US3469135A - Tone signal generator - Google Patents

Description

L. J. HAVILAND ETAL TONE SIGNAL GENERATOR Sept. 23, 1969 Filed March 24, 1967 5 Sheets-Sheet l p 3', 1969 L. J. HAVILANb ETAL 3,469,135

TONE SIGNAL GENERATOR 5 Sheets-Sheet 2 Filed March 24, 1967 I P 1969 L. J. HAVILAND ET AL TONE SIGNAL GENERATOR Filed March 24, 1967 5 Sheets-Sheet 5' United States Patent 3,469,135 TONE SIGNAL GENERATOR Lyman J. Haviland, Port St. Lucie, Fla., and Herbert E. Meinema, Barboursville, Va., assignors to Hammond Corporation, a corporation of Delaware Filed Mar. 24, 1967, Ser. No. 625,670 Int. Cl. H02k 17/42, 19/20, 19/24 US. Cl. 310-170 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention Electric tone signal generation for musical instruments Description of the prior art One of the basic types of tone signal generating systems used in electric organs and similar musical instruments is illustrated in the patent to Laurens Hammond, No. 1,956,350. The arrangement is to provide a synchronous electric motor to drive a plurality of rotating serrated discs, in this art usually referred to as tone wheels, one for each frequency in close proximity to permanent magnet pole pieces having pickup coils. The tone wheels form portions of magnetic circuits with the pole pieces and thus their rotation generates an alternating current in each pickup coil at the desired frequency. The frequency generated in each coil depends upon motor speed, the gear ratio of the drive from the motor to the particular tone wheel, and the number of serrations around the periphery of the wheel.

In the particular organization of the above-mentioned Hammond patent, there is a tone wheel for each fundamental tone signal required in the instrument and some extra wheels which supply upper harmonic structure above the fundamental limits of the keyboard.

Another system which makes use of rotating wheel type generators is illustrated in the patent application of William B. Ayres, Serial No. 560,217, filed June 24, 1966. In this system a group of twelve tone wheel generators supplies all of the signals for the top octave only of the desired group of signals, and the signals of lower frequency are obtained from these originally generated signals by means of cascaded frequency divider circuits.

SUMMARY OF THE INVENTION This invention relates to either type organization, but in the interest of simplifying the disclosure, the description will be largely limited to a generating system of the the second type, that is, a system comprising an octave module which generates the twelve tone signals for a single high octave, frequency dividers, not shown, being used to supply the lower octaves, and if desired, frequency multipliers to provide still higher frequency tone structure. By the use of appropriate driving gear ratios, and tone wheels with different numbers of teeth, and different shaped pole pieces, a plurality of the single octave modules can be used to supply all of the tone signals required in an organ of the previously mentioned Laurens Hammond type.

In view of the above, it is an object of this invention to provide a novel signal generator of the tone wheel type which is particularly adapted for organization into octave modules.

An additional object is to provide a novel tone wheel type signal generator which can be manufactured at relatively low cost, which is stable in operation and trouble free, and which has a relatively high signal output voltage.

Yet another object is to provide a novel signal generator of the above type which requires fewer elements, and particularly fewer different kinds of elements, and lower cost elements than is customary.

Other advantages and features of the invention will become apparent from the following description of a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the drawings in which similar reference characters refer to similar parts throughout the several views:

FIG. 1 is a diagrammatic top view showing the general organization of a typical generator module embodying this invention adapted for providing a complete octave of tone signals;

FIG. 2 is a transverse sectional view in greater detail which is taken in the direction of the arrows substantially along the line 22 of FIG. 1, but rotated 90 degrees;

FIG. 3 is a sectional View taken in the direction of the arrows substantially along the line 33 of FIG. 2;

FIG. 4 is a longitudinal sectional view showing details of construction of one of the tone wheels and its associated mounting and drive mechanism;

FIG. 5 is a view similar to FIG. 4, but showing a variation the construction; and

FIG. 6 is a much enlarged, somewhat diagrammatic, plan view of one of the tone wheels and the pole pieces of its associated magnetic circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a frame 10 supported upon resilient shock mounts 12 carries a synchronous electric motor 14 near one end and the tone signal generator module 16 strung along the frame to the right of the motor. The generator module 16 is made up of three essentially similar submodules 18, 20 and 22. Each of these, 18 for example, consists of a pair of parallel plates 24 and 26, carried in the frame, and provided with bearings which journal a main shaft 28. One end of this shaft 28 is connected through a flexible coupling 30 with the shaft of the motor 14, so that the shaft 28 runs at motor speed, but alignment problems are avoided and pulsations in the motor shaft speed are to some extent absorbed by the flexible coupling.

