CA1169483A - Constant speed electric motor - Google Patents

Abstract

ABSTRACT
A constant speed brushless D.C. motor of the permanent magnet outside rotor type with an internal stator wired in quadrature having a pair of Hall effect switches one of each mounted in proximity to one of a pair of opposite stator wirings to sense the direction and phrase relationship of the field in the rotor, a coarse feedback loop sensing back electromotive force in said motor including first comparator for comparing the negative generated motor voltage and motor potential said Hall effect switches operating under the influence of said first comparator to control power to said motor and thereby control its speed, a fine speed control loop including a tachometer for generating electrical pulses in proportion to motor speed upon rotation of said rotor, counter means counting said pulses and a second comparator connected to said counter means for comparing the frequency of said electrical pulses having a given frequency and the electrical pulses representing a steady desired preset motor speed providing a balancing potential difference at said second comparator thereby achieving a frequency lock to lock in the motor speed.
An electrical dynamic brake is disclosed.

Description

IMPRO~EMENTS IN A CONSTANT SPEED ELECTRIC MOTOR
~ This invention relates to a constant sp~ed direct current motor and particularly hut not exclusively to a motor for use in driving tape records andicartridge machines.
Hysteresis synchronous motors are currently used ln tape recorder and cartridge machine capstan drives. Where there is a constant frequency A,C. mains supply such drives exhibit excellent constant speed characteristics.
The use of D.C. motors for this purpose is not new; however, speed control is poor leading to serious problems with timing stability. Nevertheless D.~. motors have inherent advantages over A.C.
sy~nchronous motors:
(i) The D.C. motor is considerably more efficient than its A.C. counterpart thereby avoiding overheating problems.
(ii) The standard set speed of the D.C.
machine is variable within narrow limits whereas th~e snychronous machine is fixed. Thus small speed changes must be achieved by changing the diameter of the drive capstan (as applied to a tape machine).
(iii) The D.C. machine is unaffected by frequency changes in the mains supply and volt~ge ~ differences are éasily catered for in the control `~ circuitry.
Applicants are aware of U~.S. patent speci-fi~cations numbers 3,663,877 and 3,651,266 both to Cl~rk relating to a brushless D.C. motor including a , :: :

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s~ 3 tachometer commutation circuit and a digitalization circuit. The digitalization circuit is used to digitalize analog signals generated by Hall effect switches in the winding of the motor. The Hall effect switches do not have any effectlve direct control of the coil windings in the Clark motor as disclosed. This digitalization of the analog signals creates eight digital signals which are used to identify the instantaneous angular position of the rotor. Simultaneously, a tachometer generates a set of pulses of constant frequency which are correlated with the digital signals to produce a set of phase related switching signals. Each of the phase related switching signals has a period corresponding to a full revolution of the rotor, and this defines a unique ph~se relationship therewith.
This appears to constitute the basis of operation of the Clark invention. The Clark motor i5 primarily designed for use on machines where it is necessary to have both accurate positional and speed in~ormation, for example in editing machines where in~ormation concerning the precise position of the tape is required. The Clark motor could be used in tape recorder and cartridge machines but this would not make sense technically because the motor would produce unnecessary positio~al information, and also because the cost would be increased significantly due to the digital circuitry.
Applicants are also aware o~f U.S. patents 3,660,238 to Anderson and 3,656,039 to Konrad, which are related to each other and which disclose the use of dynamic b aklng in relation to D.C. ~otors. The dynamic braking is achieved by applying a short circuit to the armature which permits the generator voltage to be dropped across the armature to produce a dynamic braking effect.
U.S. patent 3,896,357 to Tamikoshi discloses the use of a tachometer in relation to a D.C. motor to control the speed of the motor and to eliminate electric noise. U.S.
patent 3,448,359 to Engel discloses the use of an oscil lator in the creation of an error voltage to control the operation of a D.C. motor. U.S. patent 3,754,157 to 10 Girault discloses a clrcuit generating squarewave signals which are supplied to the windings of a motor to control the speed of the motor. U.S. patents 2,810,084 to Sprando and 3,737,693 to Mishima relate, generally, to~"inside-out" motors. U.S. patent 3,541,361 to Nola 15 discloses a brushless D.C. tachometer which uses Hall effect switclles.
It is an objective of the present invention to provide a constant speed D.C. motor having good speed control characteristics and other peripheral 20 features to be explained in more detail later. For z example, good startlng torque; dynamic braking to avoid overrunning and run on characteristics.
There is provided according to the present invention a constant~ speed D.C. motor of the permanent 25 magnet outside ro;tor type with an internal stator wired in quadrature, a pair of Hall ef~ect switches one of each mounted in proximity to one of a pair of said ;
windings to sense the direction and phase relationship of~the field in the rotor, a feed back loop sensing back 30~electromotive force (negative generated motor voltage), comparator means f~r comparing ~he negative 8enerated :

