Time-proven mechanical cams are still the best choice for some applications. For others, electronic cams offer improved accuracy, faster product changeovers, and new machine capabilities. This will help you make the best selection
Providing cam motion electronically — either as a combination mechanical-electronic or fully electronic system — offers several important advantages over mechanical cams. But, before you abandon the mechanical-cam world for the electronic, examine each system’s characteristics and capabilities to determine which is best for a specific application.
A typical mechanical cam configuration incorporates a lineshaft, which distributes power from a prime mover to various elements within the machine. Cams mounted on the lineshaft provide cyclical, linear motion to mechanisms with each rotation of the shaft. Because all elements of the system are driven by the lineshaft, all functions of a mechanical cam system are inherently coordinated.
Hybrid cam systems
A hybrid system is an alternative configuration combining a lineshaft and an electronic sensor to provide the master cam-motion reference. The sensor, usually either a speed or position transducer, is mounted on the prime mover. An electronic signal from the sensor to a servo motion controller provides the information necessary to drive an actuator producing the cam motion. This motion is most often linear, although it can be rotary and can follow some prescribed functional path (sinusoidal, for instance).
The transducer in a hybrid system may be either an encoder or a resolver. An encoder is a feedback element that converts linear or rotary position to a series of digital pulses. Encoder resolution is represented as the smallest positional change that can be detected by the encoder and is equal to the number of lines per revolution times a multiplier (normally four times the line count). If you are using an encoder, your ability to precisely know your position is determined by how closely spaced the pulses are. If you are between pulses, you may have up to one count of error since there is no such thing as a “half pulse” with an encoder. Speed accuracy is determined by the number of pulses counted within a specified time frame. For example, if you are only counting ten pulses in a particular time frame, accuracy is limited because precision is usually plus-or-minus one pulse count (i.e. ±10%). For this reason, the usefulness of encoders is limited at lower speeds. At moderate speeds, encoders are highly resistant to electrical interference and are sometimes preferred for this reason.
A resolver is a rugged transducer using magnetic coupling to measure absolute rotary position. It uses an analog signal interface with special conditioning electronics, which are normally included in the motion controller. Resolvers are much better adapted to applications that must operate over a range of speeds — from slow to very fast. The accuracy of a resolver-based system is less sensitive to the conditions affecting encoder accuracy and is only limited by the accuracy of the system’s analog-to-digital converter and the inherent accuracy of the resolver itself, which is usually less than10 arc minutes.
Digital electronic cams
A fully electronic cam system involves only servo drives; there are no lineshafts in the system. All machine functions are electronically coordinated with the master servo drive without any other connections. A fully electronic cam system is digital and always incorporates a microprocessor. Low-end analog servo drives are not usually appropriate for these applications. Cost of a suitable microprocessor drive depends on its dynamic accuracy and power.
Which is best?
Purely mechanical cam systems are well known and understood by machine designers without specialized knowledge. A hybrid system is a little more complicated and requires some knowledge of servo control theory such as the function and configuration of PID control loops. In a fully electronic realm, the more you know about control-system theory, the better. Also, some knowledge of system programming is helpful. In any plant using programmable logic controllers, you are likely to find someone capable of programming a fully electronic servo system.
In applications where speeds and accuracy requirements are moderate, there is often little incentive to convert a mechanical system to either a hybrid or fullelectronic system, although the added flexibility is sometimes a reason. The hybrid approach makes sense when upgrading a machine and only some elements require greater speed, accuracy, and flexibility. This is cost-effectively accomplished by adding a transducer and servo for those upgraded elements while the rest of the machine remains mechanical.
The advantages of both the hybrid and fully electronic configurations over the mechanical include increased accuracy, flexibility and productivity. One of the appealing features of electronic cam systems is that it is not necessary to divulge proprietary process or production knowledge to adapt an electronic cam system to a machine. A system vendor can help a user select the appropriate functionality according to general application requirements, then the user can program the system to create a cam for the specific requirements of the machine or process.
With a hybrid system, because you are introducing a transducer — either an encoder or a resolver — which requires some translation, there is usually some loss in accuracy.
The accuracy obtainable with a fully electronic system is superior to that of either a mechanical or a hybrid system. Elimination of mechanical components, such as cams and followers, and their associated manufacturing tolerances and dynamic and wear characteristics provides greater accuracy with an electronic system over that of a mechanical system. Also, because it is totally digital with no need for an analog-to-digital signal conversion, which tends to degrade precision, accuracy of a fully electronic system is better than that of a hybrid. Also, any adjustments made on a digital system are numeric and therefore may be re-entered reliably on additional machines. In contrast, analog systems with potentiometers require individual adjustment.
