Feedback Devices. By John Mazurkiewicz. Baldor Electric

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1 Feedback Devices By John Mazurkiewicz Baldor Electric Closed loop systems use feedback signals for stabilization, speed and position information. There are a variety of devices to provide this data, such as the analog tachometer, optical encoder, Hall sensor, and resolver. In the following, each of these devices will be defined and explained. Tachometer Tachometers resemble miniature motors. However, the similarity ceases there. The tachometer is not used for a power delivering device, but for a signal providing device. Typical construction is shown in Figure. Figure Typical tachometer assembly Operation is as follows. The tach armature is rotated and a voltage is developed at the terminals (a motor in reverse!). The faster the shaft is turned, the larger the magnitude of voltage developed (i.e. the output signal is directly proportional to speed). The output voltage has a polarity (+ or ) which is dependent on direction of rotation. Analog, or DC tachometers as they are often termed, play an important role in drives, because of their ability to provide directional and rotational information. They can be used to provide speed information to a meter (for visual speed readings) or provide velocity feedback (for stabilization purposes). The DC tach provides the simplest, most direct method of accomplishing this. As an example of a drive utilizing an analog tach for velocity information, consider a lead screw assembly which must move a load at a constant speed. The motor is required to rotate the lead screw at 36 RPM. With a tachometer voltage constant of 2.5 volts/krpm, the voltage read on the tachometer terminals should be: 3.6 krpm x 2.5 volts/krpm = 9 volts. If the voltage read is indeed 9 volts, then the motor/load is rotating at 36 RPM. The servo drive will try to maintain this voltage to assure the desired speed. Although this example has been simplified, the basic concept of speed regulation via the tachometer is illustrated. Some of the terminology associated with tachometers includes: voltage constant, ripple and linearity. The following defines each. A tachometer s voltage constant may also be referred to as voltage gradient, or sensitivity. This represents the output voltage generated from a tachometer when operated at RPM. Sometimes this may be converted and expressed in volts per radian per second, i.e. V/rad/sec. Ripple may be termed voltage ripple or tachometer ripple. Since tachs are not ideal devices, and design and manufacturing tolerances enter into the product, there are deviations from the norm. As shown in Figure 2, when the shaft is rotated, a DC signal is produced which has an AC signal superimposed upon it.

2 Figure 2 Scope trace showing tach ripple Volts Ripple DC Volts Time In reviewing literature, care must be exercised to determine the definition of ripple, since there are three methods of presenting the data: ) Peak to peak the ratio of peak to peak ripple expressed as a percent of the average DC level; 2) RMS the ratio of the RMS of the AC component expressed as a percent of the average DC level and 3) Peak to Average the ratio of maximum deviation from the average DC value expressed as a percent of the average DC level. Linearity The ideal tachometer would have a perfect straight line for voltage vs. speed. Again, design and manufacturing tolerances enter the picture and alter this straight line. Thus, linearity is a measure of how far away from perfect the tach is. The maximum difference of the actual versus theoretical curves, as shown in Figure 3, is linearity (expressed in percentage). Figure 3 Tach linearity Volts Actual Ideal Deviation Optical encoder Speed A digital tachometer, often termed an optical encoder or simply encoder, is a mechanical to electrical conversion device. The encoder s shaft is rotated and an output signal results, which is proportional to distance (i.e. angle) the shaft is rotated through. The output signal may be square waves, or sinusoidal waves, or provide an absolute position. Thus encoders are classified into two basic types: absolute and incremental. Absolute encoder The absolute encoder provides a specific address for each shaft position throughout 36 degrees. As shown in Figure 4, this type of encoder employs either contact (brush) or non contact schemes of sensing position.

3 Figure 4 Example absolute encoder disk Example Brush Disk Rotation The contact scheme incorporates a brush assembly to make direct electrical contact with the electrically conductive paths of the coded disk to read address information. The non contact scheme utilizes photoelectric detection to sense position of the coded disk. The number of tracks on the coded disk may be increased until the desired resolution or accuracy is achieved. And since position information is directly on the coded disk assembly, the disk has a built in memory system and a power failure will not cause this information to be lost. Therefore, it will not be required to return to a home or start position upon re energizing power. Incremental encoder The incremental encoder provides either pulses or a sinusoidal output signal as it is rotated throughout 36 degrees. Thus, distance data is obtained by counting this information. The basic construction of the incremental encoder is illustrated in Figure 5. A disk is manufactured with opaque lines. A beam of light passes through the transparent disk and is picked up by a photo sensor. The photo sensor outputs a sinusoidal waveform and electronic processing transforms this signal into a square waves, i.e. a pulse train. Figure 5 Concept of basic incremental encoder Disk Photo Sensor Pickup Squaring Circuitry Light Source Grid Assembly