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motor voltage and motor potential to control drive to said switches and thereby drive said motor.
In a further aspect of the invention there is provided a constant speed brushless D.C. motor including at least a pair of llall effect switches (known per se) buried in the motor stator windings for sensing the direction and position of the rotating motor magnetic field, a feed back loop comparator means in said loop, said comparator means being adapted to sense and compare motor negative electromotive force (back e.m.f.) and motor potential whereby the speed of said motor is held substantially constant. The present motor includes two sources of speed control namely, the fi~e outer speed control loop and a coarse inner spëed control loop utilizing back e.m.f. generated by the motor in operation. ', Conveniently the motor is adapted to generate pulses and includes means for counting said pulses, comparator means for comparing frequency of said motor pulses with a standard frequency pulse (representing a steady desired motor speed) also providing a balancing potential at said comparator means.
Conveniently the negative generated motor voltage (back e.m.f.) is converted to current which is required to exactly offset the motor current fed to a comparator means whereby current to the Hall effect switches is switched off effectively removing power from the-motor windings. This speed control is relatively coarse.

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- s -Conveniently a fine speed outer control loop is- superimposed over th.e previously described coarse sp~eed control. Thus a group of integrated pulses produced by the motor dîrectly proportioned to its speed of rotation is compared with a separate group of pulses having a constant ~requency whlch e~actly equates with the motor speed desired by comparator means thus ach.leving a frequency lock. ~y use of a down/up counter or integrator error ls fed into the back e.m.f. speed 10 control to provide fine tuning.
The motor pulses are conveniently produced ~y a variable reluctance type tachometer to produce .~ . a numher of voltage pulses in each motor revolution, t~se pulses are converted to a voltage9 this voltage 15 is- compared with. one of opposite sign created by osclllator means. Th.us a fine correction current is produced if the two voltages are not exactly matched.
Additional velocity feed~ack means is also optionally provided to stabillze th.e fine control loop.
~. Conveniently control of t.he number of pu~ses produced by the oscillator allows in turn fine control of the motor speed. Thus the motor is frequency locked to the oscillator controlled fine speed control loop and can be run at any desired constant speed 25 according to the oscillator s~etting.~ :
It is understood that it is conventional to employ back e.m.~. to control the speed of standard D.C. motors, but~this has not been used to control the sp~eed o brushless D.C. motors, as in the present case.
~:~ 30 Th~ use of "coarse" and "fine" Ioops to accurately control the speed o~ the.present motor is important : 6ecaus:e of the desired use of the motor in tape recorder :
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' and cartridge machines. In such machines, it is generally not required to know the precise position o~ the tape, and accordingly, it is not necessary to incorporate any circuitry which will provide positional information regarding the tape.
The invention ~ill be described in more detail having reference to the accompanying drawings .
in which:
Figure 1 is a generalized schematic circuit 10 diagram of the motor control circuit.
Figures 2A and 2B are detailed circuit diagrams of the motor control circuit.
Figure 3 is a sectional view of the motor co~nstruction.
Figure 4 is a schematic view of the motor fields.
Figure 5 is a more detailed view of a Hall effect switch.
Referring to Figures 3 and 4, the motor is 20 o a multlple speed brushIess D.C. outside rotor type hàving a rotor 1 and a stator 2 mounted internally of the rotor. The motor is well suited to driving a capstan ' ~ wheel of tape recording apparatus. The rotor l has an open top allowing s~elf-aligning bearings 3 to be 25 mounted on the shaft in close proximity to the major w~eight~distribuéi~on of the rotor. The stator includes ; quadr~atare windin;gs~Ll, L2, L3 and L~ general~ly~indicated~
as 5~in~Flgure~ 3. The rotor includes a pair of permanent ma~gnet poles 6.~ Hall effect switches HSl and HSl are 30 m~unted within~the~;ætator wlndlngs~;aDd are responsive to th~e magnetic poles~whereby the switches sense~the location and direction of movement of the poles as the roto~r rotates about the stator. ~ ~

