EP2589745A1

EP2589745A1 - Apparatus with electronic control and automatic regulation of the rotations of a pneumatic motor

Info

Publication number: EP2589745A1
Application number: EP11187869.0A
Authority: EP
Inventors: Guido Valentini
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-11-04
Filing date: 2011-11-04
Publication date: 2013-05-08

Abstract

The disclosure refers to an apparatus (1) for controlling the rotational speed of a shaft (2) of a pneumatic motor (3) to a desired speed and its use. The apparatus (1) comprises:
- means (4, 5) for detecting the current rotational speed of the shaft (2), for example a Hall sensor device;
- means (6) for varying a flow rate of air through the pneumatic motor (3), for example a solenoid valve disposed in an air intake (8) of the motor (3); and
- electrically operated means (7) for automatically controlling the variation means (6) in response to the detected current rotational speed of the shaft (2) and depending on the desired speed, including for example a CPU.

Description

The present invention relates to an apparatus apparatus for controlling the rotational speed of a shaft of a pneumatic motor to a desired speed.
In particular, the invention refers to a control apparatus for a machine tool operating with a pneumatic motor, which, for example but not limited to, may be an excenter grinding machine, a random orbital sander or a polishing machine.
It is well known that the performance of a motor actuated by compressed air depends on the pressure and the flow rate of the supplied air. As shown in figure 1, while maintaining the pressure and flow rate constant, air motors show a linear relation between the torque and the speed. This graph also known as the torque curve V.
It is noted that a pneumatic motor can work at any point along the entire torque curve V, starting from the point of idle speed (or maximum speed v), until it stops at the point of maximum torque n, corresponding to a speed v of zero (corresponding to deadlock, blocking or stall). The idle speed is defined as the speed v at which the motor shaft rotates with no load applied to it (torque n = 0).
Anyway, simply by acting on the input of compressed air supplying the motor, thereby intervening on the flow rate and the pressure of the air, performance of speed v and torque n of a pneumatic motor can be modified.
The power provided by a pneumatic motor is simply the product of torque n and speed v. Air motors produce a characteristic power curve P, in which the maximum power p is detected at approximately 50% of idle speed v. The point p represents, in fact, the point at which the product of torque n and speed v is at its maximum. The torque z produced at this point is called 'torque at maximum power'.
The performance of a compressed air motor is expressed by the performance diagram P shown in figure 2. For each motor running at a constant pressure and flow rate of the supplying air, power P, torque V and air consumption L vary according to the speed (0 ... v).
The graph of figure 2 represents the performance of a compressed air rotary motor with vanes, which operates at a constant pressure and a constant air supply rate. In figure 2 the following abbreviations are used:

P = power curve
p = maximum power [W]
V = torque curve
v = idle speed [rpm]
n = maximum torque [Nm]
½v = speed at the point p of maximum power [rpm]
z = torque [Nm] at maximum power p
L = consumption curve
l = air consumption [1/min] at maximum speed v
l₀ = consumption [1/min] at maximum power p
ls = consumption [1/min] at deadlock (or blocking or stall)
of the motor (speed v = 0).