Four tone wheels 32, 34, 36 and 38 are mounted within the submodule 18, as will be described presently, upon separate shafts parallel to the drive shaft 28, and are driven through gear trains from the main shaft 28. In the interest of convenience of reference and understanding, the gears in these trains will be assigned characters which also indicate the number of teeth in the particular gear. For instance, drive shaft gear has 85 teeth and drives gear 78, having 78 teeth, which is on the shaft with tone wheel 32. Thus, tone wheel 32 rotates at 85/78 drive shaft speed. Similarly, drive shaft gear 82 rotates gear 71 on the shaft for wheel 34 at 82/71 drive shaft speed.

Assuming the motor speed is 3600 rpm, and that the tone wheelsall are the same in this modulehave 64 teeth, it will be apparent upon calculation that the teeth of wheel 32 will move past their stationary magnetic field poles, to be described presently, at a rate which generates the note C, at about 4816 Hz. and that similarly wheel 34 generates the adjacent semitone Cit.

The shaft 28 also drives tone wheel 36 by way of gears 104-85 which gives D, and wheel 38 by way of gears 70-54 which gives Dii.

The end of the drive shaft 28 projects through frame plate 26 and is connected to the drive shaft 40 of the next sub-module 20 through a flexible coupling 42. This submodule has tone wheel 44, driven by gears 144-83 to provide E, wheel 46 driven by gears 96-66 to provide F, wheel 110 driven by gears 74-48 to give Fit, wheel 112 driven by gears 80-49 to give G.

Similarly, the end of the drive shaft 40 is connected by flexible coupling 114 to drive shaft 116 of the third submodule 22. This submodule has wheel 118 driven by gears 100-63 to give Git, wheel 120 driven by gears 88- 48 to give a precise A at 7040 Hz., wheel 122 driven by gears 101-52 to give All, and wheel 124 driven by gears 107-52 to give B. Note that in some instances a single gear, such as gear 52 in the combinations 101-52 and 107-52 can be used in more than one place in the assembly, thereby requiring fewer separate kinds of gear parts to be manufactured than the total number of gears used.

Calculation will indicate that all of the notes generated by this module are quite accurate. A is accurate at an octave relationship to the 440 Hz. international standard, and the most inaccurate note is only off the theoretical by one part in 2,500. Also, for practical purposes, all the notes that are off the theoretical are off in the same direction, minus, the maximum inaccuracy in the plus direction being only an insignificant one part in 32,500. The inaccuracies introduced by the system are, therefore, less than can be noticed and certainly less than it is possible to avoid as a practical matter with any tunable instrument, such as a piano or pipe organ, excepting for perhaps a very brief interval following tuning.

One of the submodules is illustrated in greater detail in the remaining figures. It comprises the side bearing and mouting paltes 24 and 26 and a pair of intermediate closely spaced bearing plates 130 and 132. All these plates are held in assembled parallel relationship by bolts 134, nuts 136 and appropriate intervening tubular spacers 138. The drive shaft 28, with the drive gears 82, 85, 70 and 104 pressed thereon to appropriate position is journaled in bushings in the outside plates 24 and 26 with appropriate spacers 140 preventing excessive end play. The center plates 130 and 132 have cutouts 142 large enough to clear the drive gears to faciiltate assembly.

Each of the tone wheels and its gear, for instance, tone wheel 32 and its gear 78, are assembled upon a shaft 144 journaled at its ends in bushings secured to one of the end plates and its adjacent intermediate plate, in the case of wheel 32 and gear 78 these are plates 24 and 130.

In the example shown, all of the tone wheels are identical and have 64 teeth. They are formed in this example by pressing substantially pure electrolytic powdered iron in molds under heavy pressure and sintering at about 2080 F. in an ammonia atmosphere according to a Well understood technique. This provides a wheel having desirably soft magnetic characteristics. They are less than an inch in diameter and of a convenient thickness, about /s-inch, to work out satisfactorily within the limitations of the powdered metal method of fabrication.

