PRACTICAL DESIGN FOR RUT tenis = MOTION CONTROL PETER E. NACHTWEYFluid power offers many unique advantages. For example, hydraulic ‘actuators can it and hold heavy loads without the need for braking, move heavy objects smoothly af fast or slow speeds, and apply force of torque efficiently without the need for gear boxes, while consuming less space and producing less heat atthe actuator than electric motors. Hydraulic actuators are comparatively small even for applications that involve heavy loads. While electric motors must be sized for the maximum load that willbe applied. hydraulic pumps need to be sized only forthe average load with an accumulator used to store energy to handle peak loads ‘And when fluid power is combined with modem motion contro! technology, these benefits can be realized along with precise motion {and positior/force control, automated tuning, and easy integration with other control components. Yet some people stil avoid hydraulics ‘even where itis a great ft. Why is this? One thing Delta has observed Is that some people turn to electromechanical actuators because they understand electric motors better. Years ago, we began to notice that ‘one of the most popular downloads from Delta's website was a simple ‘one-page “Do's and Don'ts” of hydrauile motion control. We realized that there was a need for more practical knowledge on this subject Fortunately, Delta's President, Peter Nachtwey, has a deep and broad ‘understanding of hydraulic motion control on both the theoretical ‘and practical sides. He has been on-site at many diflerent industria! ‘applications - from sawmills in North America, Australia and the UK to stee! mils in Turkey and India. Peter began a series of articles, ‘on various aspects of fluid power motion control, in order to make easier to design highly successful systems. This effort eventually resulted in this Practical Fluid Power Motion Control Design Guide ‘This guide covers key components in a fluid power system and shows how to use modem motion controllers to precisely control position, velocity, pressure/force or torque in industrial fuid power or hybrid ‘machine applications, ‘We hope this Practical Design Guide will help you to make your ‘control system applications more successful - whether you use hydraulic, pneumatic, or electric control. Your choice should come down to ft — what technology works best for your application. With the knowledge gained from this Guide, you will be able to choose ‘a motion controller with the special capabiities needed for high performance hydraulic motion control. And a flexible. powerful, yet ‘easy-to-use motion controller such as the Delta RMC will also work ‘great to control electric motors, allowing one controller to be used for multiple closed-loop applications, whether itis controling hydraulic, electric, or any combination of hydraulic and electric axes.eter & Nachtivey About the Author Mr. Nachtwey has more than 25 years of experience developing hydraulic, pneumatic, electronic and vision systems for industrial applications. He graduated from Oregon State University in 1976 with a BSE and served in the U.S. Navy until 1980, He became president of Deta Computer Systems, Inc. in 1992. In adation to leading Delta's engineering and R&D programs, his articles ‘are published in industry publications such as Hydraulics and Pnoumatics and Machine Design. He has also presented technical papers for IFPE NFPA, FPDA and various technical conferences. Control 3... 1-1, On-Off ys, Closed-Loop Control Closed-Loop Position-Pressure/Foree Control 10.... 1-3. Feed Forwards Make Closed-Loop Tuning Easier 14... 1-4, Selecting the Right Control Algorithm 18, .. 1-5. Tips for Precise Pressure Control 22... 1-6, Closed-Loop Control Examples in Cyclical Testing, Connect 26... 21. Correct Cyhinder Siz System Operation is Critical to Proper Hydraulic Servo 28... 2-2. Choosing the Best Valve for Servo Hydraulic Control 33... 2-3, Avoiding the Wrong Valves 36.... 2-4, Accumulators: Unsung Heroes of Hydraulic Motion Systems 40.... 2-5. What You Should Know About Rotary Encoders 44... 2-6, Pressure Transducers Boost Precision and Reliability in Hydraulic Applications Optimize 47... 3-1, Optimized Control Requires Fine-Resolution Feedback 50.... 3-2. Electronic Filtering Solves Problems 54... 3-3, Adaptive Hydraulic Control 58.... 34, Active Damping Saves Energy 61... 