4 There are typically two output signals or channels and the relationship between these, as shown in Figure 6, informs the control which direction the motor is rotating. Figure 6 Incremental encoder signals Code Disk Light Sensor (three sensors) C B A Light Source (three lights) Three Light Beams Outer Track Middle Track Inner Track ( Pulse per revolution) A B C Zone 2 3 A B Pattern repeats 24 Times/ Revolution In utilizing an encoder, the following parameters are important: ) Line count: The number of pulses per revolution. The number of lines is determined by the positional accuracy required in the application. 2) Output signal: The output from the photo sensor can be either a sine or square wave signal. 3) Number of channels: Either one or two channel outputs can be provided. The two channel version provides a signal relationship to obtain motion direction (i.e. clockwise or counterclockwise rotation). In addition, a zero index pulse can be provided to assist in determining the home position. An encoder provides information similar to the information derived by a micrometer. The micrometer screw is turned to accurately measure the thickness of a piece. A micrometer barrel is divided so measurements can be made. The encoder does exactly the same thing with one important difference. Instead of having to read and interpolate (as with the micrometer), the encoder provides the information, in the form of electrical signals. An encoder translates the rotation into discrete electrical signals, that are directly related to the shaft position, and therefore the distance traveled. These signals are fed into a counter. A typical application using an incremental encoder is as follows (see Figure 7): An input signal loads a counter with positioning information. This represents the position the load must be moved to. As the motor accelerates, the pulses emitted from the encoder come at an increasing rate until a constant run speed is attained. During the run period, the pulses come at a constant rate which can be directly related to motor speed. The counter, in the meanwhile, is counting the encoder pulses and, at a predetermined location, the motor is commanded to slow down. This is to prevent overshooting the desired position. When the counter is within or 2 pulses of the desired position, the motor is commanded to stop. The load is now in position.

5 Figure 7 Monitoring encoder pulses to measure distance Hall sensors Hall sensors are solid state devices which are used to sense magnetic fields. As a magnetic field is passed near the Hall sensor, its output changes from on to off. In some motors, a 4 pole magnet wheel is attached to the rear of the motor shaft, and as this magnetized wheel passes by the Hall sensor, the Hall s output changes state. Figure 8 illustrates typical construction. In other motors, the actual rotor magnets are used (rather than the magnetized wheel). Figure 8 Typical Hall sensor assembly In some brushless motors, three Hall sensors are used to provide electronic commutation information. These three square wave signals, as shown in Figure 9, are phased 2 degrees apart, and provide information to the control, so the control knows which electrical winding power should be applied to. Figure 9 Hall sensor output signals

6 Additionally, since the output of the Halls look like a series of square waves or pulses, the timing between these pulses can be used to derive speed information. This can be used for speed control or speed regulation. This scheme works best at higher speeds. At very low speeds the pulses are far apart, thus it becomes extremely difficult to accurately control speed. Resolver Resolvers look similar to small motors that is, one end has terminal wires, and the other end has a mounting flange and a shaft. Internally there is a rotor and stator. A signal winding revolves inside a fixed stator. As the winding is moved (the rotor), the output of the signal changes. This changing signal is directly proportional to the angle which the rotor has moved through. The simplest resolver, as shown in Figure, contains a single input winding, and two output windings (located 9 degrees apart). A reference signal is applied onto the input winding, the stator (the primary), then via transformer action this is coupled to the rotor (the secondary). As the rotor is moved, the signal is modified according to, and proportional to, the angle the resolver rotor is moved thru. This signal then is coupled (via transformer action) back to the stator. The stator has two secondary windings which provide a sine and cosine output. These signals are then fed into the controller. Figure Brushless resolver construction Inside the controller, a resolver to digital (R to D) converter produces a signal representing the angle which the rotor has moved through, and a signal proportional to speed. There are various types of resolvers. The type described above would be termed a single speed resolver; that is, the output signal goes through one sine wave as the rotor goes through 36 mechanical degrees. If the output signal went through four sine waves, as the rotor goes through 36 mechanical degrees, it would be called a 4 speed resolver. Another version utilizes three windings on the stator and would be called a Synchro. The three windings are located 2 degrees apart. Each feedback device has its own characteristics, parameters, operating range, and advantages. The engineer designs the control package to make best use of the features and advantages of the feedback device in the application.

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