Using an Optical Commutation Encoder for

Brushless DC Motors

By George Beauchemin,

The quest for higher reliability and lower maintenance electromechani­cal devices, coupled with exciting new magnetic materials and the downward cost spiral of commuta­tion electron~cs and motor con­trollers has fueled increased usage of permanent magnet (PM) brushless DC motors (BLDC).

All BLDCs require rotor position information to effect electronic commutation. Commutation can be defined as a change in direction of current in a phase or a switching on or off of current in a phase. The most popular rotor position trans­ducers are inexpensive latching Hall effect transducers, but less popular alternative commutation techniques do exist. Examples include absolute encoders, back electromotive force (EMF) sensing techniques, incre­mental optical encoders using decoding techniques, optical encoders with commutation tracks, resolvers, sense winding techniques, synchros and Wiegand wires. This article compares the performance of Hall effect transducers with one such alternative position transducer, the optical commutation encoder. The tests were performed on the identical motor/commutation elec­tronics pair and test fixture, the sole change was substituting the outputs of the optical commutation track encoder for the Hall effect transduc­er.

The results show that there are applications where a commutation track optical encoder is a significant performance-enhancing and (05t­

effective alternative to traditional Hall effect commutation schemes.

Brushless DC Motor Overview

It is remarkable that considerable confusion exists regarding the defi­nition of BLDCs. They have been called AC servo motors, AC syn­chronous servo motors, AC PM servo motors, brushless DC servo motors, brushless servo motors, electronical­ly com mutated motors (ECM) and PM brushless DC servo motors.

A BLDC can be defined as a motor comprising three main components: a fixed (non-rotating) wound stator, a permanent magnet rotof, and a transducer or technique for ascer­taining rotor position vis-a-vis the wound stator. BLDC commutation electronics use rotor position infor­mation to facilitate electronic SWitching of the Windings vs. the mechanical brush/commutator

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-1.00 0 30 60 90 120 150 180 210 240 270 300 330 360

8ectricat Degrees

Figure 1, brush/ess DC motor torque curves.

Electrical Manufacturing· September 1989 • 49

SWitching of the ubiquitous PM DC brush motor. The commutation elec­tronics can be separate or integral. BLDCs are therefore self-commutat­ing permanent magnet DC motors.

Other motor technologies, such as steppers and certain AC motors, also eschew brushes or sliding electrical contacts but lack the favorable per­formance characteristics of BLDCs. The advantages of a BLDC include high speed capability not limited by line frequency, high starting torque, very high peak torque, linear speed/torque characteristics and variable speeds. These are also the textbook advantages of the perma­nent magnet brush DC motor (PMDC).

The absence of brushes allows the BLDC to have unique features com­pared to its PMDC relative. These include less contamination concerns (no brush dust), lower EM! and RFI (absence of brush to commutator

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Figures 2 and 3 are bmshless DC motor torque curves. Figure 2, top, shows switch­ing phases for positive torque, while figure 3, bottom, depicts effective torque ripple­three phase bipolar.

arcing), increased life/reliability, low or no maintenance, lower acoustical noise, increased performance due to superior thermal characteristics, higher speed (no brush limitation) and higher torque-to-inertia ratio. The lack of sliding electrical contact also obviates problems such as 5tart­iog voltage difficulties due to vari­able contact resistance, contact coo­tamination, corrosion, and mechan­ical wear. Sensitivity or reliable com­mutation to moisture content and the type of atmosphere (or lack of atmosphere) are also eliminated.

The BLDC does have some poten­tial disadvantages, depending on the application. The most commonly identified disadvantage can be cost. Although there are fewer mechani­cal parts with a BLDC, the addition-ai cost of the electronics has to be contended with. In addition, BLDCs generally have not gone as far down the learning and tooling curve as brush-type PMDC motors. The exception is the astronomical BLDC use in computer peripheral devices, particularly Winchester hard disk drives, floppy disk drives and fans.

A second disadvantage can be

higher torque ripple. Generally, BLDCs are three-phase devices and as such have higher torque ripple. In a sinusoidally wound motor this torque ripple will be approximately 15%. In a trapezoidally wound motor, this torque ripple can be reduced. Using sine wave winding excitation can virtually eliminate torque ripple, but at the expense of sophisticated and expensive drive electronics. PMDC motors, on the other hand, usually have a signifi­cantly higher number of phases, which directly translates into lower torque ripple.