Machine flexibility is also enhanced with an electronic servo drive system. Scaling an electronic system up or down, to meet varying production requirements, is simply a matter of changing a number or setting rather than making physical changes in the machine. Doubling a motion, for example, is simply a matter of dialing in a multiplier. This not only dramatically reduces machine-configuration time, but also eliminates the requirement to stock a variety of cams for differing requirements. Moreover, with electronic cam systems, adjustments to maintain tolerance are fewer and more easily accomplished.
With electronic cams, for example, it is possible for a processor to control both the amount of various component fluids in a mixing/filling operation to achieve a specified blend and to control the amount of blend in each container filled. Changing either of these parameters electronically is a simple, quick procedure. This degree of flexibility is impossible with a mechanical system.
There are more components in an electronic system that can fail and cause a system shutdown than there are in a mechanical system. In spite of this, electronic cam systems are no more prone to failure than are mechanical systems. But, should a failure occur, the time to diagnose and eliminate a problem is dramatically shorter for electronic systems. Modular electronic componentry speeds replacement time; and, because they are programmable, they can be quickly configured simply by downloading instructions from a computer. The refurbishing of electronic cam systems is usually a matter of minutes as compared with hours, or even days, for mechanical systems. Analog system recommissioning takes somewhat longer than digital systems because of the various adjustments that are required. Because they do not have to be lubricated periodically, less day-by-day maintenance is required for electronic cam systems, and their system environment is cleaner.
A useful characteristic of electronic cams is their ability to completely control actuator motion, both while extending and retracting. This makes it possible, for example, to extend the actuator in some pre-determined motion pattern then greatly accelerate its retraction. In a mechanical system, speed of the follower return as the cam falls away is limited by characteristics of the return spring and the system dynamics. A massive return spring that ensures fast follower return and positive contact with the cam can cause problems by requiring greater power during actuation to overcome the spring force during compression. And, if the cam is located a distance down the lineshaft, the possibility of torsional distortion may require a larger lineshaft to maintain synchronization.
The acceleration possible with electronic cams is limited by the capabilities of the servo drive and the motor, as well as the physical limitation of mechanical elements in the system. Motor acceleration is limited by its winding characteristics, the amount of current it can accept, and the current capability of the drive.
Within the constraints of torque and speed that exist, you may not be able to exactly match a mechanical cam motion electronically. But, the original reasons why the motion was chosen may no longer exist. The criteria for choosing an electronic cam motion are different from those dictating a mechanical cam shape. The basic criteria for determining a satisfactory mechanical cam shape — system dynamics and manufacturing constraints — are not considerations with electronic cam systems. The choice is usually based on system characteristics. For instance, one criteria is the digital servo timebase — how often does the servo process a point. If you have a servo with a 2 to 5 msec timebase you probably would choose an algorithm with higher fidelity than if the servo had a shorter timebase. Required accuracy is also a consideration in the design of an electronic cam. As is the case with mechanical cams, electronic system motions must also be moderated to limit the detrimental effects of jerk — extreme changes in acceleration. In machine systems with inherent elasticity — such as with belt drives — the effects of jerk can be absorbed and are not as critical as in systems that are composed primarily of less elastic elements (metal-to-metal contacts, for instance).
Electronic cam systems incorporate mechanisms to protect the machine against failures. A failure in the servo drive portion of the system will be detected by the supervisory microprocessor and an orderly shutdown initiated. If the microprocessor fails and a predetermined signal is not sent to a watchdog timer circuit, the system will shut down.
In electronic systems, a common method of stopping motors in an emergency is dynamic braking. To stop the motor quickly, the motor leads are disconnected from the drive, and resistors are connected across the motor leads. The kinetic motor energy is converted to thermal energy through the resistor, which stops the motor.
With a servo system, motors can be stopped in an emergency even faster than with dynamic braking, providing the servo drives are operational. In such cases the servo supplies a negative torque to the motor for braking. Servo stopping of motors in 50 msec vs. 1 sec for dynamic braking is common.
Michael Labarre is senior applications engineer, Atlas Copco Controls Inc., Wexford, Pennsylvania. Bernard Schneider is manager of strategic development, Socapel SA, Lausanne, Switzerland.