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-: ' ~ , ' ' ' ' HALL EFFECT SWITCHES.
- Referring to Figure 5 these switches are m~niature four terminal devices which sense the direction of a magnetic field directed perpendicular 5 to the plane o~ the switch which in the drawing is the plane of the paper.
A bias current of about 5MA is driven from terminal 2 to terminal ].. Under no magnetic field terminal 3 and 4 have no potential difference ~etween them. As the magnetic field is increased in a direction directed into the page a potential difference will occur between terminals 3 and 4.
- The magnitude of the potential difference depends ol~ the magnetic field strength and the bias current.
(Typically 200 m V /Ma / Tesla). A magnetic field in the reverse direction directed out of the page will create a reverse potential difference (-200m V/Ma / Tesla) (1 Tesla ~ 10 Gauss = 1 Weber/sq.
metre).
~ The output of the Hall effect switches is amplified to create a field in the stator windings which always leads the permanent magnet field by an average of 90 degrees and hence creates an acceleràting torque in themotor. Thus the motor 25~ would~behave as a;series~D.C. motor~, if thls~torque was allowed to act in an uncontrolle~d manner and .
the speed of th~e motor would b~e l~imited only by motor losses (say 4000-6000 Rpm). Acc;ord~ingly it is necessary ; ; to~control the speed of the motor a~nd details of such co~trol will follow. The support bar 7 and ball bearings 8 allow the motor to be operated at any desir~ed direction ~ ~ between vertical and~hori~onta~

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Slots 10 are equi-spaced around the periphery of, the rotor. In this application there are 125 slots.
Mo~nted in the support bar 7 a reluctance tachometer 9 is adapted to detect the presence of each of the 125 slots during rotation of the rotor l. Thus 125 voltage pulses are produced in every revolution.
SPEED CONTROL.
To control the speed of the motor, two feedback loops are employed which comprise a coarse speed control inner loop and a fine speed control outer loop. The coarse loop is designed to hold motor speed to about 1% to 2% regulation while the fine loop reduces this regulation to about 0.1%. The speed co-ntrol device will be described having reference to Figure 1 which shows a b~lock diagram of the clrcuit.
The field windings 2 (L , L , L , L ) of the motor stator are alternatively sw21tched by the magnetic Hall effect switches 3 thereby creating a rotating field leading the rotor field thereby forcing the rotor to rotate. Switching transistors are controlled by the ~lall effect switches 3 to effect switching of the four field windings.
The controller includes an inner Ioop 11 .
which utili~es the motor back electromotive force as a measure of the DIotor speed. This circuitry will be described in greater detail later.
The o~lter loop incorporates a tachometer 9 mounted on the motor giving 125 pulses per motor rævolution. A stable oscillator 12 combines with a programmable divider 13 to measure the speed error as indicated by the number of pulses generated by the tachometer and those of the oscillator. An error signal 14 is generated where there is a speed error which is fed back to the inner loop su~ming junction.

