GB2110432A - Vegetation cutters - Google Patents

Abstract

A vegetation cutter, e.g. an air- cushion supported lawn mower, driven by an electric motor such as a commutator motor is fitted with an electronic motor control unit effective to limit the maximum no-load speed of the motor, to stabilise motor speed over a predetermined range of motor loads, and preferably to enable the motor to rotate at another lower speed which is also constant over a predetermined range of motor loads. The lower speed produces a reduction in the cutting height of the mower. The speed of the motor is sensed by a Hall effect device, where output is fed to a frequency-to-voltage converter and ramp generator connected in the trigger circuit of a triac. The desired speed is manually set.

Description

SPECIFICATION Improvements in or relating to vegetation cutters This invention relates to vegetation cutters and has particular application to lawn mowers of the rotary and air cushion supported types.

Many forms of such mowers are powered by commutator motors, usually series wound universal motors and it is an object of the present invention to reduce substantially some of the disadvantages that arise from such use.

According to the present invention, a vegetation cutter powered by an electric motor includes an electronic circuit controlling the speed of rotation of the motor in such manner as to limit the speed of rotation of the motor under no-load conditions and to provide a constant speed of rotation within predetermined limits of motor load. The control may also provide at least one other lower speed of rotation of the motor that is less than the said constant speed.

By way of example only, an air-cushion supported lawn mower embodying the invention will now be described in greater detail with reference to the accompanying drawings of which: Figure 1 is a schematic perspective view of the mower, Figure 2 is a vertical cross-section, on a larger scale, through a part of the mower, Figs 3 and 4 are explanatory graphs, and, Figure 5 is a circuit diagram of a speed control circuit for the motor of the mower.

The air-cushion supported mower shown in Figure 1 is generally of conventional appearance having a cutter housing 1 enclosing a cutter blade and impeller driven by an electric motor accommodated in a housing 2 mounted upon the upper surface of the housing 1. Motion of the mower is controlled by a handle 3 on the cutter end of which an on/off control 4 that controls the supply of electricity from input lead 5 joined to a source of electrical power to a lead 6 connected to the input terminals of the motor.

Figure 2 is a cross-section on a larger scale of the cutter housing 1 and motor housing 2.

The cutter housing 1 has an upper surface 8 with a depending peripheral wall 9 outwardly upturned as indicated at 10.

The upper surface 8 has a central circular aperture 11 surrounded by a wall 12 integral with the casing 1. The edge of the aperture 11 is upturned as indicated at 13.

Positioned centrally on the aperture 11 is the annular part 14 of a support for a motor 15.

Extending from part 14 are spaced radial arms (not shown) by means of which the support is secured to the cutter casing 1.

Secured to the part 14 is an end mounting 16 of the motor 15. The mounting 16 has a central boss 1 7 that locates in the central opening on the annular part 14. The boss 1 7 has a through passageway as shown and on which is secured a ball bearing support 18 for the lower end of armature shaft 1 9 of the electric motor 1 5.

The end mounting 1 6 is fixed to the end face of the field-lamination stack 20 of the motor 1 5 by bolts 21 which secure to the other end face of the field-stack the ring-shaped base 22 of a second end mounting. From the base 22 extend inwardlyinclined spaced arms 23 that carry support for brush boxes 24 containing brushes 25 whose inner ends are in electrical contact with the commutator 26 of the motor 1 5. Also supported by the arms 23 is a spherical bearing 27 for the upper end of the armature shaft 19.

Also supported by the arms 23 is a speed control module 28 that will be described in detail below.

The housing 2 enclosing the motor 1 5 includes a lower part 29 with an outwardly inclined wall 30 which overlaps slightly the wall 12 surrounding the aperture 11 and is spaced therefrom to form an entrance 31.

The lower part 29 is formed to provide an annular pocket 32 located between the central wall of the part 29 and an intermediate wall 33 radially spaced from an inner wall 34 that surrounds the field-stack 20 and is substantially coextensive therewith. The inner wall 34 has an inturned flange 35 that locates on an out-turned flange 36 on end mounting 1 6 to position the part 29 of the motor housing 2.

The annular pocket 32 accommodates an air filter 37 of a suitable foamed or porous plastics material that is held in position by radial ribs 38 that extend inwardly from the curved wall 39 of the domed upper part 40 of the housing 2.

The top wall 41 of the part 40 has a central aperture covered by a sight glass of a clear plastics material through which the end of the armature shaft 1 9 can be observed as a check on the rotation thereof. The top wall 41 also has a stepped portion 42 with an aperture (not shown) whose purpose is described below.

Secured to the lower end of the armature shaft 1 9 is an impeller 43 whose blades are located very close to the aperture 11 in the cutter housing 1. Also secured to the armature shaft 1 9 and positioned below the impeller 43 is a cutter bar 44.