3-5. Modeling and SimulationCONTROL me, Miiith 1-1. On-Off vs. Closed-Loop Control The decision to use on-off contro! one can get by using only open-loop vs. closed-loop control requires control with the same feedback options consideration of several factors. In as with an on/off valve. Finally actuality, comparing on-off control there are high-end systems that use (sometimes called “bang-bang” an encoder or MLDT for feedback control because of the noise that “closing the loop" with a servo valve an actuator makes when it stops or servo-quality proportional valve abruptly) to closed-loop control is The decision options can be arrang not an “apples-to-apples” comparison in a table with the feedback options because on-off control only specifies on one axis and the control options on the output whereas closed-loop servo another (see Table 1) control implies linear feedback and The two options marked "NA" in : linear output control Table 1 are not applicable because closed-loop control cannot be done Factors to consider without continuous feedback (limit switches or photo eyes are not For instance, one can use on-off valves with no feedback, with discrete limit switches or photo eyes to provide coarse position information, or they Feedback cean be used with continuous position Obviously the least expensive or velocity feedback. Continuous feedback method ts to use none at feedback can come from an analog This "no-feedback" control is often feedback device, an encoder, or a augmented by operators that “close the megnetostrictive linear displacement g their vision and reflexes transducer (MLDT), Even if one is actuators back and forth. , using a ser¥0 oF proportional valve, For aulomation, this method provide ted flexibility in that the only enough), there are many applications wherepositions that can be reached with Certainty are the ends of the cylinder The actuator will also travel at varying speeds dependent on changes in load, pressures and how warm the oil is. However, in many applications this method is adequate, Discrete feedback (limit switches and photo eyes) is the next step up. This option uses either on-off valves or proportional valves to drive the hydraulics. Although itis possible to hard-wire limit switches and photo eyes directly to the valves or use mechanical relays, the downside result is a very inflexible system. Changing the wiring can be very expensive and the locations where the limit switches ‘and photo eyes are mounted may have to change with different manufacturing needs. Also, it ean be very difficult to physically mount the limit switches and photo eyes where they need to be. So, to provide the needed flexibility, computers or programmable controllers are normally placed between the detectors and the valves. Because of this, a large cost that isn’t immediately obvious, but must be considered, is the cost of programming. The most flexible form of feedback is the one that provides continuous position feedback. MLDTs mounted inside or along-side of cylinders provide excellent position feedback With continuous feedback, velocity can be determined by detecting how fast the position changes. This information is critical in applications where motion ‘must be accurate and repeatable, Options for controlling outputs Open-loop control with on-off valves Using on-off valves is the simplest, but most limited way to control hydraulic flow. The ability to control or synchronize motion is difficult; however itis possible to use multiple bang-bang valves for rough control of speed. In the past, it was ‘common for systems to use high- and low-flow valves, mounted in parallel Decreasing the flow rate of oil to decrease the speed of the actuator required shutting off the high-flow valve as the actuator approached the set point. This left the low-flow valve oon, eausing the actuator to ereep close to the set point when it, too, was shut off. This high-speed/low-speed valve approach could achieve relatively ‘good accuracy with litle complication ‘except for the feedback device. On-off valves can be used with limit switches or continuous feedback devices depending on the requirements. Back in the late 70’s and early 80"s, it was ‘common to see positioning systems for sawmills that used high- and low-flow valves with MLDT feedback. Open-loop control with propor- tional valves Accuracy and throughput can be improved by replacing the high- and low-speed on-off valves ‘with a proportional valve. Even simple systems can run in open-loop until ‘a “ramping zone” is reached. When the actuator gets close, the output can bbe ramped down as a function of the distance remaining. This is not strictly closed-loop control because the error Figure 1-1. Control system diagram shots veloc fedorward(P). and acceler aon ‘motion contr (ic. the distance between actual and target position) is not being used as feedback, but can be very effective if Toads are relatively constant. Proportional or servo valves can be used in either open-loop or closed- loop mode. Open-loop control is sometimes all that is required. If the process is repeatable enough, one can be fairly sure a given output will result in a desired speed. This is easy to implement because one can use a simpler controller or PLC with an analog output. An analog output from the PLC or computer in the range of =10 to +10 volts can be used to cause the valve spool to shift in a continuous ‘manner as the voltage to the valve changes. This allows one to roughly control the flow and therefore the speed, as long as variables such as the pressure across the valve and the load stay constant. ‘A little more sophisticated variation is to use feedback not to close the loop, ‘but to change the output as a function of distance. Ramping of the control tow proportional (ner ihoward (ts Coie epee oeoutput as a function of distance is accomplished using a PLC or a simple motion controller. This form of control works very well in applications sping down the output to reduce speed in a smooth manner. Generally, no precise tuning of the ‘motion is required in these systems. Closed-loop control ‘The main reasons for using closed- Joop control are flexibility, accuracy, speed and the ability to maintain precision with changing conditions (eg., loads). As productivity demands Position feedback signal Figure 1-1-2 Closed-loop hydraulie control system diogram. The mot increase, more and more applications and processes require more ‘sophisticated closed-loop controllers. "At the high-end of control is the combination of continuous feedback ‘nd closed-loop control. Closed-loop control compares the feedback position from the feedback device to a desired position. The difference between the desired and actual feedback is multiplied by a proportional gain (a ratio that converts the error in position units to output in volts or milliamps). The greater the error, the greater the output is to correct the error. Higher-end controllers augment the controller receives {eedback from mayretotictive linear displacement transducer (MLDT) amd pressure transducers ‘and control he proporonal servo abe. n Ere! teroce ono PECs, is and enterprise computing Stems" we mation conor ><) DELTA proportional (P) gain with integral (1) and derivative (D) gains and feed- forwards. The control diagram in Figure 1 shows how the gain factors ‘combine to implement precise closed- loop control. Feed-forwards are really just open~ Joop gains that are used as predictive factors in combination with closed- loop control. For example, if one knows the actuator will move at 2 inches per second per volt, then 4 volts should be applied to go 8 inches per second. Of course, this assumes that hydraulics respond in a linear manner, ‘when in fact it doesn’t. However, ifthe feed-forward gain is used to provide an output that approximates the desired value, then the PID gains can correct any non-linearity or changes in load not predicted by the estimate. Without feed-forward gains, the PID gains would have to compensate for a larger error, resulting in an increased likelihood of system lag and instability. The combination of PID, feed- forwards, continuous feedback, and a servo valve or servo-quality proportional valve allows the controller to go to positions using velocities, accelerations and decelerations that the user can program. The programmable accelerations and decelerations reduce the wear and tear on the hydraulics and mechanical systems. In the case of presses, one can ramp down the velocity so that when the tooling hits the work piece the press will have just the right amount of kinetic energy to do the necessary work. Figure 1-1- 2 shows a system diagram of the hydraulics used in a high-end press, with position feedback provided by an MLD and differential pressure (force) feedback provided by two pressure sensors mounted in the cylinder. Control is provided via a proportional servo valve, Sometimes a combination of open- loop and closed-loop is the best. In the press application above, it may not be necessary to synchronize four axes Many applications, such as single axis presses, do not need precise controluntil the tooling gets close to the work piece, In this case the actuator can be ‘commanded to move down at a h speed using open-loop control. Once the actuator gets close to contacting the material, control can be switched to closed-loop position control. Upon contact with the material, a logical decision can be made to seamlessly transition from closed-loop position control to closed-loop pressure or force control by using pressure feedback Communications A good controller should have good networking support in order to fully take advantage of its control capabilities. It makes no sense for a controller to have the ability to change set points on-the-fly if these set points can’t be updated from some external source quickly and easily Also, a closed-loop motion controller is not very flexible if it has only a handful of inputs that can tell it to go to only a small number of pre- programmed positions. The more advanced controllers have EtherNet, PROFIBUS, PROFINET, Modbus, or other fieldbus interfaces that make interfacing to PLCs, HMIs, PCs and DCSs