As is apparent from the power curve P of pneumatic rotary motors with vanes, the number of rotations (speed ½ v) corresponding to the maximum power p is well below the number of rotations reached by the motor in its idle state (speed v). For example, a machine tool under load which at the point p of maximum power reaches 6,000 rpm, in its idle state (no load) it reaches about 12,000 rpm. It is noted that the air consumption of a pneumatic motor is proportional to the motor speed and, therefore, it is at its maximum (air consumption 1) at the idle speed v. Consequently, at the point p of maximum power the air consumption l₀ is lower (l > l₀). Also, in the deadlock condition (i.e. when the engine is totally arrested due to the application of the maximum torque n) the engine still consumes a quantity ls of air. This consumption is due to the air losses and leakages in the pneumatic motor. According to the current state of the art, in order to obtain an appropriate torque under load, it is accepted that the tool turns at a number of rotations much higher than actually needed, and that it is the user to eventually manually vary the flow rate of the supply air for the machine by means of a manual controller, based on the required torque. Disadvantageously, a continuous change in the number of rotations of the motor can be noted during the intended use of the pneumatic tool depending on the load applied, with an unreliable and inaccurate control exclusively depending on the user's experience.
There are known purely mechanical systems capable of limitting the number of rotations (for example by mechanical means taking different positions depending on the centrifugal force to which they are subjected to) in case the machine rotates at idle speed, and capable of automatically controlling the supply of the compressed air to the motor. However, these systems are mechanically very complex, expensive and inaccurate.
As already mentioned, the method most commonly used for varying the torque/speed curve V of a pneumatic motor or tool, respectively, is to apply a manual controller at the inlet for the supply air in order to decrease or increase the flow rate according to power requirements. Placing the controller at the inlet of the supply air (rather than at the engine's exhaust air duct) has an economic advantage, since the air consumption is greatly reduced by changing the torque curve V.
Figure 3 shows how the torque curve V changes due to progressively increasingly throttling the flow of supply air. This increasing throttling produces a progressive decrease and a shift of the torque curve to the left, from the curve V to V₁, V₂, V₃, reaching the respective idle (or maximumum) speeds, v, v₁, v₂, ½v. The throttling leads to a decrease of the passage section for the compressed supply air at the air intake, and consequently to a reduced flow rate.
In view of the prior art the object of the present invention is, therefore, to overcome the aforementioned drawbacks, by proposing an apparatus for controlling the number of rotations of a pneumatic motor, which is able to avoid waste and keep the number of rotations of the motor substantially constant, even during variation of the load applied and torque requested, in particular trying to operate the motor constantly at the maximum power.
In accordance with the invention this object is achieved with an apparatus for controlling the rotational speed of a shaft of a pneumatic motor, in particular for a pneumatic machine tool operating with an air motor, characterized in that it comprises:

means for detecting the current rotational speed of the shaft, for example a Hall sensor device;
means for varying a flow rate of air through the pneumatic motor, for example a solenoid valve disposed in an air intake of the motor; and
electronically operated control means for automatically controlling the variation means in response to the detected current rotational speed of the shaft and depending on the desired speed, for example some kind of calculating means, in particular comprising a CPU (central processing unit).