The tooth form is essentially invlute, but somewhat shortened or stubbed to provide a generated output waveform which is sinusoidal and very low in harmonic content. The modified tooth shape also simplifies the molding operation. Powdered metal wheels have no grain orientation, therefore, the magnetic reluctance across the wheel diameter does not change as it rotates. If the wheel is formed from sheet steel in the customary fashion, the grain orientation pattern produced when the sheet steel is rolled causes the magnetic reluctance of the wheel to be lower along the diameter parallel to the grain and higher across the grain. As the wheel rotates this change in reluctance generates a low frequency signal which is twice that of the wheel speed. Under some conditions this low freqeuncy voltage can cause considerable difiiculty, particularly with the higher frequency generators. The usual solution is to provide high pass filters for the generators affected, usually those above about 600 Hz., to remove these spurious frequencies. The use of powdered iron wheels which have random grain orientation has been found to eliminate the need for these high pass filters for each note.

It is not practical, using present pressing methods, to hold the center hole in a molded tone wheel of this type concentric with the periphery of the wheel within acceptable limits for this specialized purpose. The solution we have found for this problem is to make the center hole oversize and to provide the shaft 144 with a collar 152 having a narrow annular ridge 154 which engages the face of the wheel. The shaft and gear wheel are then set up in a fixture which establishes precise alignment and the collar 152 is soldered, as at 156, to the [face of the wheel. This assembly, it has been found, results in a very accurately running wheel.

Tone wheels of this small size have very little rotational inertia and care is necessary to avoid transmitting the rotational pulses from the motor shaft to the tone wheels. This is more of a problem at the low frequencies where the wheels run at lower speed. The solution shown for the low frequency wheels is to provide a compound coil spring 158. The small diameter portion of the spring slips over and grips the shaft 144 near the wheel 32 and transmits drive from the spring to the shaft 144. The larger diameter concentric free spring portion provides the necessary softness in the drive system and the free end of the large diameter portion extends outwardly and is formed at its tip to engage a hole in the gear 78. To establish alignment, the last few turns of the large diameter spring portion slip over a hat shaped collar 159 which loosely fits the shaft 144 and engages the face of the gear. The gear 78 is loose and floats upon the shaft 144 and is held by the spring 158 lightly against a positioning collar 162. Thus, drive is from the main shaft 28, by way of gear 85 to gear 78, and thence through the light spring 158 to the shaft 144 and wheel 32. A preferred arrangement is to match the spring compliance to the tone wheel rotational inertia such that the torsional resonance period is at a vibrato rate, about 7 Hz. This effectively isolates the tone wheel from pulses in the drive system which otherwise might produce tone flutter.

This drive system also can be used for the high frequency wheels, but has been found to be an unnecessary refinement, since the pulsations are more rapid and of less amplitude. A simple inexpensive expedient which works well at higher frequencies is to replace the spring 158 with a soft disc or short sleeve of stable resilient plastic sponge material 164 such as polyurethane sponge, for instance, which is clamped under slight compression between the gear 78 and the tone wheel 32. The sponge frictionally engages both the wheel and the gear and prevents slippage, since the torque to be transmitted is low, but the sponge is sufficiently torsionally nonrigid so that the high frequency pulsations are absorbed therein and do not reach the tone wheel.

The field pole pieces 166 are formed in U shape of sheet iron stock, which is considerably thinner than the tone wheels for a reason to appear presently, and are riveted at the midpoint to the ends of nonmagnetic, brass for instance, posts 168 which are secured to the plates 24 or 130 or the equivalent by soldering, for instance. The post length is such as to accommodate a slotted permanent magnet '170 between the pole piece and its mounting plate. It is preferred to use permanent magnets 170 formed of a resilient rubber-like material, such as Plastiform Nitrile R2 supplied by Leyman Corporation, Magnetics Division, for example, but other materials could be used if desired. Such permanent magnets can simply be slid into place since their thickness can be quite uniform, and will be retained by magnetic attraction, The slot also permits the magnets to be moved in or out a bit relative to the pole tips so as to adjust the voltage outputs of the various generators. Although these magnets are of generally horseshoe shape, they are magnetized transversely, so that the opposite poles are on the opposite large fiat faces. When installed they are oriented with the south pole of one magnet and the north pole of the other against their pole pieces 166. The magnetic circuit, therefore, includes the pole pieces, the tone wheel and the mounting plate closest to the wheel. Thus, the magnetic circuit is short and essentially complete in magnetic material and no two tone wheels use the same mounting plate to complete the magnetic circuit. This, together with the shielding effect of the mounting plates, reduces interference to a minimum. By making the tone wheels considerably thicker than the field pole legs, the wheel edges are remote from the magnetic flux path between the pole legs and the wheels as compared with the centers of the wheel rims and thus, any inaccuracy at the edges of the wheels does not modulate the magnetic circuit and cause noise. The; wheels also have a strong magnetically induced tendency to run well centered relative to the field poles.