The third disadvantage may be esoteric - a BLDC application requiring integral commutation elec­tronics and haVing extremely high or low ambient temperatures that exceed the operating temperature limitations of the integral drive elec­tronics.

The successful commutation of the brush less motor is nothing more than knowing where the rotor is, in electrical degrees and haVing the proper switching (commutation) scheme. This is illustrated by the bipolar example that follows. Engi­

neering the individual phases of a three-phase BLDC would generate, as a function of the electrical degrees of rotation, a torque curve like the one in figure 1. Each phase would be 120 0 electrical apart. It should be noted that electrical degrees are sim­ply mechanical degrees multiplied by the number of pole pairs of the rotor.

SWitching the individual phase currents at the proper electrical posi­tion generates positive torque, as illustrated in figure 2. With the proper selection of phase energiza­tion, the resultant torque output of the motor is illustrated in figure 3.

Since no position transducer or technique is perfect, what are the effects of position error? The effects include increased power dissipation due to torque sensitivity attenua­tion, torque ripple increase, torque sensitivity that changes from clock­wise to counterclockwise (reversing error) and increased internal losses.

Electrical error can be thought of as a function directly proportional to the number of rotor pole pairs. As figure 4 illustrates, small diameter motors and high pole pair motors will be significantly more handi­capped by sensor error.

Hall Effect Transducers

Latching Hall effect transducers are bipolar sensors that turn on when their magnetic field reaches a certain positive magnet flux level and turn off when the field reverses to a certain negative magnetic flux level. When used in BLDCs, they have many advantages. Commuta­tion adjustment is possible if the Halls can be "dialed" or rotated for optimum unidirectional or bidirec­tional commutation. They consume little space, particularly if they can be nested into the space between the motor's lamination slots. They are fairly high temperature devices, par­ticularly the newer "military" Halls. Simplicity, ruggedness and low cost are also attributes.

However, disadvantages do exist. Invariably, Halls will have electrical phase errors because it is extremely difficult and impractical to achieve true 1200 electrical phasing between the three Halls (in a three-phase BLDC). Hysteresis, a variabie "sweet spot" for triggering and difficulty in mechanical alignment must are also potential difficulties. The bottom

SO • Electrical Manufacturing. September 1989

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line is that even if the Halls could be placed in a mechanically perfect fashion in the motor and if the com­mutation magnet were perfectly magnetized (no mean feat) and per­fectly mounted, neither 1200 electri­cal commutation nor freedom from hysteresis reversing error are guaran­teed. Also, EMI sensitivity requires a separate commutation magnet when higher powered motors are con­cerned. Such features add cost and length to the motor.

Optical Commutation Encoder

In general terms, an optical com­mutation encoder is a variation of conventional optical encoders. It possesses a light source, code disc, mask and signal processing electron­ics. The salient difference is that the disc is constructed and mounted so that the switch points from low to high are exactly in phase with the zero crossing of the back EMF of a BLOC and the number of tracks is equal to the number of phases. An example would, be when a BLOC is driven clockwise as a generator. Track 1 (corresponding to phase A) would switch from on to off as the generated AC voltage crossed zero volts and continued on to negative. The encoder outputs mimic latching Hall effect transducers in a BLOC.

The advantages of the optical commutation encoder are many. Oifferential cost can be small if an incremental optical encoder is already to be employed for velocity and/or position feedback. The drive interface is trivial, for the commuta­tion electronics "thinks" it is seeing a Hall output (albeit a very precise Hall transducer output).

The benefit for the BLOC is real­ization of true 1200 electrical sensor output. True sensor phasing with the zero EMF crossover will maximize bidirectional torque sensitivity, and balance for sensor hysteresis is nil. The result is that torque ripple is reduced, motor losses and power dis­sipation are minimized and efficien­cy is maximized.

