GB2124712A - D.C. motor and diaphragm-pump assembly - Google Patents

Abstract

An assembly for reducing load- induced torque ripple of a diaphragm pump 20 driven by a D.C. motor 12 and operated by a plunger rod 38 that is coupled to the diaphragm at one end and to an eccentric mounting post 18 coupled to the shaft 16 of the motor at the other end comprises a coiled spring 56 coupled at one end 58 to a fixed point relative to the frame of the motor and at the other end to the end 40 of the rod. Thus as the pump operates the spring will be stretched and will gain energy as the plunger is pulled upwardly to expand the pump chamber. In this manner the load-induced torque ripple which would occur without the presence of the spring is greatly reduced, and the load on the D.C. motor approaches a constant torque load. <IMAGE>

Description

SPECIFICATION D.C. motor and diaphragm-pump assembly The output of a D.C. motor at low speeds is generally assumed to be constant. However, the actual torque often oscillates measurably above and below an average torque value. This is due usually to the switching action of the commutator but is sometimes due to the armature reluctance torque. The torque ripple induced in this manner is generally a very small percentage of the rated output torque and generally does not create problems.

When a D.C. motor, however, is being used to operate a mechanism such as a pump in which the load varies greatly from the low load intake portion of the cycle to the compression, or pressure, portion of the cycle, this varying load can create a substantial torque ripple effect on the D.C. motor. In order to reach high pumping pressures it has generally been the practice to employ motors which have relatively high input load currents. A substantial percentage of this input power was employed to offset the load induced torque ripple caused by the accelertion and deceleration of the armature.

In applications where the size of the pump is an important consideration, and especially in applications which are portable where a battery and the pump must both be carried by a person, a power drain which requires a larger than necessary battery is a decided disadvantage. One such application of a D.C. motor driven pump is found in ambulatory blood pressure monitoring systems, wherein the motor operates the diaphragm pump to provide the pressure necessary to pressurize the cuff employed in the blood pressure monitoring.

According to the invention, there is provided a D.C. motor and diaphragm pump assembly which employs a reciprocatory plunger rod that is coupled to the diaphragm of said pump at one end, and is mounted to an eccentrically rotating means coupled to the motor drive shaft on said motor at its other end, comprising resilient means connected at a first end to a fixed point, and at a second end to an eccentrically rotating point so that said resilient means acquires energy during the expansion portion of the operating cycle of said pump and releases said acquired energy during the compression portion of the operating cycle of said pump which acts to reduce the load-induced torque ripple effect on said motor that is caused by said pump.

The employment of a resilient means, such as a coiled spring assembly, in the manner of the present invention greatly reduces the load required from a battery, and at the same time enables the pump to achieve higher operating pressures than are obtainable without the use of the coiled spring assembly.

An embodiment of the invention will now be described by way of example and with reference to the drawings, in which: Figure 1 is a perspective view which shows a diaphragm pump and D.C. motor assembly according to the present invention; Figure 2 is a side partial-sectional view of Figure 1 taken along the lines 2-2 of Figure 1 in which the base portion of the pump is shown in cross-section; Figure 3 is a load current versus pumping pressure graph which shows the operation of the pump with and without the coiled spring assembly for a twelve volt rated D.C. motor operated at seven volts D.C.; and and , 4 shows the load current versus pumping pressure curves of the same motor operated with and without the coiled spring assembly at six volts D.C.

A D.C. drive motor and diaphragm pump assembly 10 is shown in Figure 1 which may be employed in portable applications, such as ambulatory blood pressuring monitoring systems, for example. The motor 12 has a coupling block 14 mounted on its output shaft 16 which has an eccentrically mounted pin 18 on it. The diaphragm pump assembly 20 is supported on the feet 22, and has an outer housing 24 which covers and encloses the pump assembly.

The motor is supported by the bracket 26 which extends upwardly from the upper surface 28 of the housing of the diaphragm pump. The screws 30,32 support the motor in a cantilever fashion from the rear face of the bracket 26.

