The invention relates to reducing the growth in speed of a DC motor, which ordinarily occurs as the brushes of the motor become seated.

BACKGROUND OF THE INVENTION

Motors used in windshield wipers in automotive vehicles are typically of the DC type, and are equipped with brushes, which deliver current to a rotating armature, via a commutator. In a newly manufactured motor, each brush makes contact with the commutator at a very small region. As the brushes become seated, however, the commutator wears cylindrical arcs into the brushes, and the region of contact grows. An increase in motor running speed accompanies this seating process. This speed increase is not desirable.

OBJECTS OF THE INVENTION

An object of the invention is to provide an improved DC motor.

A further object of the invention is to provide a DC windshield wiper motor having a lessened increase in speed due to brush seating.

SUMMARY OF THE INVENTION

In one form of the invention, the contact face of a brush is designed such that, as seating occurs, the effective contact angle changes, and in a direction which opposes an increase in speed.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A - 1C illustrate an equivalent circuit of a motor armature, and a simplification of the circuit.

Figure ID illustrates a prior-art motor for a vehicular windshield wiper.

Figure 2 illustrates an equivalent circuit for a DC motor. Figures 3 and 4 define angles used in certain equations given in the Specification.

Figures 5 - 7 illustrate the limits of integration used in certain equations given in the Specification. Figure 8 illustrates how motor speed changes during brush seating, an is attained by computations based on Equation 7.

Figure 9 resembles Figure 8, but a narrower brush was used in the computation. Figure 10 shows selected data taken from

Figures 8 and 9.

Figure 11 illustrates one form of the invention.

Figure 12 illustrates part of Figure 11, in greater detail.

Figure 13 provides dimensional information for the brush illustrated in Figure 11.

Figure 14 illustrates principles used by the invention. Figure 15 is a plot of a computer simulation of the form of the invention shown in Figure 11.

Figure 16 illustrates a comparison of some principles used by the invention with two superficially similar situations.

DETAILED DESCRIPTION OF THE INVENTION

Figure ID is a schematic of a prior-art windshield wiper motor. During low speed operation, current is delivered to the ARMATURE by brushes L and C.

During high speed operation, current is delivered to the ARMATURE by brushes H and C.

The advanced position of brush H (with respect to brush L, and against the direction of rotation) , in general, causes the speed increase, when brush H is used, as compared with brush L.

Armature Model

The brush-armature system of Figure ID can be modelled as shown in Figure 1A. Resistances Rl and R2 represent the resistances of the windings in the armature. Voltage sources e _gl and e _g2 represent the voltages induced in the windings. Resistance r _t represents the terminal contact resistance. Voltage source V _b represents the voltage drop across the brush.

Simplification of Armature Model

Equivalent Resistance This model can be simplified by use of the Thevenin equivalent shown in Figure IB. Figure IC shows why this simplification is justified. The top part of Figure IC illustrates how the equivalent resistance, r _eq , is obtained: the two voltage sources, e _gl and e _g2 are both set to zero, resulting in the short circuits indicated. The equivalent resistance is the parallel sum of Rl plus R2, as indicated in equation 1, below.

R _λ R ₂ eg R ₁₊ R ₂ (1)

Equivalent Voltage

The equivalent voltage V _eq in Figure IB is obtained as shown in the bottom part of Figure IC. Using

the principle of superposition, the voltage sources e _gl and e _g2 are set to zero, one-at-a-time, and an output voltage (V _outl and V _out2 ) is obtained for each case. These output voltages are each computed using the voltage- divider rule. The equivalent voltage, V _eq in Figure IB, is the sum of these two voltages, and is given in Equation 2, below.

^e gl ^R 2 ^+e g2^2

^V a= (2)

R +R

Speed Equation

The simplified model of Figure IB can be used to obtain an expression for motor speed. Applying Kirchoff's Voltage Law (KVL) to Figure IB produces Equation 3, below.

Vdc = ^r tl _a ~ ^V b ~ a ^{" V} eg (3 )

Equations 4 - 6 state well known motor relationships. In these equations, "k _v " is an equivalent armature constant,

"T _a " is developed armature torque,

"n" is motor gear ratio,

"77" is gear box efficiency,

"T _out " is output torque, and "T _D " is drag torque.

^ ⁼ 7? ⁽⁴⁾

^T oυ _t = ^nr lT _a -T _D (5)

Equations 4 - 6 are substituted into Equation 3, which is then rearranged, to produce Equation 7.

