How to Design a Better Servo Control System

How to Design

A Better Servo Control System

Lee Stephens, Danaher Motion

Servo control system engineers design systems that involve complex motion, such as this ﬂ at-panel display processing machine that incorporates a high-precision gantry system from Danaher Motion.

Servo control system engineers typi- gineers might have to redesign an exist- One major factor to consider in the cally work designing new systems or re- ing system that typically comprises older new system is calculation time. A digital designing existing systems for convey- analog controllers, sensors and drives. system works in three serial, quantifi able ors, pick and place machines, gantries, Some might expect that designing a steps: measure, calculate and output. Th e machine tools or other equipment. Of new system would be more diffi cult than controller requires specifi c time slices to course, the new design task starts with replacing an old one, but often, this is not run through these three functions. Th e a clean sheet of paper. Since this system the case. Although the necessary loads, calculation period might be so long as did not exist before, all the specifi ca- speeds and torques might be known, a to let the system wander out of control. tions must be derived from calculations, newer digital controller behaves diff er- An analog system does not have this simulations, or actual measurements on ently than the old analog system using particular drawback to the same degree. existing machinery. Engineers can mea- brushed DC motors that it replaces. By comparison, the analog system “cal- sure feeds, speeds, loads and torques on New control laws often enter the equa- culates,” measures and outputs almost similar equipment that operates without tion, and when the designer is not aware simultaneously and continuously. Typi- servo controls, or they can simulate them or does not anticipate these diff erences, cally, the lag time is not as severe. with a variety of software packages. On the fi rst system off the drawing board Both new and replacement systems the other hand, servo control system en- might not live up to expectations. follow the same basic laws of physics 30 powertransmissionengineering fall 2007 www.powertransmission.com but diff erent control laws, so the de- Th e bandwidth is usually defi ned clude DisSim, Mathcad, and Motioneer- sign approach and hardware shopping in terms of its –3 dB point and the 45° ing, which is available from Danaher. A list could be very diff erent for each sys- phase shift. Don’t prefer one parameter Motioneering model for a typical PID tem. For example, a new system design over the other; exceeding the –3 dB controller lets designers examine the sys- can be defi ned in two ways. Th e fi rst is point and the 45° phase shift indicates tem behavior with numerous gains and straightforward, where the controls en- that the system is out of control. For ex- other parameters. It can show a plot of gineer designs a system totally on his ample, if the system is at –3 dB but it this behavior with certain perturbations. own from the ground up. He completely has a 60° phase shift, it was signifi cantly VisSim also is a widely used simulation defi nes the system and orders the com- out of control long before it hit the –3 software package. A limited trial version ponents needed to do the job. He alone dB point. may be downloaded free of charge from is responsible for the outcome. In digital systems, other functions www.vissim.com. Th e trial version can On the other hand, a new system aff ect the phase angle, which sometimes run and modify a model, but it cannot might involve a client that has a resident surprises the customer. It concerns cur- save the model. However, the full pur- engineer who helps defi ne the system rent-loop bandwidth. It is calculated chased package performs the complete parameters and selects the components. digitally, and the calculation delays be- job. Motioneering is free and determines Th e consulting motion controls expert come apparent. Th e delay is worse than how much current and power the mo- may help design the client’s new system a phase shift, but it is essentially the tor needs to function properly. Mathcad after its resident engineer had already same thing. Phase is time in the fre- is another modeler that many designers selected a few key components. Th e quency domain. Th e digital system may use. resident engineer may have determined not have optimized control algorithms When modeling, it is sometimes loads, speeds and torques from actual to calculate current, velocity or position. hard to decide when to stop. When de- measurements, calculations or simula- Th en the digital system may have more signers gain suffi cient experience, how- tions based on a few assumptions. He of a calculation delay than the system ever, they can recognize when some of also