easier. This allows parts manufacturing machines to take advantage of the flexibility of the controller so new set points can be downloaded when ‘making new part types. In some cases, such as lumber processing, new set points are downloaded for each log, cant, or board because each piece of lumber is different, Another valuable advantage of motion controllers with fast communications such as EtherNet or serial ports is diagnostics. The best controllers have the ability to graph the desired and actual motion profiles This ability greatly eases tuning and troubleshooting of the entire motion system. Choosing between closed-loop and ‘open-loop control ultimately depends on the requirements of the specific application. In some cases, using continuous feedback and proportional valves with open-loop control may be a reasonable answer. To summarize, that you go with an appropriate mot hydraulic motion control. whichever route you choose, it is best controller that can be optimized for pe Calculating a big move? RMC150. Motion Controllers We'll do the heavy lifting. Delta's new RMC Motion Controllers make it easy to move heavy objects precisely. RMCTools Software Whether you have hydraulic or electric 205, our new RMCIs0 gives you precise closed-loop control. And our RMCTools eases the load of setup, program: ming, and tuning. So don’t sweat over ie rote calculations or vaand-eror tuning, ==" instead let an RMC:50 Motion Controller do the heavy ling, i's one powerful, ht work easy to-use controller that makes of all your big moves. More details at deltamotion.com. DE LTA COMPUTER SYSTEMS1-2. Closed-Loop Position-Pressure/Force Control Hydraulic power has always offered the benefit of being able to lift and move heavy objects. Hydraulic systems can apply great amounts of holding force requiring very little power. In contrast, motors use more power because current must be flowing through the armatures to apply torque. In the past, hydraulic motion was not. thought of as being extremely precise: Most hydraulic valves were of the on/ off variety and control was typically ‘open-loop, or used imprecise feedback mechanisms such as limit switches or ‘operator joysticks, With the advent of precise position feedback devices such as magnetostrictive displacement transducers (MLDTs) and closed-loop electro-hydraulic motion controllers, machine designers and integrators could achieve precise positioning of hydraulic actuators, but something ‘was still missing. Many applications, such as presses, require both position and pressure/force control. In the past, presses used pressure relief valves to limit the pressure used to apply force, but there isa flaw in this technique due to net force considerations. Pistons have two sides and a pressure relief valve only reacts to the pressure on one side of the piston. Since there is pressure on both sides of the piston, the applied force is not equal to the pressure on one side of the piston. Rather, the applied force can be either measured directly with a load cell mounted between the piston and the load (as in Figure 1-2-1), or approximated by calculating the net force. The net force is derived by taking the difference of the pressure readings on either side of the piston and scaling them by the areas on each side of the piston (Figure 1-2-2) The decision to use a load cell or Figure 1-2-1. This mrion controler, used in am application tha dos flex testing of leaf springs controls force by interfacing lad cell (Formula 1-2-1) Net Force = Area A Pressure A Area B x Pressure ‘two pressure transducers depends on the force ranges and environment. Load cells may be needed in applications where the applied force is very low. When the force to be ‘measured is less than the amount of force that must be exerted to overcome the seal and rod friction, then using a load cell is preferable because the load cell reads this applied force rather than the force across the piston, (Formula 1-2-1 provided above for net force across the piston does not take into account internal friction). However, load cells can be difficult to mount and ‘may not survive in harsh industrial environments. Using two pressure Sea transducers works well when the forces are sufficiently high that the internal friction can be ignored. Pressure transducers are relatively inexpensive and can be mounted on the cylinder (as, in Figure 1-2-2) or on a manifold away from the end of the cylinder rod. The addition of accurate force feedback allows the controller to precisely control applied force. This has several advantages: 1, Pressure spikes that damage sensors and cause leaks can be reduced or eliminated. This decreases maintenance costs and extends the life of the machine 2. More consistent motion resultsPosition feedback signal Figure 1-2-2. Ths portion ofthe system schematic from a powder metal press shows connections