When the pole pieces 166 are riveted to the posts 168, a thin brass U-shaped strip 172 is also secured by the heading operation on top of the pole piece 166. This brass strip has transversely extending short tabs 174 near the terminations of the pole pieces, the purpose for which will be pointed out presently. A terminal tab member 175 is also formed at the bight of the U and is bent outwardly to make it more accessible.

Two sets of these pole pieces and magnets are provided for each tone wheel and they are mounted across the axis from each other to provide four main pole legs 173. As shown, each of these pole legs is subdivided at its free end to form three teeth 176 which have double the spacing of the wheel teeth. Note, however, that at one side of the wheel, as the three teeth of one pole leg are aligned at a certain instant with wheel teeth, the three teeth of the adjacent pole leg are aligned with the valleys between the wheel teeth. The pole arrangement is similar at the opposite side of the wheel, so that pole legs having similarly aligned teeth are opposed across the wheel axis (see FIG. 6).

The principal advantages of this arrangement are as follows. As the wheel rotates, the teeth on one pole of a pair will be approaching alignment while those on the other pole will be receding. If the flux through the teeth is changing at a sinusoidal rate then the total flux through the wheel-pole structure will remain constant. The wheel teeth will not attempt to line up or lock in with the pole teeth when at rest and thus the only force required to rotate the wheel will be that necessary to overcome bearing and gear friction and wheel rotational inertia. Starting torque requirements are therefore very low. Radial motion of the wheel due to bearing clearance and wheel unbalance will not produce a change in total fiux and therefore will not develop or generate a spurious signal in the electrical output. Also frequency modulation due to eccentricity or out-of-roundness of the wheel will be considerably reduced. Because of the confined path of the alternating flux in the pole structure, cross coupling of alternating flux between adjacent wheels is greatly reduced as is the effect of stray magnetic fiields such as that developed by the driving motor. Since the direction of magnetic flux across any wheel periodically reverses, the wheels do not acquire a permanent magnet set, which is a frequent source of low frequency noise in prior art wheels. The wheels magnetically center themselves in the plane of the poles and there is no side motion to cause a change in the output amplitude or change in frequency due to end thrust friction in the hearings.

Each of the four legs of the field assembly is provided with a coil 180, all of which are connected in series. In the interest of convenience and cost saving, these coils in the present example are wound upon a form which is somewhat larger than the pole legs and all four coils are wound as one continuous interconnected unit. The wire runs between the individual coil sections are bent as necessary and the coil sections are then slipped over their pole legs and the tabs 174 previously mentioned are bent up to retain the coils in place. Note that if the wire runs between the coils are bent as illustrated, the signals generated in the individual coils are in series aiding relationship. If desired, the four coils can be wound as one continuous closed coil which is then pulled apart at the appropriate places to form the four separate coils with the intervening connections.

One end lead of the coil string is soldered to one of the terminal elements 175 which is, of course, grounded. The other coil end lead is soldered to a sheet metal terminal member -177 which is crimped around the other terminal element 175 with a short piece of insulating sleeving 179 intervening so as to electrically insulate the terminal 177 from ground.

A complete octave module is made up of three of the submodules, each of which provides four notes, the drive interconnection between the submodules being effected by way of the flexible couplings 42 and 114 previously mentioned. If it is desired to provide a module for the octave below the one shown, the two modules can be alike but connected by a two to one reduction gear ratio, such that the lower octave wheels run at half speed. The next still lower octave module, if provided, seems to be best set up by using tone wheels with 32 teeth rather than 64, with successive octave modules having wheels with 16 teeth, 8 teeth and so on. Also, it may be desirable to arrange the field pole pieces with fewer teeth at the lower frequencies, but this is largely a matter of design preference.