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'3~3 g The error signal 14 is differentiated to provide an acceleration feedback signal which is also applied to the system summing junction to improve the stability of the system.
A dynamic brake 15 is provided and is used whenever the speel of the motor exceeds that normally encountered, e.g., a practical excess speed of say 2~ will cause the brake to operate. This is normally encountered when the selected speed i5 reduced or the motor is switched off.
ReEerring to Figures 2A and 2B, these show a detailed circuit diagram of the motor and control circuit .
Coils Ll, L2, L3 and 14 generally designated

2 are the four quadrature windings on the motor stator.
"HaIl effect" switches HSl and HS2 are buried in the quadrature windings to sense the direction and position of the permanent magnet rotor field. Figure 1 shows the phase relationship of windings relative to said Hall effect switches. Complimentary Darlington amplifiers Q5- - Ql, Q6 - Q2, Q7 - Q3 and Q8 - Q4 amplify the Hall effect switch outputs to create a field which always leads the permanent magnet field by an ayerage of 90 degrees and heDce creates an accelerating tor~que on the rotor.
Uncontrolled, this motor would behave as a series D.C. motor, where speed is ~limited only by motor losses and friction (typically 4000 - 6000 R.P.M.). To control the speed of the motor, two feedback loops are

3~0 e~ployed, via coars~e inner loop~ and a fine outer lo~p. ~The cpastan ~not shown3 tape drive diameter is chosen to give a~surface speed of 7~ inchlsec. (19.05 m.m./sec.) when c~he capstan speed~is lO revs.lsec.
which is equivalent to a motor speed o~f 600 R.P.M.
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-The inner loop is a voltage loop generatedby diodes Dl to D4 which pick up the negative generated motor voltage, the magnitude of which is proportional to the speed of the motor. This negative "generated"
voltage is converted to a current by Rll, which must exactly offset the current from resistors 8, 9 or 10, before comparator U3B cuts off drive to Q9 base and hence removes dr:ive from the Hall effect switches, which in---turn removes power from the motor field windings. Put another way referring to Figures 2A and 2B to control the speed of the motor the negative peaks of the generated voltages in Ll, 2, 3 and 4 are rectified by diodes 1, 2, 3, and

4 and therefore draw current from the summing junction of U3 B (pin 9) via resistor Rll and thermistor THl. At the same time resistors R8 or R9 are sourcing current to the junction.
When the sink current is equal to the source current, the bias current to Hall switch~s HSl and HS2 is reduced via Q9 which is driven from the output of comparator U3 B. This procedure effectively changes t~e "series" type motor to ~ "shunt" type motor. Each of the resistors R8, R9 or ~10 repres~ent a speed setting, high, medium and low.
The fine control (outer loop) involves an~analog comparison of frequency between a tachometer and a stable oscillator 12 (Ule and Uld). As explained :
in more detail hereinbelow, the tachometer 9 generates 125 pulses per revolution of the~ motor. The stable oscillator 12 generates a separate group of pulses ~havlng a constant frequency which;exactly equates with the motor speed desired. As explained below, : ~ ~
these pulses are summed at a summi~ng junction, and an analog error~ signal is produce;d. ~This analog error s~ignal is then used to control the speed of the motor to the fine control outer loop circuit. It ~.
can be appreciated that although the error signal is .

