By Jack L. Johnson, P.E.
Historically, management and engineering viewed issues of high production quantity and high quality as diametrically opposed. This double-Q dilemma maintains that if product quality is paramount, then production rates must suffer, resulting in lower quantity. However, if production is stepped up, scrap must increase, and quality generally, must suffer.
But there are implications beyond this simple double-Q quandary. If processes are accelerated, production machinery becomes noisy, less reliable, and produces inconsistent parts or services — not to mention the increased down time due to overstressed machine members and parts. Management was in a constant struggle to increase production for maximum profits, but without the risk of alienating good customers. Such was the double-Q dilemma, but no more!
The motion control solution
Motion control technology was developed specifically to address and
defeat the double-Q dilemma. With
motion control technology you can increase
speed and increase both product
quality and consistency. Machines
can operate faster, with higher
reliability, less noise, and
achieve longer life. Furthermore,
with reduced machine
stress levels and noise, safety
for both man and machine are
improved. And, of course, with
higher productivity, greater
quality, and higher reliability,
operating costs are lower, so
profits go up. The benefits also
help build customer loyalty
because more predictable and
reliable production allows justin-
time-inventory methods.
 |
The motion control star shows that motion control
technology can be used to make divergent goals
converge.
|
The designer can use motion control to take total and complete control of the machine. No motion is left to chance. The machine is “guided by the nose” from cradle to grave, from inception to resurrection. The motion control star represents the idea that motion control technology eliminates the mutual exclusivity of the double-Q dilemma, and instead, creates mutual inclusivity in not only quality versus quantity, but in other performance capabilities as well.
Motion control technology is not a separate academic discipline, such as integrated circuit design and manufacturing, computer architecture, or electrical engineering. Instead, it is a convergence of many existing, but diverse, technologies to solve systemic problems of all types.
The purpose here is to present an overall view of the technology and, in the process, cover the constituent technical disciplines to make you more aware of motion control and what is needed in order to be successful in it.
Elements of motion control
Motion control encompasses hydraulic,
pneumatic, and electromechanical
technologies. However, the
focus of this column hydraulics. Hydraulic
actuation provides the power
needed to move the load. The diversification
taking place at both the distribution
and manufacturing levels has
brought integrated electromechanical
components into motion control systems
that traditionally had been all hydraulic.
As more designers recognize
the need for the “optimum solution”
to motion control problems, more
suppliers now offer product lines that
include hydraulic, pneumatic, and
electrical actuation methods.
Interestingly, it is the actuator that separates the hydraulic motion control system from the electric. To some extent, the sensors are unique, but the separation really is dictated by the selection of the actuation method. The rest of the system elements are common from one actuation medium to another.
Analyzing the load
Load analysis is one of the more
challenging elements of motion control
technology. Any useful motion
control system will move a load. The
load may be constant, for example in
the case of a flying cutoff. Or it may
vary considerably, as with robot axes
and elevators. It may not be clear to the uninitiated, but the load is “inside
the control loop.” The load seriously
affects all aspects of motion control
performance.
Newcomers to the technology have too often been burned by having the first installation go well because the actuator had a light load, but disaster struck with the next application because the load was severe. The load must be taken into account if, for no other reason than to determine the extent to which it dominates, potentially limiting the performance of the motion control axis.
Analog and digital signals
There is no denying that motion
control contains a strong electronic
element. The electronic element must
include both analog and digital components.
First, sensors are mostly
analog, and the fluid power and electromechanical actuation devices certainly
are all analog. But the motion
controller itself, more often than not,
is a digital computer of some sort. It
may be an industrial PC, but, more
likely, it will be a programmable logic
controller (PLC).
In the high performance arena, the PC or PLC will be fitted with a dedicated motion control card using a digital signal processor (DSP) that provides the speed needed to service one or more servo loops. However, at least one manufacturer offers the inverse of this — a motion controller with an auxiliary PLC.
Integral with the motion controller card will be analog-to-digital converters (A/Ds) and digital-to-analog converters (D/As). Additionally, sensors will have signal conditioners, special purpose amplifiers to drive the coils on servo/proportional valves, and communications bus link-ups as industry increasingly “gets wired” into enterprise-wide networks.
Software plays an increasing role, because every digital device needs some kind of program to run. This is true of the PC, PLC, motion controller, and data acquisition card. Unfortunately, the programming of motion controllers is a proprietary endeavor. All card designers and manufacturers have developed their own instruction sets and have made no attempt to create a universal motion control language. In fact, different product lines within a single company can have different instructions. The user may have to learn a different set of instructions for each different card.