So the proposed controller automatically increases or decreases the flow of supply air on the basis of the load applied, power required and/or torque requested. In order to achieve a constant speed during the variation of the applied load or torque, respectively, the flow of supply air through an air intake of the motor is automatically increased (or decreased), which allows - for example - to quickly switch from curve V₃ to curve V (or vice versa) until the maximum power p (or the minimum consumption l₃) is reached. The motor speed is advantageously regulated by means of an electronic closed loop control.
As described above, throttling the air flow in the intake duct of the pneumatic motor results in a reduction and shifting of the torque curve to the left, see figure 3. This allows a reduction of the supply air needed for running the pneumatic motor while - at the same time - still providing the power required and/or torque requested. Further, the pneumatic motor equipped with the apparatus according to the present invention may rotate at a reduced speed, thereby reducing vibrations.
The detecting means may detect the rotations of the pneumatic motor shaft per time unit in any desired way, in particular by means of a tactile system or contactless, for example optically, by means of a capacity or inductively. Furthermore, it is possible that the detecting means do not directly detect the rotational speed of the shaft but rather of some other component of the motor or assigned to the motor (like a gear mechanism), the speed of which being indicative of the shaft's speed.
According to a prefered embodiment of the invention, the detecting means detect the rotational speed of the shaft by means of magnetic induction caused by the rotating shaft in at least one sensor element for detecting a magnetic field. For creating magnetic fields changing with the rotation of the shaft, it is suggested that the shaft be equipped with at least one pair of ferromagnetic elements of opposite polarisation. The more ferromagnetic elements the shaft is provided with the finer and more precise the current rotational speed of the shaft can be determined. The at least one sensor element preferably comprises a Hall sensor. The sensor element is preferably static and the ferromagnetic elements move past the sensor due to the rotation of the shaft. Due to the opposing polarisation of the ferromagnetic elements, during rotation of the shaft the at least one magnetic sensor detects magnetic fields with changing polarities and a current having a value depending on and corresponding to the rotational speed of the shaft is induced in the at least one magnetic sensor.
According to another preferred embodiment of the invention, the means for varying the flow rate of air through the pneumatic motor are disposed in an intake duct for the supply air of the pneumatic motor. This has the advantage that the flow rate of air through the pneumatic motor can be reduced, thereby also reducing the energy consumption. Of course, the variation means could also be disposed in an exhaust for the air after it has passed through the pneumatic motor. Preferably, the variation means comprise a valve, which can selectively increase or decrease the cross section of an air duct of the motor vehicle, thereby reducing or increasing the air flow rate through the motor. Preferably, the valve is a solenoid valve electrically actuated by the control means.
According to a further preferred embodiment of the invention, the control means are an electronic control unit (ECU) comprising at least one processor (CPU) on which a computer program for performing the function of controlling the rotational speed of the shaft of the pneumatic motor is executed, thereby generating electrical control signals for the variation means, in particular the at least one solenoid valve disposed in an air duct of the pneumatic motor. Such control means can be realized easily and cost efficient. Changes in the controlling mechanism can be flexibly programmed and easily implemented. This allows the use of the present invention in a plurality of different pneumatic tools simply by adapting the control program running on the CPU.
Preferably, the control means are adapted to constantly maintain the rotational speed of the shaft of the pneumatic motor at the desired speed, independently from the requested torque and the load applied to the shaft. If the requested torque increases because the load applied to the tool increases, for example because the tool is pressed to the workpiece more firmly, the apparatus according to the invention automatically increases the flow rate through the pneumatic motor, thereby trying to achieve a higher speed, which however is prevented by the higher torque, in the end resulting in a constant speed.
The apparatus according to the present invention is preferably used in a pneumatically driven tool. This has the advantage that these tools are not driven by an electrical motor causing sparks which in some environments are dangerous or even prohibited. For example in humid or dusty environments the use of pneumatically driven tools may be particularly advantageous or even mandatory. The electrical energy for operating the control apparatus according to the present invention, in particular the sensor means for detecting the rotational speed of the shaft, the means for varying the flow rate of the supply air and the ECU, can be provided by a battery making part of the apparatus. The battery could be a conventional non-rechargeable battery to be exchanged for a new one when it is consumed. Preferably, the battery is rechargeable, for example via an electrical cable or by placing the entire pneumatic tool on a charging station when the tool is not in use. The charging station and the pneumatic tool could be equipped with inductive charging means allowing a contactless recharging of the battery.
Alternatively, a generator can be provided in the intake duct for the supply air of the pneumatic motor, rotating as a result of a supply air flow and generating an electric current for recharging the battery and/or operating the control apparatus. Preferably, the rotating shaft makes part of a dynamo for producing electric energy which is used for recharging the battery and/or operating the control apparatus. For example, the ferromagnetic elements disposed on the shaft and primarily making part of the sensor for detecting the rotational speed of the shaft could also be used for producing electric energy for recharging the battery and/or operating the control apparatus. In that case the sensor and the dynamo could be one single integral component adapted for detecting the rotational speed of the shaft as well as for providing electrical energy to the control apparatus.
The ECU and the other electrical parts of the control apparatus preferably operate at a low voltage between 2.5 V and 12 V, so the ECU and the entire tool can be used in humid, wet and/or dusty environments without danger of electric shocks and explosions due to sparks.
The tools are preferably hand-held tools used in the field of bodywork of motor vehicles or boats, and in the building industry, in particular for wood and stone working. For example, the tools may be but are not limited to hand-held tools like a sander, in particular a random orbital or orbital sander, a polisher, in particular a rotating polisher, a grinder or a drill.
It is preferred that the tool comprises a device for selecting the desired speed. Preferably, the device is a switch, in particular a multi-way switch, allowing the selection of one of a plurality of different speeds as the desired speed. It is even possible that an almost infinitely variable selection of the desired speed is possible, for example by means of a rotary control device. It is preferred that the device for selecting the desired speed is disposed such that it can be easily reached and actuated by a user of the tool, in particular during the intended use of the tool, when working the workpiece with the tool. It is suggested that the device for selecting the desired speed, comprises a selecting position which corresponds to a speed equal to zero. This allows the user to stop the pneumatic motor, when interrupting or finished with the use of the tool. Being able to reach the selection device during the intended use of the tools allows the user to adjust the desired speed of the pneumatic motor to the current working situation, for example the material of the workpiece or the currently used sander, grinding or polishing element.
These and other features of the present invention will be made more obvious by the following detailed description of an example of its practical implementation described without limitation in the attached drawings, wherein:

Figure 1: a speed/torque-curve V of a pneuamtic motor;
Figure 2: a performance of a pneumatic motor;
Figure 3: various speed/torque-curves V of a pneumatic motor for a varying flow rate of compressed air supplying the pneumatic motor;
Figure 4: caracteristic graphs of a pneumatic motor comprising various speed/torque-curves V, and corresponding power curves P and air consumption curves L;
Figure 5: a modified characteristic graph for controlling the speed of a pneumatic motor with an apparatus according to the invention;
Figure 6: shows a schematic view of an apparatus according to the invention;
Figure 7: shows a plan view from above of a ferromagnetic element comprising a Hall sensor according to the invention;

Figure 8: shows a sectional view of a portion of a supply line for compressed air with a regulator valve; and
Figur 9: a tool according to the invention comprising an apparatus for controlling the rotational speed of a pneumatic motor.

With reference to the attached figures, and in particular to figure 6, an apparatus 1 for controlling the rotational speed of a motor shaft 2 is shown, especially for a machine tool 20 (see figure 9) running with a pneumatic motor 3.
The apparatus 1 comprises a device 4, 5 for detecting the number of revolutions per time unit of the motor shaft 2, a device 6 for varying said number of revolutions per time unit, and a device 7 for controlling the detecting devices 4, 5 and the variation device 6.
The device 4, 5 for detecting the number of revolutions per time unit advantageously comprises a ferromagnetic element 4 associated with the rotation of the schaft 2 of the pneumatic motor 3 and provided with at least two magnetic polarizations 10 opposed to each other (north poles and south poles). The ferromagnetic element 4 is advantageously a cylindrical element fastend by wedges to the motor shaft 2.
The device 4, 5 for detecting the number of rotations further comprises at least one Hall sensor 5 associated with the ferromagnetic element 4, or the cylinder 4, in order to detect the speed of rotation of the motor shaft 2, according to a known technique. This technique is based on the known Hall effect, which means that under certain conditions an electric current is generated by way of magnetic induction in a circuit comprising the Hall sensor 5 by means of a magnetic field generated by the polarizations 10. From the number of current pulses generated per time unit by the ferromagnetic cylinder that rotates in front of the Hall sensor 5 integral with a frame (not drawn) one can deduce the number of revolutions per time unit of the shaft 2.
The apparatus 1 may also comprise a battery (rechargeable or not) for providing electric energy to the control apparatus 1. For example, the battery may be recharged by a dynamo comprising ferromagnetic elements fixedly connected to the rotating shaft and a static coil of electric wires surrounding the shaft and ferromagnetic elements. Upon rotation of the shaft and the ferromagnetic elements in respect to the coil, an electric current is induced in the coil. The ferromagnetic elements of the dynamo could be those ferromagnetic elements 4 also used for the sensor device 4, 5. In that case the sensor device 4, 5 would be an integral part of the dynamo or vice versa.
The device 6 for varying the number of rotations of the engine shaft 2 is advantageously, and only illustrative and not limited to, a throttle input of an air intake 8 of the air of the pneumatic motor 3, of a type also known as for example a solenoid valve 6. As can be seen in figure 8, in a duct of the air intake 8 a valve 81 is disposed which is controlled by an electric or electromagnetic actuator 82 acting as to obstruct the passage of supply air. A spring 83 surrounds the valve 81 and holds it in a position of partial blocking of the duct of intake 8, the actuator 82 acting in a direction opposite to the force applied by said spring 83 to "liberate" the conduit 8 from valve 81 and, thus, to provide more flow of compressed supply air. Contrariwise, it is possible to provide a valve 81 normally outside of the conduit of intake 8 and forced by the actuator 82 to block the intake 8. In figure 8 the valve 81 is in a position in which the air intake 8 is half way closed. Starting from this position, the passage of the air intake 8 can be opened or closed according to the desired power and load applied to the tool 20. Of course, it is also possible that the passageway of the air intake 8 is completely open, that is when the valve 81 is completely removed from the intake 8.
The control device 7 may advantageously be embodied as a CPU (central processing unit) making part of a ECU (electronic control unit). A computer program for performing the function of controlling the rotational speed of the shaft 2 of the pneumatic motor 3 may be executed on the CPU, thereby generating electric control signals for the variation means, in particular the at least one solenoid valve 81, 82 disposed in an air duct 8 of the pneumatic motor 3. Changes in the controlling mechanism can be flexibly programmed and easily implemented.
With the apparatus 1 of figure 6 according to the present invention the tool 20 is made work at a constant speed of ½v at a variable power between points A and p according to any request of torque going from 0 to z .
It is important to note that so far we have assumed, for the sake of clarity of the description, that the speed at which the regulator is set, was only a single velocity, and for being able to provide the maximum power corresponding to the speed ½v. In fact, however, according to the present invention, it is also possible, depending on the type of tool 20 to provide with the regulation apparatus 1 according to the invention, to decide to equip the automatic regulator with multiple positions, the speeds of which ranging between 0 and v in figure 1, so that the end user can choose between the various configurations. It is assumed, for example but not limited to, a speed regulator for multiple speeds including the speed 0, i.e. the off position.
The apparatus 1 according to the present invention, therefore, allows the operator to optimize air consumption based on the needs of power requested automatically and with precision guaranteed by the use of electronic devices.
The caracteristic graphs of figure 4 show how the air consumption curve L and the power curve P vary as a result of a variation of the torque curve V following a progressive throttling of the air flow through the pneumatic motor's air intake duct 8.
By way of example, the following five settings of the air flow rate used in practice are mentioned:

an air flow of zero, when the tool is turned of and the pneumatic motor does not rotate,
a maximum air flow of v, with the motor rotating at maximum speed and consuming a maximum amount of air,
an optimal air flow of ½v, when the motor provides the maximum power p at a reduced air consumption, and
two intermediate settings 1/4v and 3/4v of the air flow rate, for example 1/4v used at the beginning for slowly distributing a polishing crème on a surface to be polished.

For descriptive simplicity we assume here only four different settings of the air flow, but of course there may be many more:

the maximum flow of air that allows the engine to reach the maximum speed at no load, called v,
a reduced flow of air that allows the engine to reach a top idle speed at no load equal to ½v,
two intermediate positions referred to as v₁ and v₂,

It is noted that a controller of the flow of air can change the torque curve V so that the speed of the pneumatic motor 3 can be at any point between zero (maximum throttle) and the point of maximum speed v (no throttling).
In the example shown in the figures, each torque curve, V, V₁, V₂, V₃ corresponds respectively to a power curve, P, P₁, P₂, P₃ and a consumption curve, L, L₁, L₂, L₃ decreasing with decreasing speed.
Preferably, the present invention proposes an automatic electronically controlled regulator for a pneumatic motor 3, such that by intervening in increasing or decreasing the flow of compressed air at the intake 8, the motor's speed is maintained constant at a predefined value, varying the torque and, consequently, the power.
For example but not limited to, if the regulator is applied to a rotary polishing machine, which with maximum air flow without load applied to it reaches the maximum speed v and has the maximum consumption l, while the maximum power p is delivered at a speed of about ½v, which is evident from the graph in figure 4. Hence, starting from a maximum speed v and a maximum air flow l, the regulator would reduce the air flow and the speed of the motor according to the load applied and power requested until the current speed reaches a desired or predefined speed value. Then the speed is held at the desired speed value.
In a further mode of realization, the motor can already start at a reduced speed corresponding to a desired or predefined speed value, in which case the regulator would increase the air flow according to the load applied and the power requested (see figure 5, to be explained in further detail below), in order to maintain the desired speed value.
If the automatic regulator is set to maintain the tool 20 always (even with no load applied to it) at a constant speed of for example ½v, then, when initially activated the tool 20 is without any applied torque (n = 0), the automatic regulator will progressively reduce the flow of air until the desired speed value is reached at which corrispondingly the torque is zero (see point A along the torque curve V₃) and the consumption, l₃, is significantly less than l. With other words, initially, in the tool's idle state, the apparatus switches to torque curve V₃ and the corresponding power and consumption curves P₃ and L₃. When the tool 20 is used on a surface to polish, the applied load is increased and, consequently, more torque is requested as a result of mechanical work and friction, that tends to reduce the speed from the point ½v rising along the torque curve of V₃.
In response to the automatic detection of this reduction in speed by the means 4, 5 for detecting the current rotational speed of the shaft 2, the apparatus 1 according to the invention, in particular the control means 7, will command the flow variation means 6 to gradually increase the flow of air supplied to the motor 3, until the speed is brought back to the pre-set value, in this example ½v, but with a torque greater than zero and equal to x, a power that goes from zero (on power curve P₃) to p₂ (on power curve P₂) and a consumption equal to l₂ (on consumption curve L₂), see point B along the torque curve V₂. With other words, the apparatus switches to curve V₂ and the corresponding power and consumption curves P₂ and L₂.