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~enerated from a dlgital source, namely the puls~s of the tachometer 9 and the stable osclllator 12, the operation of the fine control outer loop using the error signal is entirely analoge and does not require the presence of digitalization circuitry.
The tachometer 9 is a variable reluctancie type which generates 125 pulses per revolution of the motor. These pulses are amplified by U3d to deliver ~ a square wave at test point 4 (TP4) having a frequency lQ of 1250 Hz when the motor is running at 600 R.P.M.
On the negative going edge of each pulse, capacitor C4 and diode D9 inject a precise negative charge onto capacitor C6, so that the faster the motor spins the more negative the voltage on C6.
lS Capacitor C4 "differentiates" this square wave to produce 1250 Hz pulses, and diode D9 removes or absorbs the negative going pulses tending to charge capacitor C6 negatively~ At the same time capacitor C3 is "differentiating" a square wave 20 derived from oscillator ~ld and Ule and transistor Qll. Thus the positive going pulses from C3 are removed or absorbed by D8 tending to charge capacitor C6 positively once per oscillator period.
Comparator U3A (comparator 9), connected 25 as a high gain operational amplifier, amplifies the voltage on capacitor C6, to give an overall fine correction voltage at test point TP3 which ls - supplied via resistor R13 to thesumming junction o~
comparator &3b. ~K~t- effect is that if the motor 30 speed is low the output from the comparator U3a at test po~int TP3 is high (lOV), and conversely, if the motor sp'eed is too high the voltage at TP3 is low (OV). t Additional velocity feedback is als~o supplied via capacitor C9 and resistor 1~l to stabilize the fine

5 control loop. Oscillator Ule and Uld is a very stable temperature compensated C~OS oscillator whose frequency $s-adjusted by RVg.

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As the motor controller has been designed ~o operate at three harmonically related speeds (1:2:4), the oscillator 8 must be divided by 2 when operating at the medium speed and divided by 4 when operating at the low speed.
- This division is performed by U2a and U2b and the desired frequency is selected by D6 and D7 and Qll.
Inverter Ulf is wired as a buffer to 10 illuminate LEDl to indicate when fine control loop has "frequency locked" the motor.
DYNAMIC BRAKI~G OPERATION.
Brushless D.C. motors do not have the facility of "powering" speed reduction but can only rely on 15 mechanical losses to reduce speed.
In a high inertia load environment, i.e., capstan motors, it can take 10 to 15 seconds to reduce the speed from 30 revolutions per second which represents (fast forward speed) to 10 R.P.S~ (normal speed).
20 To overcome this dynamic or eddy current braking is employed according to the invention by shunting windings Ll, 2, 3 and 4 via diodes Dl, 2, 3 and 4 whenever the taehometer frequency exceeds the oscillator frequency.
This change in frequency is detected by 25 measuring the voltage at test point TP3 using comparator U3C. The voltage at point 4 of U3C is held at 1 volt.
Whenever the voltage at TP3 connected to the positive input of U3C dro~s below 1 volt, the output of U3C
at point 2 is at 0 volts. The low o;utput of U3C at ,-30 point 2 has two effects: ~
(a) ; Removes drive on the Hall effect devices HSl - and HS~2 by earthing Pin l4 of U3B via Dll.
(b~ ~ Shorts out windings Ll, 2, 3 and 4 via diodes Dl, 2, 3~;and 4, by turning Q12 and Q13 on.
35 The dynamic brakes are applied as long as the tachomater frequency is higher than the oscillator frequency or the RUN line connected to point 4 of U3C is taken to .
a high voltage of say 12 volts.
This occurs in normal operation say when a ~ 40 lower speed is selected.

