. The large breadboard area on the interface board is available for any auxiliary wiring or supplemental electrical components that need to be added. Use it if you need to. Terminals Analog Outputs Analog Inputs Servo Amp Analog Inputs Analog Input Interface Board Figure 2 - Servo Amp and Interface Board Servo Amplifier The servo amp is a custom board manufactured by Parker to drive its servo valves. It is common for a manufacturer to produce an electronic board to drive a servo valve, a motor, etc. The servo amp is the left-hand green board on the lab backplane. It is powered by 110 VAC/60 Hz electricity. For our purposes, the servo amp is simply an amplifier that converts a control signal into it into an output current signal to drive the servo valves. The input signal is ±10 VDC. The output signal is ±50 ma-dc. You can assume that the input/output relationship is linear.

7 You will find the connection terminals on the servo amp on its right side. The important terminals are given in Table 1. Terminal Connection A External Power Ground B 120 VAC + C 120 VAC - G +15 VDC H Signal Common I -15 VDC J Command Input (±10 VDC) N Current Return from Valve O Current to Valve All voltages in the control loop should be measured with reference to Signal Common, not the External Power Ground. Design Procedure You have been given a description of the parts and pieces that make up this system as well as its purpose and how we propose that it be run. Consulting with each other, use this information to arrive at a wiring diagram for the system, providing all connections to make the system work. Besides this, build a conceptual block diagram (see chapter 1 in the text) of the system. Provide as much detail as you can. If you can supply an exact transfer function of a component (like the linear pot), do so. If you cannot, at least identify the input and output signals and then name the component for which a transfer function is needed. Deliverables You should produce and hand in these items: 1. Electrical schematic of the data acquisition and control system for this experiment. Wire the sensors so that they are supplied with the power they need (if any). The output from the sensors should be connected to the control computer. The control signal out to the real world should be shown in its entirety. Components should be shown and clearly labeled. Label also the signal lines to show their minimum and maximum voltage or current. You may need to add resistance at certain points to get the voltage levels you need for the DAQ. Calculate these resistance values and report them as needed. Give wire and connector types for all electrical connections. With your drawing, I should be able to wire up a working system. 2. Conceptual block diagram of the control loop. Give exact transfer functions if

8 possible. For those components for which you do not know the exact nature of the transfer function, label them and at least show their input and output. Put minmax ranges on all signals in the block diagram that you can. 3. Data collected to characterize potentiometer. Also include your calculations and an estimate of the pot s transfer function. 4. Also fill in the following table. You should attach calculations or reasoning used to acquire calculated values. System parameters Value Units Range of linear motion Pressure actuation area of hydraulic cylinder Pressure actuation area of pneumatic cylinder Mass of lead mass Mass of other moving parts Stiction force of cart