may have purchased some major can tolerate. It did not achieve the in- the parameters are not relevant enough components such as motors and trans- tended bandwidth. to consider. In the beginning, try ev- missions based on these determinations Modeling erything on the list. Th en narrow it before hiring the consultant. Th e con- Some of the design work is carried down to the few parameters needed to sultant’s initial posture is to assume that out by modeling with a variety of soft- adequately and suffi ciently model the the components that the resident engi- ware packages. Systems may be mod- system. Any adjustments after that are neer selected are perfectly suitable. Un- eled in a digital format to determine not usually minor. Stop when further detail- fortunately, sometimes, this is not the only the bandwidth, but also the posi- ing does not make the model any better case. Th e assumptions may have been tion accuracy. Th e model can become as or does not gain any more advantage in made under static conditions when they detailed as necessary. the design. should have been dynamic, particularly Widely used software packages in- Often, designers return to thecontinued model regarding the load. Th en the consultant has no choice but to revise the model to include the proper parameters. Strategy Calculation lag time is especially troublesome when a control system con- tains multiple axes. In a three-axis pick- and-place system, for example, the x, y and z axes all must converge simultaneously on a particular point. If a lag appears in any one or more axes, the com- ponent part could be set in the wrong place. To avoid this, fi rst determine the bandwidth of the system. Measure the load inertia, determine how fast it must move, and more importantly, how fast it needs to settle. Settling time re- ally dictates the bandwidth. Bandwidth does not determine the speed, but how quickly and precisely the load stops or Engineers can use software such as Motioneering from Danaher Motion to model their follows a contour. systems before building them. www.powertransmission.com fall 2007 powertransmissionengineering 31 after the system did not meet specifi - meet expectations, sometimes the prob- when they need a secondary feedback cations, and they fi nd errors. Th ey go lem is a miscalculation; the system was device. For instance, a system may re- through the system and might fi nd that expected to operate under certain cir- quire a feedback device mounted directly they should have considered something cumstances, but maybe it could not. Th e on the motor and another closer to the else that was more important. Always model should have shown how much load. Th e feedback device on the motor try to validate the model. It is only as headroom there was and how close it could be used for velocity control, but good as the information that goes into was. If not, then it may be necessary to the feedback device on the load would it. Some information is diffi cult to de- construct a Bode plot or run an FFT be used for fi nal position. It doesn’t termine mathematically and has to be (fast Fourier transform) to fi nd out why sound too diffi cult, but as the feedback done empirically. However, the em- the system is vibrating, resonating, or device moves farther from the transmis- pirical information determined for the not rejecting the disturbance. Th e ability sion device (which is the motor here), model may not be suffi cient, so use mul- of a control system to reject disturbanc- more items will be enclosed in the loop. tiple formats, including the frequency es is a fi gure of merit that goes beyond For instance, when a resonance appears domain and the time domain. VisSim a number; it is intuitive. It is through at a certain frequency that is outside the and Mathcad, for example, work well feedback that the disturbances will be intended control bandwidth, having the together for this, so take advantage of rejected. Th e proper feedback loop will feedback device outside of the load is them. let the load reach the intended position. certainly going to help move the load to Validation Th is might involve velocity, position or its fi nal position. However, this system is After the model is completed, the torque feedback. Th e nature of the feed- extremely diffi cult to tune. Tuning this hardware may not yet be available to back loop depends on the function that system means getting the position sys- work with. Th e customer may have the is specifi cally needed, such as torque or tem up to the desired performance level. breadboard and prove that the supplier’s current for machining operations. Th erefore, when a very high-bandwidth products will work in his system. How- Feedback position loop is needed, a secondary en- ever, the consultant may not be in con- Th e feedback system complexity coder is typically placed at the end of trol of how close the breadboard repre- varies