Equation 7 gives angular rotational speed, ω _r , as a function of several variables. These variables include V _eq (given in Equation 2) and r _eq (given in

Equation 1) . These variables are, in turn, computed from intermediate variables given in Equations 8 - 14. (Of these, Equation 11 defines a constant.) Table 1, below, indicates how the intermediate variables (of Equations 8 - 14) correspond to those needed in Equations 1 and 2, which are used to compute V _eq and r _eq , and which are needed in Equation 7.

For example, the first line of Table 1 indicates that, under low speed operation, the variable e _gl (shown in Figure 1A and used in Equation 2) equals e _ga which is computed in Equation 13. Under high speed operation, e _gl equals the sum of e _ga plus e _gb , computed in Equations 13 and 14.

Variables in Equations: Graphical Explanation The variables given in the two right-hand columns of Table 1 are defined in Figure 2. The angles "β" in Equations 8 - 14 are defined in Figure 3. In general, these angles are defined with respect to the contact points of the brushes, as indicated.

The angles "or" in Equations 8 - 14 are defined in Figure 4. These angles indicate the lengths of the arcs which the commutator cuts into the brushes, as the brushes seat. Of course, in a new, unseated brush the 5 angle "of" is zero (in theory) .

Figures 5 - 7 illustrate, in graphical form, the limits of integration stated in Equations 8 - 14. Figure 5 refers to Equations 8 and 12. Figure 6 refers to Equations 9 and 13. Figure 7 refers to Equations 10 0 and 14. These Figures are obtained in a straightforward manner, by applying the limits of integration to Figures 3 and 4.

^r _a =] _n Zr _coil d =Zr _coil [2τt+β _c -β ₁ -α _c - ₁ ] (8)

= ^α ^r _coil dφ=Zr _coil [β ₁ -β _h - ₁ - _h ] (9)

CN,

Z = (11)

2π

e _αa = ^»2 2 ^π π ⁺ + ^P β ^c _c ^" - ^β α ^c _c ZΦ _m ω _r sin(φ-γ)dφ = ZΦ _m ω _r [cos (β _c -α _c -γ) -cos (β ₁ +α ₁ -γ) ]

(12)

^{b =} _h ' ^{"Z fflωrSin(Φ"γ) d} -cos(β _Λ+ α _Λ -γ) ^{( 3)}

^e c = " ^'ah ZΦ _m ω _r sin(Φ-y) dΦ -ZΦ m _m ω r _r [cos ⁽ β ₁ -α ₁ -γ) -cos(β _c +α _c -γ) ⁽ f 4

TABLE 1

Characteristic Low Speed Operation High Speed Operation (Figure 2)

^e gl ^e ga ^e ga ^{+ e} gb

^e g2 ^e ga ^e gc

Rl r _a r _a + r _b

R2 r _a r _c

Equations 8 - 14 are simplifications, and assume that both the resistance of the armature coils, and the induced voltages, can be treated as though continuously distributed over the circumference of the armature.

Plot of Equation 7

The inventors ran a computer simulation, based on Equation 7, using brushes having widths of 4.5 mm. (Width is defined as dimension W in Figure 3. ) The computation was done for three load points, namely, 1.0,

2.0, and 3.8 Newton-meters of torque. Figure 8 illustrates the results. One can see that speed monotonically increases as the brushes become seated.

The inventors repeated the simulation, but with a different brush width, namely, 4.0 mm. Figure 9 illustrates the results.

Figure 10 emphasizes selected data from Figures 8 and 9. Figure 10 indicates that, upon full seating, for each load, the final speed of the narrow (4.0 mm) brush, is less than final speed of the wide brush. The differences range from 0.7 to 0.9 rpm.

Analysis

The independent variable ω _r in Equation 7 depends upon its dependent variables in a complicated manner. However, the Inventors surmised that one significant factor causing the speed increase was the increase in the angles a (see Figure 4) , which occurs as the brushes seat.- In order to compensate for this change in of, the Inventors proposed causing the angles β (see Figure 3) to shift as the seating occurs.

One approach to causing this shift is illustrated in Figure 11. A brush B is offset from a radius R by dimension e. As the brush moves into contact with the commutator C, it initially contacts at point PI.

As the brush seats, an arc becomes worn into the brush. The arc terminates at points PI and P2, as indicated.