Two-for-one offer for readers of Hydraulics & Pneumatics
The Designers’ Handbook for Electrohydraulic Servo and Proportional Systems covers electrohydraulic system design, motion control, and system analysis by teaching applications principles and reviewing practical problems from the real world instead of endlessly discussing complicated theories. It has been written by and for the practicing applications engineer and technician. The textbook: • covers everything from fundamental
hydraulic circuit
analysis to dynamic testing of
motion control systems,
Special offer to H&P readers
The book sells for $99.95, plus shipping and handling. Readers of Hydraulics & Pneumatics who order the Designers’ Handbook directly from IDAS Engineering will also receive a free copy of the newly revised and expanded Lexicon – Electrohydraulic Motion Control. This is a $19.95 value at NO additional cost to you. The Lexicon contains more than 700 terms and their definitions, covering all aspects of electronic components and circuits, analog and digital electronics, transducers, hydraulic principles related to electronic control, and much more. The Lexicon relies upon scores of analogies that relate electronic devices to your own command of fluid power components, circuits and systems, making it both a reference book and a learning book. Both books are ideal, as well as essential, references if you are considering certification. Check out what readers of the Designers’ Handbook have said about it at www.amazon.com/tag/electrohydraulics/forum. For more information, including a detailed table of contents describing all topics discussed, call IDAS Engineering Inc. at (414) 226- 0152, e-mail jack@idaseng. com, or visit www.idaseng.com. The Designers’ Handbook for Electrohydraulic Servo and Proportional Systems is also available from Barnes & Noble, and Amazon but must be ordered directly from IDAS Engineering to receive the two-for-one offer. |
Control engineering
Overlying the various engineering
specialties is a blanket of mathematics, which is dominated by control
system theory. This subject has been
developed to a high degree, especially
since the ubiquitous use of digital
computers has advanced as a mathematical
tool.
Classical control theory covers such subjects as frequency response (Bode and Nyquist analysis), root locus, and Laplace transform theories. Modern control theory is more general, thus more powerful, but not generally used until graduate school in engineering. It makes use of matrix methods (linear algebra) and direct time-domain solutions, but uses the classical concepts as special parts of it.
I have avoided covering the general theories of control system math in this text because they are not broadly practiced. The skills required to use them become quickly rusty if allowed to lay dormant for too long. This is not to suggest that such skills are not needed. Quite the contrary. The serious practitioner of the motion control art will be well served by developing and honing the skills associated with both modern and classical control system theory.
Back to basics
All technological disciplines are
based in physics. This includes load
analysis, electronic circuit analysis,
and hydraulic circuit analysis. If
you want to simulate motion control
systems, then certainly, physics becomes
the basis from which all the
components and their interactions are
described analytically. If you want to
develop and synthesize mathematical
models and processes, then a strong
background in both physics and mathematics
is essential.
“Motion Control” is aimed at exploring the methods for designing so that divergent goals can be made convergent. To introduce the concepts, it is necessary to review some basic electrohydraulic designs. Two conventional hydraulic circuits will be used to illustrate the point:
1. discrete, or so-called “ bang-bang,”
on-off control, and
2. open-loop control using a proportional
valve.
But these two methods produce only limited success. The “ultimate solution” is the electrohydraulic positional servomechanism. When the electrohydraulic servomechanism is properly designed, and the control system is suited to the task, the result will be a true motion control system that meets all the divergent goals.
Computer control, using a modern motion controller, streams to a motion control command profile that literally guides the servomechanism through the entire motion cycle. In this way, acceleration, velocity, and position are totally and simultaneously controlled, and along with it, accuracy, repeatability, and smoothness will be enhanced while reducing shock, vibration, and noise. By creating the proper motion control command profile, the machine cycle rate, and therefore machine productivity goals can be assured.
Easy-to-use software simplifies control system design, and it makes command profile synthesis and generation very intuitive, all with great likelihood of success.
Motion control design methodology begins with the required productivity rate (speed), a thorough analysis of all the loading possibilities, and then proceeds to size and select the hardware that will ensure success in the application. With proper implementation of motion control technology, we can have both high quality and quantity (productivity), sufficient to be profitable while satisfying all application demands.
Details of the design methodology are too extensive to be covered here; however, they will be covered in future editions of “Motion Control.” But these two will lead to the “ultimate solution,” the electrohydraulic positional servomechanism. It will be shown that when the electrohydraulic servo is properly designed, and the control system is suited to the task, the result will be a true motion control system that meets all the divergent goals.