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Thus in operation diode Dll removes drive from the power stage while dynamlc braking i8 in process.
~he input to diode D10 (RUN) will start the motor if LOW
or stop the motor lf HIGH. Start lnput via R12 can be us~ed as a "phase lock" input if motor was used in a "p-ilot tone" type recorder to ensure the pllot tone on the tape is replayed at the correct frequency, or as an additional drlve to the sulnming ~unction to avoid "START
WOW" when pressure roller is engaged.
EXTERNAL CONTROLS AND INPUTS.
POWER SU_PPLY (PIN 20) The motor requires an unregulated 30 to 40 volt 1 amp supply to operate. An in circuit 12 volt regulator supplies regulated power to-critical circuits.
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~15 RUN PIN 24 is connected to a cartridge tape micro switcb (not shown) and requires a - voltage of less than 1 volt to stop the motor or greater than 12 volts to start it.
START PIN 22 receives a high voltage (20-24V) when the pressure roller (not shown) is engaged. This gives a slight increase in operating power via R12, to compensate for : friction losses due to loading of pressure roller on motor.
SELECT HIGH SPEED (PIN 27) A low voltage (less than 3 volts) on this point causes UlC output to be at 12 volts, enabling potentiometer RVl and disabling counters U2A and U2B thus delivering 2500 Hz to the comparator as the reference frequency. The compara~or (U3A) is only - stable ~7hen the tachometer is delivering - 2500 Hz i.e. 20 revolutionslsec. or capstan speed of 15 inches/sec.
SELECT MEDIUM SPEED (PIN 26) A low voltage (less than 3 volts) on this point enables medium speed potentiometer ~RV2) and disables counter U2B (via D7), thus delivering 1250 Hz frequency to ~ 3~

comparator V3A which i8 only stable when capstan motor ls running at 10 revolutions/
sec. or 7~ inches/sec. capstan speed.
SELECT LOW SPEED ~PIN 25) A low voltage (less than 3 volts) on this - point enables potentiometer RV3 and allows divider U2A and U2B to divide osclllator (UlE and UlD) frequency by 4, thus delivering 625 Hz to the comparator U3A which is stable when motor speed is 5 revolutions/sec.
(3-3/4 inches/sec. capstan speed).
SET HIGH SPEED
RVl Is used to preset ~he fast forward speed to 20 revolutions per second (15 inches/sec.
capstan surface speed).
SET MEDIUM SPEED
RV2 is used to preset the coarse control loop speed at 10 revolutions per second (7~ inches/sec. capstan surface speed).
SET LOW SPEED
RV3 is used to preset for low speed (3-3/4 inches/sec. capstan surface speed).
SET FREO~UENCY
~ RV5 in the oscillator 12 is used to trim the normal speed to the required accuracy.
BALANCE
RV4 is used to balance the drive to the two Hall effect pairs HSl and HS2 this is set to mini~ize wow and flutter.
EXTERNAL INDICATION
A light emitting diode LED6 is a useful device to ascertain at a glance the ~ performance of the sys~tem at normal speed via:
~ ~a) LED at half brilliance is correct setting ..

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(b) LED flashing indicates SET SPEED is set too high or SET FREQUENCY RV5 is too low.
(c) I.ED fully ON lndicates SET SPEED is , set too low or SET FREQUENCY RV5 is set too high.
The invention has been particularly described when having reference to a three speed machine. However, there are two speed applications, e.g., in the playing of tape cartridges and such operation is achieved with only minor modification of the circuit.
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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEDGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A constant speed brushless D.C. motor of the permanent magnet outside rotor type with an internal stator wired in quadrature having a pair of Hall effect switches one of each mounted in proximity to one of a pair of opposite stator wirings to sense the direction and phrase relationship of the field in the rotor, a coarse feedback loop sensing means for comparing the negative generated motor voltage and motor potential, an outer fine speed control loop superimposed over said coarse feedback loop, said outer loop including means for producing a group of integrated pulses representing a constant frequency which is substantially exactly equated with a desired motor speed thereby achieving a frequency lock, said Hall effect switches operating under the influence of said comparator means to control power to said motor and thereby control its speed.

2. A motor as claimed in Claim 1 wherein said means for producing said integrated pulses representing the speed of the motor is a variable reluctance type tachometer capable of producing a number of said pulses in each motor revolution, means for converting said pulses to a voltage of predetermined polarity, oscillator means producing a voltage of opposite polarity from said separate group of integrated pulses, said second comparator means being adapted to compare said voltages such that a correction current is produced if said two voltages of opposite polarity are not matched.

3. A motor as claimed in Claim 2 wherein said means are provided to vary the number of integrated pulses in said separate group so as to vary the level of the voltage of said opposite polarity.