DC Tachometers (Electric Motors)

10.3.8
Tachometers are used as velocity feedback sensors on speed-control systems. These devices generate an electrical signal which is proportional to the angular velocity of the motor shaft. They are used for monitoring open-loop systems and as the primary feedback element in velocity control systems. As shown earlier, they can also be used for inner-loop stabilization in position-control systems, of which there are three basic types, iron core, moving coil, and brushless, the most predominant type being the iron core.
Terms
dc generator Used interchangeably with dc tachometer. Kg Tachometer voltage sensitivity, V/krpm
Ripple Noise voltage which can be assumed to be superimposed upon a linear output.
Principles of Operation. A tachometer is the opposite of a motor. A motor converts electrical energy into motion, and a tachometer converts mechanical motion
into electrical energy. The output of the tachometer can be modeled as a main signal and the ripple component. The main signal is directly proportional to the rotor angular velocity, much like the counter-emf generated by a motor. The constant defining this proportionality is termed the gain Kg. Typical values for Kg range from 0.3 to 25 V/krpm.
Methods of Fabrication. Iron-core tachometers are made using rotor laminations, much like a motor.The rotor for a tachometer has more slots than a rotor for a motor would. Because of this use of iron, these devices have significant inertia. The moving-coil tachometer is a wound-coil rotor with magnets on the stator. These have very low inertia. For both iron-core and moving-coil tachometers, the current generated in the individual windings is routed to the output terminals via a commutator and brushes. Once again, this is similar in a fashion to a motor, but the current flow is in the opposite direction. The current flows out of the tachometer, and into the motor. Brushless tachometers are like iron-core devices, but instead of brushes they use optical or magnetic commutation circuits for current steering.
Output Signal Qualities. Tachometer outputs are bipolar, and are positive for one direction and negative for the other. A perfect device would have a completely linear relationship between rpm and output voltage, with zero ripple. This is never the case, however, and voltage ripple defines the quality of the output. The output voltage sensitivity is affected by the load the tachometer sees.
The tachometer output is dependent upon the load resistor value by
tmp121-23_thumbTachometer circuit.
FIGURE 10.24 Tachometer circuit.
Accuracy and Resolution. The accuracy of a tachometer is determined by its linearity, ripple voltage, and temperature stability. Accuracy values needed vary depending upon the application. For the paper and pulp industry, accuracy values of ± 0.03 percent are standard. Linearity of 0.5 percent is normal up to 3000 rpm. At higher velocities, nonlinearities become more apparent. This can result from brush bounce, commutator eccentricity, and brush skew due to directional changes at high
speeds. Linearity is also affected by eddy current and hysteresis losses in the armature due to shorting during switching.
A good value for voltage ripple is 1 percent. With ripple values this small, small-order effects like shaft eccentricity can be seen if present. Levels this low are generally achieved only by moving-coil types because of their very low inductance. Ripple is composed mainly of noise created by brush transition between commutator segments. The signal is periodic and is related to the number of commutation segments
and the shaft velocity. The frequency of this second-order contributor to ripple is armature eccentricity. It causes low-frequency amplitude variations of the same frequency as the rotor rpm. Finally, inductive effects in the windings can also affect the tachometer output. This contribution is generally very high frequency, however, and can be easily filtered out.
Temperature stability can also have some effect on tachometer performance. A thermal stability of 0.01 percent per degree Celsius is the best available, while the low-end commercial grade can be as poor as 0.2 percent. For a particular application, look at the temperature differential expected during operation. If the servo needs to maintain 2 percent of set point over this range, then 2 percent divided by range equals the percent stability required. Choose a tachometer with a temperature stability rating that is better than this value.
Application Considerations. In order to eliminate backlash, tachometers are generally mounted directly onto the motor shaft. In some cases, the tachometer is actually built right into the motor. This manufacturing approach is cost effective, but leads to coupling between the motor and the tachometer due to interaction between their magnetic fields. The electromagnetic coupling between the motor and the tachometer will be stronger at higher frequencies, so it is a sort of high-pass filter. The phase relationship of the electromagnetic coupling with respect to the motor voltage depends on the angular orientation of the tachometer to the motor. If the tachometer is improperly aligned to the motor, the combination of the high-pass filter characteristics and an improper phase relationship can result in instabilities in the motor/tachometer outputs even when used in an open-loop system. Many manufacturers provide motors with integral tachometers, and when properly built, they can be used for systems with servo bandwidths in the 15- to 30Hz region. However, above this frequency, magnetic coupling between the tachometer and the motor will not be manageable, and the two devices should be separated.
Tachometers are not designed to provide any significant output power. In order to maintain commutation quality, they should be terminated into a load resistor which will maintain current levels on the order of 1 mA. If linearity is the prime consideration, the RL should be chosen to be at least 100 times the dc resistance of the tachometer. If the intended application will run at very low speeds, such that this will be difficult to maintain, a silver commutator should be used.


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