Total inertia is reduced when com­pared to employment of a commuta­tion magnet. There is the additional possibility of a shorter stack length

-The "Comtrac" series modular incremen­tal optical encoder, by BEl Motion Systems Co., Cqmputer Products Div., San Marcos, Calif

motor (and commensurate lower motor rotor inertia), due to the increased performance realized.

Motor cost is also favorably impacted with the elimination of the Hall transducers, ancillary print­ed circuit (PC) boards, wiring and labor and the usual commutation magnet. Encoder mounting is simple and motor case length may be reduced.

However, if an optical encoder is not necessary, the differential cost may preclude the optical commuta­tion encoder from consideration. One also cannot align an indepen­dent zero index (if the optical encoder has one) in relation to the motor shaft. "On the fly" commuta­tion advance/retard capability, prac­tical with some incremental encoder decoding schemes, is not possible. For unidirectional applications, however, the optical commutation encoder can be adjusted to achieve true commutation optimization for a given load, speed and direction. Extreme temperature ranges, severe shock and vibration could also pre­clude the optical encoder from con­sideration.

Table 1 compares the accuracy of

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Figure 4, top, error in electrical degrees vs. mechanical sensor error. Figure 5, bot­tom, no load current comparison Amps at 1,000 rpm.

Electrical Manufacturing. September 1989 • 51

Sensor mechanical error ±D.DOS Inch

-----------------~~~~~-====~~~~~~ 2 4 6 8

Pole Pairs

0.63 Amp

017 Amp

the optical commutation encoder with Hall effect sensors. Both sensors are compared operating on approxi­mately a I" radius.

Test Setup

The BLOC tested was an off-the­shelf BLOC type 01l38-25-900E. This 8-pole inner rotor design uses mold­ed samarium cobalt magnets, a sepa­rate commutation magnet and a 36­slot laminated stator that reduces cogging to virtually imperceptible levels. Continuous stall torques of up to 120 oz.. in. and the maximum recommended rotor speed of B,OOO rpm is possible. The commutation electronics was a driver developed in-house for the laboratory experi­ments.

The optical commutation encoder was a prototype version of a modu­lar incremental optical encoder.- In addition to the traditional X and Y quadrature tracks and Z zero index tracks there were three commutation tracks on the code disc designed for an eight-pole (4 pole pairs) BLOC. For motor loading a Magtrol motor tester, P.N. 4636, was used. Ancillary equipment included an oscilloscope,

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Figure 6, torque constant comparison, tested vs. computed from back EMF.

averaging digital current meters, dig­ital rpm strobe and power supplies.

Once the encoder was mounted to the rear mounting surface of the BLOC, the motor was driven by a synchronous motor at 1,800 rpm counter clockwise (from the shaft end). The generated wave form of the back EMF of phase A was aligned with sensor 1 output from the encoder. Alignment was simple, requiring that the generated voltage of phase A be observed relative to commutation track 12 on the sec­ond channel of the oscilloscope. The encoder body was rotated until the encoder commutation switched from high to low exactly when the back EMF was decreasing and cross­ing the zero volt level.

In order to ascertain motor and test fixture torque loads, no-load current was recorded clockwise and counterclockwise with the motor detached and also coupled to the dynamometer fixture at a rotor speed of 1,000 rpm. The no-load current of the motor not attached to the fixture will determine motor vis­cous and coulomb torque losses. The no-load current figures when attached to the fixture are necessary to determine test fixture and motor/fixture coupling torque loads.

200

175

150

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75

50

40 55 70 85 100 115 130 oZ.4in.

To determine "observed torque sen­sitivity" an operating point of 1,000 rpm and 50 oz.. in. of loading torque from the dynamometer were used.

Analy.i. of Results

It is important to define the differ­ence between "observed" and "pub­lished" torque sensitivity. A BLOC torque sensitivity definition has its roots in its brush-type motor her­itage. The usual way to determine torque sensitivity is to measure the back EMF Vp [volts] at a known angular velocity of (J) [Krpm] and cal­culate torque sensitivity. The back EMF constant is a sinusoidal func­tion with a peak defined as:

The peak torque sensitivity KTP [oz.·in./Amp] is related to the peak back EMF KEP [Volts/Krpm] by:

1.3524 KEPKTP =

For a sinusoidally wound bipolar BLOC the average back EMF con­stant KE will be the peak back EMF constant KEP multiplied by the con­stant 0.955.