The diaphragm 34forthe pump is connected by the coupling 36 to the lower end of the reciprocating plunger rod 38, the upper end of which has an eyelet 40 integrally formed thereon which receives the eccentric pin 18 therein. Thus, as the motor shaft rotates the pump is operated by means of the reciprocating action of the rod 38, and when the eccentric pin 18 is at the upper extent of this motion at A the chamber 42 has its greatest volume, and atmospheric air is drawn in through the tube 46' and the inlet valve 46. At this time the outlet valve 48 is closed. During the upward stroke of the rod 38 from the point B to the point A the motor is under a relatively light load.If an average torque value is assumed for the entire cycle, therefore, the upper stroke portion of the rod between the points B and A corresponds to a torque load that is lower than the average value. On the down stroke of the plunger rod 38 from the point A to the point B, the torque load on the D.C. motor will be greatly increased over the average cycle value as the plunger is driven downwardly to compress the chamber 42. The output valve 48 will then be open allowing air to be communicated through the tube 50 to an output device such as a pressure cuff of an ambulatory pressure monitoring system where the pressure must be built up. The inlet valve 46 is then closed.

Prior to the present invention, this cyclic operation of the diaphragm pump in fact did create a large torque ripple in which the torque load would be reduced upon the upstroke of the plunger rod and greatly increased upon the downstroke. This caused a continual acceleration of the armature of the motor during one-half of the cycle and a deceleration of the motor during the other half of the cycle. The forces created by this repeated acceleration and deceleration of the armature caused a substantial amount of input energy to be wasted so that it did not contribute to the development of the desired pressure. Input current is supplied to the D.C. motor 12 from a battery through the wires 52 and 54.In an ambulatory blood pressure monitoring system it is desirable to keel: the battery size as small as possible, and, therefore, it is desirable to employ a battery which operates at no more than about 6 volts D.C. to supply input current through the leads 52 and 54.

A D.C. motor and diaphragm pump assembly which is suitable for supplying pressure to an ambulatory pressure monitoring system, and which generally resembles the pump and motor assembly described so far by reference to Figures 1 and 2, is commercially available. One such motor and pump assembly is commercially sold under the name Romega Model 080. In an ambulatory blood pressure monitoring system it is desirable to keep the battery voltage low on the order of 6 volts D.C. A motor and pump assembly with a 6 bolt rated motor would thus conventionally be used with a 6 volt battery. In order to employ a 6 bolt D.C. rated motor and to achieve pressures of over 200 millimeters of mercury in a blood pressure cuff, a relatively high battery drain is imposed.For example, with a Romega Model 080 6 volt rated D.C. motor and pump, to achieve 260 millimeters of mercury it was necessary to use 4.5 watts of power. Also this same motor and pump when operated at an idle speed consumed 2.7 watts of power. This relatively high idle speed consumption was required so that the motor could reach its operating output torque under the load of the pump to reach the desired pressure.

The present invention makes it possible to reduce the idle wattage and to minimize the drain on the battery, since this does not contribute to the pumping efficiency of the system. In order to accomplish this a D.C. motor with an operating voltage which is substantially higher than the battery voltage is selected. For example, if a 12 volt D.C. rated Romega Model 080 motor and pump is selected and operated at 6 volts then it will have a wattage consumption of only 0.6 watts at idle. Of course the idle speed will be reduced, but this is of no consequence as long as the desired operating pressure is reached. The 12 volt D.

Crated Romega Model 080 motorand pump without the torque assist provisions of the present invention operated at 6 volts can achieve a pressure of 210 millimeters of mercury in the ambulatory pressure monitoring cuff with a wattage dissipation on the order of 2.4 watts. Thus when relatively low pumping pressures are dictated, the employment of a higher rated D.C. motor for driving the diaphragm pump will substantially reduce power dissipation and increase efficiency.

In ambulatory blood pressure monitoring systems, however, it is desirable at times to achieve pressures higher than approximately 260 millimeters of mercury. For example, pressures up to 500-600 millimeters of mercury may be desired.

Without the torque assist provisions of this invention a 12 volt rated D.C. Romega Model 080 motor will not be able to operate at 6 volts and still achieve an operating pressure substantially above 300 millimeters of mercury despite the desired reduced wattage dissipation that results from the use of the 12 volt D.C. motor at 6 volts D.C. Prior to the present invention the attainment of high operating pressures indicated that a motor rated at the battery voltage, for example, 6 volts should be used despite its increased operating wattage dissipation, and its corresponding higher idle wattage rating.

The torque assist provisions of the present invention are provided by the coil spring 56 which is expanded and acquires energy as the plunger rod 38 is driven upward, and the eccentric pin 18 moves from the position B to the position A. The spring 56 then releases its energy, assisting the downward motion of the plunger rod 38, as the eccentric pin moves from the position A to the position B on the compression stroke of the pump. The lower end of the spring has a hook 58 on it which engages a loop 60 on the bent section 62 of an elongate rod 64, which is preferably also constructed of spring material. The ends 66 and 68 of the rod are secured to the top surface 28 of the housing by means of the nuts 70 and 72 screwed on the threaded screws 71,73.