As so far described the mower operates in the conventional manner. Energisation of the motor 15 rotates the impeller 43 which draws in air via the entrance 31 and aperture 11 to create an air cushion beneath the cutter housing 1 to support the mower. Air is also drawn in to the upper part 40 of the housing 2 via apertures in the latter (not shown). Such air passes through the filter 37 which cleanses the air of any entrained material and the air then flows upwardly to enter the space between the motor armature and the field-stack 30 so helping to cool the motor. The flow of cooling air exits via apertures (not shown) in the end mounting 1 6 and joins the air flow from entrance 31 before entering the impeller.

Energisation of the motor also rotates the cutter bar 44 whose outer ends cut any vegetation in their path.

In Figure 3, curve A is the speed-torque characteristic of a conventional series connected commutator motor powering an air-cushion supported mower of the construction shown in Figures 1 and 2. The characteristic is typical of such motors and shows that as torque increases the speed decreases. Such a reduction in motor speed deleteriously affects the smoothness of the cut obtained with the mower, also, in long and/or wet grass, which imposes considerable load on the motor, the reduction in speed may be large enough to prevent the mower cutting at all.

To ensure that effective cutting is maintained under heavy load, it may be thought that all that is necessary is to use a motor of higher power than would normally be used and thereby ensure that an adequate speed is maintained over a greater torque range and this may be achieved by so designing the motor that it runs at a higher speed.

However, the use of such a motor carries with it the penalty that the speed at no load is excessive and associated with this excessive speed is an undesirable increase in power consumption and noise level.

Curve B of Figure 3 is the speed torque characteristic of such a higher power motor. The full line curve is the characteristic of higher power motor when electronically-controlled whilst the dotted line indicates the different portion of the characteristic that applies to a higher power motor without electronic control. The electronic control produces a reduced and constant speed over the initial part of the speed-torque characteristic--the reduced speed being sufficient to maintain effective cutting in long and/or wet grass.

In that way, the problem of excessive speed under no-load conditions is overcome together with the excessive noise level and excessive power consumption associated with that excessive speed. Furthermore, there is available an increase in torque to meet heavy load conditions.

A further and important advantage is that the constant speed obtained over the initial part of the curve B speed/torque characteristic produces a smoother cut as compared with that produced by a mower powered by a motor without the speed control.

A further reduction in noise level and a further saving in power consumption can be achieved by incorporating into the electronic control means that can be set by a user and which further reduce the motor speed when conditions permit. For example, during the summer months, the rate at which lawn grass grows is reduced and therefore a "light" cut only is needed and this can be carried out at a lower motor speed.

Associated with the use of the lower speed is another advantage. The height at which the mower is supported by the air cushion is slightly reduced with the result that a closer cut is achieved.

In Figure 3, curves C and D relate the speed and torque of the higher power motor at two different reduced speeds.

Curves A, B, C and D of Figure 4 relate torque and power input to the motor. Curve A refers to a conventional series connected commutator motor powering an air-cushion supported mower of the construction described above with reference to Figures 1 and 2 and corresponds with curve A of Figure 3.

Curves B, C and D of Figure 4 are the torque/power input characteristic of the electronically-controlled higher power motor when controlled to rotate at speeds represented by curves B, C and D respectively of Figure 3.

It will be observed that in the case of curves B, C and Figure 4-there is a substantial drop in the power consumed under no- or low-load condition as compared with curve A. The power reduction, in the case of curves C and D is maintained over a considerable part of the range of torque values.

Figure 5 is a diagram of an electronic control circuit for the mower motor and provides the facilities described above and others as hereinafter described.

The control circuit incorporates an integrated circuit IC2 of the semi-custom type providing a soft-start ramp generator, a frequency-to-voltage converter, a control amplifier, a trigger pulse generator and a current limiter. The power to the motor is controlled in response to signals received from a tachometer associated with the motor. The speed of rotation of the motor is thereby maintained at a constant value over a wide torque range, for example, as indicated by the horizontal initial part of curves B, C and D, Figure 3.

The tachometer comprises a magnet and a Hall effect device, the latter being shown schematically in Figure 5 as block 1C1 The magnet may be a bar magnet or a ring magnet mounted in a disc-like housing with the magnetic poles adjacent the periphery of the disc. The housing is secured to the armature shaft of the motor for rotation with the shaft and may be located adjacent the motor commutator. A suitable Hall effect device is that known as Type TL 170 CLP manufactured by Texas Instruments Inc. The device is positioned close to the periphery of the ring magnet described above, such that the Hall effect device can sense the magnetic fields produced by the magnetic poles at the periphery of the ring-magnet. The Hall effect device thereby generates a train of pulses whose frequency is proportional to motor speed.

This train of pulses is input to the frequency-tovoltage converter which converts the signals from the tachometer into a d.c. signal whose value is proportional to the frequency of the tachometer pulses thence to the motor speed.

The output of the soft-start ramp generator is compared with the output of the frequencyvoltage converter by a control amplifier. This amplifier provides an output signal that controls the timing of a trigger pulse generator used to trigger a triac CSR1 and thereby to determine the firing angle thereof. The triac CSR1 is in series connection with motor M across electricity supply terminals L and N.

The integrated circuit also includes a current limiter connected to the soft-start ramp generator to limit the maximum current drawn by the motor.