.-- Encoder Commulaled

The average torque sensitivity therefore is:

"Observed" torque sensitivity is defined as the ratio of a change in motor load torque ~TL divided by the change in the average current ~IA to the motor:

The measured peak back EMF aver­aged (cw and ccw) 17.64 V/Krpm, corresponding to a theoretical calcu­lated torque sensitivity peak of 23.86 oz.·in./Amp.

In comparing the results, in­evitable hysteresis of the Hall effect transducers was noted. Although quite good and relatively neutral when going clockwise vs. counter­clockwise they are never "on the money, /I leading in one direction and lagging in the other. A perfect motor and perfect commutation electronics will still have less-than­ideal commutation. But is this all solely for the theorist and of no practical value.

The results of no-load current at 1,000 rpm was most surprising. As figure 5 illustrates, the no-load cur­rent reduction was almost a factor of 41 Such a reduction can be extremely beneficial in battery-powered appli­cations. Further study and testing is reqUired to determine why there as such a significant no-load current increase. If the results were com­pared to the change in "observed" torque sensitivity, a similar percent­age difference would have been pre­dicted. Clearly, internal losses are lower for the optical encoder-com­mutated motor configuration.

Although not as dramatic as the no-load current reduction, torque sensitivity at 50 OZ.·in. averaged a significant 11.5% increase (figure 6). Since winding losses are proportion­al to the square of current, the Hall commutated motor will have almost 20% increase in "I2R" losses, even ignoring the no-load current increase. For this particular operat­ing point the Hall effect motor had almost 67% more heating to do the same job. It is important to note that with a sinusoidally-wound BLOC the "observed" torque sensi­

Figure 7, winding temperature vs. load torque using test results for parameters. tivity cannot be within 1% of the

52 • Electrical Manufacturing. September 1989

25

calculated peak. Although the com­parison between the Hall commutat­ed encoder and the optically com­mutated motor are valid from a rela· tive point, there may have been a calibration error in the dynamome­ter. The closest the optically commu­tated motor should have been capa­ble of attaining would have been approximately 95.5% of the theoret­ical peak.

Figure 7 is a simulation comparing the calculated effect of "observed" torque sensitivity difference in this motor using the published values for thermal resistance of 1.2°C/W. The no-load current was added to the current necessary to drive the load. Average torque sensitivity was used and the ambient temperature simu­lated was 25°C. Clearly the simula­tion illustrates the significant ther­mal benefit of the optical commuta­tion encoder.

Conclusion

The optical commutation encoder is a viable alternative to traditional latching Hall effect transducer-based commutation schemes. The differen­

Definitions

Note: Electrical Error in Degrees is Mechanical Error Multiplied by Number of Pole Pairs.

Commutation Transducer Trigger Point Error Phase Error Hysteresis Error

TABLE 1 ­ ACCURACY IN MECHANICAL DEGREES, 1" RADIUS

Hall Effect +/-1.0° +/_2.0° +/_3.0° Encoder +/-0.025 +/-0.025 None

Triggen'ng Point Accuracy: Mechanical Error in Degrees From Ideal Switch Point of Transducer to Actual Switch Point. Phase to Phase Error: Mechanical Error in Degrees From Ideal 1200 Electrical Phase Dif­ference Between Adjacent Switch Points of Transducer. Hysteresis: Mechanical Error in Degrees From Ideal 1200 Electrical Phase Difference Between Adjacent Switch Points of Transducer.

the designer to actually select atial cost can be small to none if an

incremental optical encoder is to be part of the motion control package to begin with. The optical commuta­tion encoder is simple to mount and a ready substitute for the Hall effect transducer. Its increased precision provides significant performance increases for exactly the same BLDC as measured by an increase in appar­ent torque sensitivity, a decrease in no-load current and potential weight and space savings. The increased performance may allow

smaller BLDC to do his job, further cutting cost, weight and space.

Editor's note: This article is based on materials presented at Motion Expo '89 in Boston, Mass. The author has since lett BEl (or other pursuits, and can be reached at 619/723-2954.

Electrical Manufacturing. September 1989 • 53