The upper end of the spring 56 also has a hook 74 on it which engages a loop 76 on a spring support member 78. The end of the spring support member 78 is secured in the eyelet 40 of the plunger rod 38 so that this spring is expanded and compressed as the eccentric pin 18 moves.

The effect of the expansion and compression of the spring is to store energy as the pump is driven through its intake, or expansion portion, of the cycle, thereby increasing the load during this portion of the cycle. The spring then releases its energy as the pump is driven into its compression cycle, thereby decreasing the torque required by the motor during this portion of the cycle. The spring 56 thereby tends to reduce the torque ripple effect on the motor 12 that is load-induced by the operation of the pump 20.

With a Romega Model 080 12 volt rated D.C. motor and pump a coil spring of an unstretched length of 0.65 inches (16.5mm), a spring wire diameter of 0.020 inches (0.5mm) and a coil diameter of 0.185 inches (4.7mm) may be employed. Figures 3 and 4 illustrate the results of the use of such a spring with this motor and pump assembly when the spring is stretched to about 0.80 inches (20.3mm) when the eccentric pin 18 is at point B, with the spring under about 1.1 pounds (500g) of tension, and with the spring stretched to about 0.95 inches (24.1 mm) when the eccentric pin is at point A, with the spring under about 1.7 pounds (772g) of tension. The maximum tension to minimum tension ratio is, therefore, about 1.5.

Figures 3 and 4 show a comparison of the input current versus millimeters of mercury developed by the pump 20 for 7 volt and 6 volt D.C. operations, respectively. In Figure 3 the curve 82 represents operation without the torque assist spring 56 while the curve 84 represents operation with the spring. In Figure 4 the curve 86 represents operation without the spring and the curve 88 represents operation with the spring. It is seen from these figures that the greatest effect of the spring occurs beyond the crossover points 90, 92 where the pumping pressure is at least 200 millimeters of mercury.Although the loop 58 of the spring 56 is shown as being permanently connected to the loop 60 of the rod 64 under tension, it is within the scope of the present inven tion to move the bent section 62 upwardly, thereby relieving tension on the spring when the pump is operating at pressures below the crossover points 90, 92; where the pump has exceeded the pressures represented by these points, the bent section 62 may be moved downwardly to increase the tension on the spring 56 sufficiently so that it will then absorb and release its energy in the manner described above to smooth out the undesired load-induced torque ripple effect on the D.C. motor.

While the present invention has been described with reference to a particular embodiment it will be recognized by those skilled in the art that the invention as claimed is applicable to other embodi ments that come within the scope of the appended

Claims (11)

claims. CLAIMS

1. A D.C. motor and diaphragm pump assembly which employs a reciprocatory plunger rod that is coupled to the diaphragm of said pump at one end, and is mounted to a eccentrically rotating means coupled to the motor drive shaft on said motor at its other end, comprising resilient means connected at a first end to a fixed point, and at a second end to an eccentrically rotating point so that said resilient means acquires energy during the expansion portion of the operating cycle of said pump and releases said acquired energy during the compression portion of the operating cycle of said pump which acts to reduce the load-induced torque ripple effect on said motor that is caused by said pump.

2. An assembly as claimed in claim 1 wherein said resilient means is a coil spring.

3. An assembly as claimed in claim 2 wherein said coil spring has a maximum tension to minimum tension ratio of substantially 1.5 over the operating cycle of said pump.

4. An assembly as claimed in claim 2 or 3 wherein the location of the fixed point is adjustable so that the spring is not placed under appreciable tension until the pressure output of the pump has reached a predetermined value.

5. An assembly as claimed in any preceding claim wherein said eccentrically rotating point is a point on said other end of said rod.

6. As assembly as claimed in any preceding claim wherein the D.C. motor is operated at a voltage substantially below its rated voltage.

7. An assembly as claimed in claim 6 wherein the operating D.C. voltage of the motor is approximately one-half of its rated D.C. operating voltage.

8. An assembly as claimed in any preceding claim wherein said pump drives a pressure cuff of a blood pressure monitoring system.

9. An assembly as claimed in claim 8 wherein such blood pressure monitoring system is portable and said motor is driven by a D.C. battery of approximately 6 volts D.C.

10. An assembly as claimed in claim 9 wherein the D.C. motor is rated at 12 volts.

11. A D.C. motor and diaphragm pump assembly substantially as hereinbefore described with reference to the accompanying drawings.