Using DC Tachometer Generators - the simple velocity feedback solution
The current thinking in motion control these days is "digital". Computer generated movement routines, encoder feedback, PWM amplifiers, stepper motors, etc. Digital technology has brought many good things to motion control. But sometimes the old ways are better. If you are working with a motor controller that will accept analog feedback, why would anyone want to go digital? While encoders may seem at times to be more cost effective, there is more than meets the eye to consider. Encoders, or most other feedback devices, require an external power supply, and sometimes an excitation voltage as well. Converting encoder output to a speed, for example, requires a frequency counter or D/A conversion. Encoders require that their code wheels be very closely axially and radially positioned in order to function correctly. This makes them extremely sensitive to shaft run out, concentricity of shaft to encoder location, and axial torque, acceleration, or magnetic thrust of the driving element. Correct coupling of the encoder to its driving element is critical. Further, encoders must usually be closely tailored to the specific needs of a digital system. In an off-the-shelf unit, while the line counts may be OK, the commutation, complements, line driver compatibility, or other electrical factors may not. Changing many of these things requires a different code wheel and sensing arrangements within the encoder, both time-consuming and expensive, particularly on prototyping.
A DC analog tachometer, on the other hand is easily modified for different voltage constants, current capability, internal resistance, in short, all the electrical parameters of a DC Tachometer Generator. Such modifications are, more often than not, a simple matter of easy and inexpensive factory adjustment at the time of manufacturing, instead of re-engineering. The result? Quicker, less expensive prototypes and vastly simplified changes when things aren't right on the nose the first time. DC Tachometer Generators are also self-powered. No external power or excitation is EVER required. Their output is inherently linearly proportional to speed, and they are true zero-speed devices, which encoders could never be. You don't have to wait for a tachometer to hit the next line to determine direction. With a DC tach, ANY rotation is instantly recognizable as one direction or the other, and changes in speed are also instantly recognizable. Real time doesn't get more real than with a DC Tachometer Generator. A DC tachometer generator can easily be used for speed monitoring as well, simply by reading its output on a voltmeter, either analog or digital. If accurate calibration on a meter is required and the DC Tachometer Generator is not already set to give the required reading, it is an easy matter of a simple resistive voltage divider network to obtain direct "volts" to RPM (eg, 7.00 volts equals 7000 RPM). Once again, self-powered, no external supplies or excitation needed. Voltage constants can range from less than one volt per 1000 RPM to 50 volts per 1000 RPM, accommodating a wide range of input or monitoring requirements. DC technology is well established, no rocket science or guesswork, just simple, easily predictable solutions.
DC Tachometer Generators are very tolerant of mounting arrangements. They can handle some thrust of the driving element, inexpensive couplings will work adequately in many applications, some eccentricity or misalignment can be tolerated in many instances, and the variety of mechanical mounting configurations is extensive. And for those few instances where there is no suitable mounting arrangement available from stock or catalog, often a different combination of stock mechanical parts will provide the required arrangement. Again, it is a quick and inexpensive prototyping. The DC Tachometer Generator presents few engineering problems. You can get "bare bones" low cost tachometers with limited environmental, electrical, and mechanical performance capability to super-precision, highly reliable and rugged models, or anything in between. In short, there are few feedback or speed monitoring needs that can't be simply and inexpensively handled by a DC Tachometer Generator
SERVO-TEK PRODUCTS COMPANY, INC. 1086 Goffle Road, Hawthorne, New Jersey 07506,Phone (973) 427-3100 FAX (973) 427-4249
E-mail: sales@servotek.com