with the application. It can be as the position loop. sents the real system. He may use the simple as an incremental encoder or a Redesign model as supplied, or he may revise it resolver, and there are diff erent reasons During tuning, sometimes the con- and fi nd changes in pulley ratios, mo- for using one or the other. A resolver is trols engineer will fi nd that rather large tors, inertias or miscalculated inertias. extremely robust and can tolerate harsh modifi cations must be made to the However, assume the physical bread- environments well. It can be a sine/co- system to gain the intended control. board is suffi cient to model and validate sine encoder, which can handle very He may even have to change his entire the system. In the next step, the custom- high bandwidth, but typically, it does theory regarding the method needed to er now implements the alpha or beta not survive well in a high-vibration, tune the system. stage of the project and buys the needed high temperature environment. Th e simplest control methods, either components. When the system does not Some systems are more complex PID (proportional, integral, derivative) or PIV (proportional, integral, velocity), usually operate in a great number of systems. For example, a standard PID control system is relatively easy to tune. It can be used for either a new system or an old one. Here, the velocity is a derivative of position and used for damping. Th e proportional term is a gain factor. Also, a math calculator using a pole/ zero function or a post fi ltering biquad fi lter method can help in the design. But either has certain advantages and disad- vantages. Th e PID is probably the simplest to understand and implement using classical control laws. During testing or any time the system is in production, however, some control laws may need changing. After some use, however, the Motioneering helps determine how much current and power a motor needs to functionfunction fi rst single PID system may not appear properly. to work suffi ciently well. It may require 32 powertransmissionengineering fall 2007 www.powertransmission.com two PID systems, a biquad system, or For more information: Danaher Motion additional low-pass or high-pass fi lters. 1500 Mittel Blvd. Stiff ness Wood Dale, IL 60191 System stiff ness, or a lack of it, seems Phone: (540) 633-3400 to be a major, chronic problem. For in- Fax: (540) 639-4162 stance, resonance problems indicate that Internet: www.danahermotion.com a system is not stiff enough. Moreover, backlash in a system is another serious problem. Here, the customer may have a Motion Engineering linear motion control system that specifi es a rack and pinion transmission on a Software precision axis. However, such a single- format gearing system produces trouble- The following resources some backlash. Even anti-backlash gears provide software that may cannot solve the problem; often they still be valuable in developing have enough backlash to create instabil- motion control systems. ity. Th ey typically contain two gears in parallel. One is spring-loaded against the other, and the teeth are clamped Mathcad from two diff erent directions. Th at ar- PTC rangement does not guarantee zero 140 Kendrick St. Needham, MA 02494 backlash, only that the backlash is taken Phone: (781) 370-5000 up by another mechanism. Th ey have a Fax: (781) 370-6000 spring rate to contend with, which is a Internet: www.ptc.com dilemma when trying to control a frequency in the domain of the spring rate. It is a common problem, usually found Motioneering in a system designed by someone lack- Danaher Motion ing controls experience. 1500 Mittel Blvd. To overcome these problems, con- Wood Dale, IL 60191 duct Bode plots and LaPlace transforms Phone: (540) 633-3400 in the frequency domain. Compare the Fax: (540) 639-4162 Bode plot performance with the La- Internet: www.danahermotion.com Place transforms and tune the system based on that information. Observe the VisSim frequencies, disturbances and ampli- Visual Solutions Inc. tudes, then determine the best method 487 Groton Rd. of attack to eliminate the disturbances, Westford, MA 01886 or insert compensation to reject it. In Phone: (978) 392-0100 addition, stiff en the system to elimi- Fax: (978) 692-3102 nate resonances, and raise the frequen- E-mail: [email protected] cies above the frequency of disturbance. Internet: www.vissim.com Also, sometimes the system may be damped, but this could also aff ect the compliance. High frequency damping usually does not add compliance, but at low frequencies, damping certainly cannot be added because it adds com- Lee Stephens is a sys- pliance that exacerbates the disturbance tems engineer with Danaher itself. Try using acceleration feedback, Motion in Wood Dale, IL. a Lowenburger observer—a relatively complex algorithm—or select a suitable fi lter.

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