If a current above the present maximum is obtained, the current limiter reduces the output of the soft-start ramp generator and the motor speed is thereby reduced, thus reducing the motor current.

The Hall effect device IC1 and the integrated circuit IC2 are powered from the input terminals L and N via a half wave rectifier D1 and a voltage dropping resistor R8, filtering being provided by electrolytic capacitor C9. Resistor R7 together with components in IC2 provide a shunt-stabiliser to maintain a constant voltage across IC2 and the Hall effect device despite variations in the electricity supply voltage. An additional electrolytic capacitor C8 provides an energy reservoir for the current pulse required to fire triac CSR1.

Control over the speed of rotation of motor M is provided by potentiometer RV1 connected between the zero volts line and pin 8 of IC1.The slider of potentiometer RV1 may be operated by a knob or lever positioned on the housing 2 and diagrammatically indicated at 7 in Figure 1. The knob or lever preferably has preset positions of which one selects the higher speed of the motor as referred to above whilst the others select the lower speeds. Of course, if desired, continuous variation of speed may be provided. The knob 7 has a shaft that extends through the aperture referred to above in the stepped portion 42 of the top wall 41 of part 40 of the housing 2.Preferably the shaft comprises a series of resilient fingers that close together to permit passage of the finger through the aperture and that then return to their original positions in which they prevent easy withdrawal of the shaft. Additionally, the end of the shaft is so contoured that it will engage mating surfaces on the shaft of the potentiometer RV1.

Referring now in more detail to the circuit of Figure 5, pins 1 and 4 of IC2 relate to the control amplifier and are interconnected by capacitor C3 which provides an integrating function.

Capacitor C4 connected to pin 6 of IC2 determines the slope of the "soft" start, i.e. the rate at which the motor accelerates to the speed selected by the setting of potentiometer RV1 connected to pins 7 and 8 of IC2.

Capacitor C5 connected to pin 10 of IC2 and resistor R2 connected to pin 9 of IC2 determine the conversion factor of the frequency-to-voltage converter. The resistor-capacitor combination C1, R2 acts as a filter circuit for the output of the frequency/voltage converter. Capacitor C2 provides additional filtering.

Pin 12 of IC2 is joined to the capacitor C6 which provides the timing ramp for the trigger pulse generator. The inputs to pins 1 3 and 14 of IC2 via resistors R6 and R5 respectively provide voltage and current synchronisation inputs for the trigger pulse generator.

Average current limiting is provided by an input at pin 1 8 of IC2 obtained via resistor R3, capacitor C7 providing the averaging function.

The resistor-capacitor combination R9, C10 in parallel connection with the triac CSR1 limit the rate of voltage rise at the latter and so reduce the possibility of false triggering when the triac turns off.

The values of relevent components are indicated in Figure 5.

The electronic control circuit may conveniently be located in the housing 2 and will be positioned so as to be cooled by a flow of air for example by clean air passing to the motor for the purpose of cooling the latter.

In the construction shown in Figure 2, the integrated circuit IC2 and its associated components are mounted upon a printed circuit board that is included in the module 28. The Hall effect sensor 45 associated with the tachometer is mounted upon an extension of the module 28 whilst the rotating magnet assembly is indicated schematically at 46.

It will be understood that some advantages accrue simply from the use, with a conventional electric motor whose speed/torque characteristic is of the form shown at A in Figures 3 and 4, of an electronic control circuit acting to limit the noload speed of the motor and also to maintain a constant speed up to a certain motor load (determined by the motor design), although in this case the increased torque for heavy loads will not be available.

The electronic circuit may also provide at least one other different speed of rotation of the motor that is also maintained constant within predetermined limits of motor load.

It will be appreciated that the vegetation cutter may, alternatively, be of the rotary type, i.e., the type in which a wheeled chassis carries a cutting blade rotatable about a vertical axis by an electric motor, or of the cylinder type. The cutter may also be of a construction employing a length of filament as the cutting element or a plurality of cutters pivotally attached to a carrier that may be the impeller that is rotated by the motor.

Claims (6)

Claims

1. A vegetation cutter powered by an electric motor and which has an electronic circuit for controlling speed of rotation of the motor characterised in that the circuit acts to limit the speed of rotation of the motor under no-load conditions and to provide a constant speed of rotation within predetermined limits of motor load.

2. A cutter as claimed in Claim 1 characterised in that the circuit also acts to provide at least one other speed of rotation of the motor that is different from said constant speed and that is also constant within predetermined limits of motor load.

3. A cutter as claimed in claim 1 or 2 characterised in that the motor is of a higher power than would normally be employed in a similar vegetation cutter.

4. A cutter as claimed in claim 1,2 or 3 characterised in that the electronic circuit comprises a module that is mounted in a housing accommodating the motor and in a position in which the module is exposed to a flow of cooling air.

5. A cutter as claimed in claim 4 characterised in that the flow of cooling air is established by an impeller rotated by the motor and which also provides an air cushion to support the cutter when in use.

6. A vegetation cutter substantially as herein described with reference to and as illustrated by the accompanying drawings.