However, the initial point PI is located nearer to the EDGE1 than to EDGE 2. Consequently, the wear-in causes the arc to reach ^' EDGE 1 before reaching EDGE 2. Once the arc reaches EDGE1, the angular position of PI does not change. However, P2 does continue to move as wear-in progresses.

Because P2 is moving, while PI is fixed, the midpoint M of the arc moves, changing the brush angle, β . Figure 12 illustrates this change in greater detail.

Initially, the brush B contacts at a single point PI. The brush contact angle, βl , is indicated. As seating occurs, an arc is cut into the brush, which terminates at points PI and P2. As the arc is being cut, PI moves left, and P2 moves right. However, when PI reaches EDGE 1, PI stops moving. Now, as the arc continues to be cut, the midpoint M, which defines the angle β , moves to the right, because P2 is moving rightward. The angle β2 is different from βl .

Figure 13 provides dimensional information for the brush illustrated in Figure 11.

Figure 14 gives an implementation of some of the principles discussed above. In Figure 14A, brush placement is done as in the prior art, with the exception

that initial β _h is advanced against the direction of rotation, by about ten degrees.

This advancement causes, as expected, a small increase in initial speed. In particular, this advancement, using narrow (4.0 mm) brushes, was found to give an initial speed which was about equal to the final, fully seated, speed attained using wide (4.5 mm) brushes.

Upon final seating, the invention causes β _h to become retarded (in the direction of rotation, or counter-clockwise, CC ) .

Plot of Invention

Figure 15 illustrates a plot of a computer simulation of the invention shown in Figure 11. Clearly, the speeds peak at about 40 % of seating. Further, the pairs of (initial speed, final speed) are approximately given by

(64.5, 66) at 3.0 Nt-m (69.0, 70.5) at 2.0 Nt-m (72.0, 73.0) at 1.0 Nt-m. Further still, the peak speeds are about 67, 72, and 75, running from heaviest to lightest loading.

Percentage Computations

Given these data, the maximum percentage peak in speed is about four percent:

(75 - 72) / 72 = 0.042. The maximum final percentage rise in speed is about 2.3 percent :

(66 - 64.5) / 64.5 = 0.023

Distinction

The change in β caused by the brush geometry shown in Figure 12 should be distinguished from the superficially similar situations shown in Figures 16A and 16B. In these Figures, two brushes Bl and B2 are shown. The contact surfaces SI and S2 are both flat.

In both brushes, at the initial instant of contact, each flat surface SI and S2 contact the commutator C at a single point P, and are tangent at that point. (Reason: by definition, any straight line which contacts a circle at a single point is tangent at that point. )

Then, so long as the brushes move in a straight line (indicated by either arrow Al or A2) , the contact surfaces SI and S2 will remain parallel to the initial tangent line. (Reason: if the brushes move in a straight line, angle A in Figure 16B remains constant. The surfaces SI and S2 remain parallel to their former positions. )

Since the surfaces SI and S2 remain parallel to a the initial tangent, as they move, the CHORD defined by

Sl and S2 will always remain bisected by the radius which runs through the initial contact point P.

Therefore, the angle β (defined in Figure 3) remains constant. β is defined by the centroid of the arc cut into a brush. This centroid remains at P.

However, in contrast, if the arc being cut reaches an edge of the brush, as in Figure 16C (and Figure 12) , then the left end of CH0RD1 in Figure 16C terminates at that EDGE, but the right end continues to follow P2 as the arc is cut. The centroid of the arc, indicated by point P5, moves clockwise, away from initial point P. β changes.

Use of appropriate curved surfaces, in different positions, instead of flat surfaces SI and S2, can control how the centroid P5 moves over time.

Definitions

The angles β in Figure 3 can be called "effective brush angles," or "effective brush contact angles." Initially, for a new brush, the angle is determined by a single point of contact. For a seated, or partially seated, brush, the angle is determined by the midpoint, or centroid, of the arc cut into the brush. The term "brush" is a term-of-art. Present-day brushes take the form of abrasion-resistant blocks of carbon or graphite, which may be sintered with a binder.

The term "brush" is believed to be applied to such structures for historical reasons.

Additional Embodiment

The preceding discussion has implied that the invention is applied to the high-speed brush alone. However, in some situations, this restricted can be lifted, and the invention can be applied to two, or all, of the brushes shown in Figure 2.

Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.