In setting up the pole pieces 166, it has been found that a good way of insuring proper spacing and orientation, relative to their wheels, is to surround the particular tone wheel with a close fitting steel master gauge ring of proper thickness, and then to set the pole pieces loosely in position with small bosses at the ends of the posts 168 in short slots in the mounting plate. The ends of the pole pieces are attracted magnetically against the gauge ring and the posts 168 can then be soldered, or otherwise secured in a manner which does not disturb the alignment, to the mounting plate. Subsequently, the gauge ring is removed and precise locating of the poles will have been achieved.

The voltages developed by this generator are high as compared with older systems, particularly because the magnetic circuits are short and do not require a return path through the air. Additionally, the field pole legs, the field teeth and pickup coils are all multiplied over the usual practice. Also, each of the generator units is well shielded and uses material in its magnetic circuit which is individual to the particular signal frequency, and this avoids the usual problems associated with crosstalk and other forms of interference.

For the reasons previously explained, the unit is easily started and very free running and smooth in operation, thereby reducing the driving power requirements and avoiding acoustical noise. The unit is compact and rugged and servicing problems are largely avoided. Furthermore, the various individual elements of the generator are relatively small and inexpensive to manufacture in quantity and the assembly operation is straightforward without the usual necessity for minor individual adjustment of the generators.

From the above description of the invention, it will be apparent that variations may be made in the structure which forms the illustrative embodiment without departing from the scope and spirit of the invention and that the scope of the invention, therefore, is to be measured from the scope of the following claims.

We claim:

1. A musical tone signal generator comprising a rotary drive mechanism, a main shaft coupled to said drive mechanism, a plurality of tone wheels, means for driving said tone wheels from said drive mechanism, each of said tone wheels having a toothed periphery, a pair of pole members mounted across from each other relative to each wheel, means providing a magnetic circuit such that said pole member pairs have opposite magnetic polarity, each of said pole member being subdivided to form two pole legs, a pickup coil for each of said legs, said coils for all of said legs of each pair of pole members being connected in series signal aiding relationship, each of said pole legs having one or more teeth opposing the wheel teeth, the teeth on one of said pole legs aligning with the teeth of said wheel when the teeth on the other pole leg of the same pole member are aligned with the valleys between the wheel teeth, the teeth of the pole legs opposing each other across the axis of a particular wheel having the same degree of alignment with the wheel teeth, said tone wheels being formed of pressed and sintered powdered iron and said tone wheels being substantially thicker than said pole legs, such that any uneveness at the edges of the tone wheels is remote from the magnetic fiux path from said pole legs to the central portions of the wheel rims.

2. A musical tone signal generator comprising a plurality of tone wheels, means for rotating said tone wheels, each of said tone wheels having a toothed periphery, a pair of pole members mounted across the axis from each other relative to each wheel, magnetic circuit means for maintaining the pole members of each pair at opposite magnetic polarity, pickup coil means for said pole members, whereby rotation of said wheels between the magnetically oppositely polarized opposed pole members subjects each of the tone wheels to a perodic reversal of magnetic flux through said wheels, said tone wheels being formed of pressed and sintered powdered iron and said tone wheels being substantially thicker than said pole members, such that any unevenness at the edges of the tone wheels is remote from the magnetic flux path from said pole members to the central portions of the wheel rims.

References Cited UNITED STATES PATENTS 2,417,868 5/1943 Glass.

2,415,022 7/ 1943 Morrison 318-171 2,314,496 3/1943 Hammond 310- 2,159,505 5/1939 Hammond 310-170 2,130,251 9/1938 Richards 310-170 2,012,207 8/1935 Walton 310-164 1,956,350 4/1934 Hammond 310-170 XR 1,295,691 2/1919 Cahill 310-170 XR 3,087,080 4/1963 Isaacson 310-44 XR 1,160,087 11/1915 Nevland 310-169 X FOREIGN PATENTS 384,681 3/1932 Great Britain.

385,926 10/1932 Great Britain.

MILTON O. HIRSHFIELD, Primary Examiner MARK O. BUDD, Assistant Examiner

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