TechArchive

This article was originally written in the period 1995-2000

Dr Lothar Wilhelmy, Chairman of Hubner Elektromaschinen of Berlin and Gerhard Vock of sister company Thalheim-Tachometerbau look at the six most widely used devices for speed detection.

Since the introduction of regulated electronic drives, it has been important to control speeds with maximum accuracy, while maintaining a favourable price.

At higher demands of speed accuracy, the sensing of the actual value and its feedback are essential. The sensor of the actual speed attains a particular significance, as it influences the accuracy of the control loop – no drive is more accurate than the feedback of the actual value.

AC tachogenerator: a permanent magnet multi-pole rotor induces polyphase voltages in the armature winding. Amplitude and frequency are proportional to the speed. The ac voltages are rectified with diodes which normally are incorporated in the ac tachogenerator.

DC tachogenerator: in a multi-pole rotor that rotates in a permanent magnetic field, ac voltages are induced which are proportional to the speed. They are rectified by the commutator and the brushes which contact the commutator segments in phase with the peak of the ac voltages.

A change in direction of rotation causes a change of the polarity of the tacho voltage. Critical dimensioning of the armature and the magnetic field is required to obtain ac voltages with a flat curved form, so that the commutation of the ac voltages results in a dc voltage with a low ripple rate.

Hubner’s silver tracks embedded in the commutator retain good contact characteristics under adverse environmental conditions, so that the tacho voltages in the mV range which are generated at crawling speed are reliably controlled.

Pure graphite brushes exhibit excellent sliding characteristics and offer dc tachogenerators a maintenance-free life, comparable with the performance life of lubricated ball bearings.

Brushless tachogenerators: in the 1980s, motors with permanent magnets and square wave commutation of the stator currents were introduced. Although they are maintenance-free, they exhibit a ripple of the torque because, with a six-pole rotor and a three-phase stator, only 18 commutation steps per turn are possible. The stator currents have to be abruptly raised and lowered in the armature.

Brushless tachogenerators follow a similar construction The magnetic flux produced by the permanent magnets is shaped so that within each stator pole range it remains constant. These “trapezoidal voltages” are electronically rectified in a similar manner to a DC tachogenerator.

Control signals delivered from an adjacent rotor-position sensor using Hall-effect elements synchronise with the overlapping constant amplitudes of the trapezoidal tacho output signals, producing a constant dc voltage. However, this process is very limited at low crawling speeds when the signals are in the mV range, due to interference. The rotor position sensor signals can also be used for brushless dc motor commutation control.

Resolver: in a typical 2 to 6-pole resolver, a constant voltage between 5-10kHz is applied to a primary rotor winding. Acting as a transformer, two stator windings induce output voltages electrically phase-shifted by 90ø. The amplitude of the output voltages represent directly the sine and cosine of the angle of the rotor position.

Specialised electronics determine the rotor position for use in position control. However, at high speed changes, a new speed value can only be obtained after a time lag to allow the calculating electronics time to set the new value. Additionally, very slow speeds also present problems.

Digital-Tacho (incremental encoder): an incremental disk with light-dark zones (lines) is detected opto-electronically and presented as high-low pulse counts. The number of pulse counts represents angular displacements (distance travelled) and the pulse count over time (frequency) represents speed. Digital-tachos are therefore similar to resolvers as speed and position measuring devices.

In order to calculate speed information, particularly at low speeds, a high number of lines is needed (resolution). However, the opto-electronic detection system and signal line transmission system upper frequency level and forces a compromise between the minimum and maximum detectable speed.

In an incremental digital-tacho, normally two channels of pulses are produced phase-shifted by 90øC with one leading the other depending on direction. A separate third channel of one pulse count per turn is optional, which is used as marker pulse for count verification.

Sinus-Digital-Tacho: in digital tachos, the detecting of the lines produce a triangular waveform and square-wave pulse counts are generated via a Schmitt trigger. Speed and position can only be determined at the change or crossover of the high- low logic states. In order to derive continuous data throughout the waveform period, a Sinus-digital-tacho attempts to attain a pure sinusoidal waveform from the triangular signal.

Hubner has developed a new low harmonics technology which is said to virtually eliminate the nuisance harmonics inherent in this process to below -12dB. The resulting high quality sine and cosine signals make available precise information between the zero crossings of the waveforms, thereby offering continuous speed and position data from zero speed.

• AC tachgenerators with diode rectification are preferred with drives with few variations of speed and without the need to sense the direction of rotation. The frequency and the zero crossing of the ac voltages offer an alternative to digital- tachos (incremental encoders) but with less resolution.

• DC tachogenerators offer an almost delay-free actual value, wide speed range from crawling speed, a broad temperature range, direction indication, robust construction and low cost 2-wire signal transmission. No other actual value sensor can offer this combination of characteristics.

• The principal application of brushless tachos is with permanent magnet motors (brushless motors or electronically commutated motors). For smooth running, these drives are inferior to conventional DC drives. However, they have the advantage of overload capacity and very high speeds.

• Resolvers suit applications in PM motors with square or sinusoidal commutation, because they can deliver speed and position information. Time lag of the speed value calculation and resolution are adequate for many drive tasks, but not for the high demands of modern servodrive dynamics and precision at very slow speeds.

• Digital tachos combine precise positional data and the possibility of calculating the speed. This makes them ideal actual value sensors for synchronised multimotor drives, regardless of the motor type, whether ac or dc. Also, they provide data suitable for direct use with digital control systems without conversion.

• Sinus-Digitaltachos with 1024 sine/cosine counts per turn offer a resolution 1024 times higher than a two-pole resolver which gives only 1 sine/cosine per turn. Therefore precision drives which demand zero and crawling speed control need Sinus-Digitaltachos. In addition, 1024 lines allows a very high maximum speed. Opto-electronic detection typically limits at a frequency of 200kHz, corresponding to a speed range of zero to 12,000 rpm.

The problem with resolvers at slow speeds

A resolver detects rotor position only, so rotor speed must be calculated from the change between two rotor positions. To do this, a good resolver offers 12 bits per turn (4096 steps). Even a moderately fast drive controller has a cycle time of 1ms and is thus capable of 1000 measurements per second. For continuous drive information, the speed sensor must also be able to sense a difference between measuring step. To produce 1000 different measured values in one second, a good quality 12 bit resolver must rotate almost a quarter turn, equating to a speed of 15 rpm. Below this speed, the drive system speed feedback will operate with lower resolution

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