Further increasing the torque applied to the tool 20 will provoke an analogous reaction from the control apparatus according to the invention. In fact, the detection of the decline of the speed will make the flow of air progressively further increase in order to bring the rotations of the motor shaft 2 to the desired value, in this example equal to ½v, with a consequent increase in torque from x to y and an increase in consumption from l₂ to l₁, see figure 4, point C along the torque curve V₁. At the same time, the power increases from p₂ to p₁. With other words, the apparatus switches to curve V₁ and the corresponding power and consumption curves P₁ and L₁.
Finally, when further increasing the torque the engine is requested to deliver, at the maximum air flow its maximum power corresponds to point p, where the speed is always held equal to about ½v, and the consumption is equal to l₀ (corresponding to the consumption at the maximum power p), still less than l. The torque delivered by the engine moves from y to z, which corresponds to the torque value at maximum power p. See point D along the torque curve V in figure 4. With other words, the apparatus switches to curve V and the corresponding power and consumption curves P and L.
Of course, the pneumatic motor's 3 rotational speed can be regulated to any desired value other than ½v, too, even though other desired speed values may not be optimal in respect of power provieded and air consumption.. The value of ½v has been chosen because it provides a maximum power p at a reduced air consumption l₀.
The invention has the advantage, that during the idle speed (no load applied to the tool 20) the air flow through the motor's air intake 8 is regulated to a lower value, thereby reducing air consumption. The compressed air used for activating the pneumatic motor 3 is usually provided by a compressor and stored in a buffer storage tank, which provides the motor 3 with compressed air at a certain constant pressure. Consequently, the invention also leads to a reduction of energy consumption, because less air is consumed by the pneumatic tool 20, and the compressor has to be activated more rarely and at larger time intervals. With the present invention a larger number of pneumatic tools 20 can be operated by the compressor and the tank or, alternatively, the size of the compressor and/or the tank can be reduced. Only when a load is applied to the tool 20, i.e. as soon as the tool 20 is brought into its working position in contact with a workpiece to be treated by the tool 20, the air flow is increased by the regulator. However, it is to be noted that the air flow is increased only to an extent that is strictly necessary, so that the pneumatic motor 3 can provide the requested torque at the desired speed.
Having said this, due to the action of the automatic electronic controller 7, one can consider ½v as a speed limit, beyond which the engine 3 cannot go, or at least it makes no sense to have the engine 3 go beyond that speed limit, because no further increase of power provided by the motor 3 can be achieved. Going beyond the optimal speed limit ½v would only lead to an increased air consumption and a waste of energy. This allows to limit the conventional torque curve V of figure 2 to speeds from zero to ½v. As can be seen from the graph in figure 5, the new torque curve now runs through points ADn, the power curve runs through the points Ap0 and the consumption curve L through points l₃l₀ls.
Therefore, it can be advantageously ascertained that, for a torque demand going from zero to z, due to the automatic variation of the adjustment of the flow of supply air at the intake 8, the speed remains constant and equal to ½v, the power varies from zero to p, and the consumption from l₃ to l₀.
While, for a torque demand going from z to n, a progressive decrease in speed, a progressive decrease of the power along the power curve Ap0 and a decrease of the consumption along the curve l₃l₀ls can be ascertained until the motor 3 reaches the point of blocking (or deadlock or stall), in which the power and the speed are equal to zero and the torque is at its maximum n, and the consumption is equal to ls, as shown in the graph of figure 5.
Figure 9 shows an example of a tool 20 according to the present invention. The shown tool is a pneumatically driven hand-held random orbital sander. Of course, the invention is not limited to this type of tool. The invention may also be realized in connection with a stationary tool, and any other kind of tool using a pneumatic motor 3. The sander 20 comprises an orbitally oscillating supporting member 21 upon which a sander element 22 can be releasably fixed. The tool 20 is brought in contact with a workpiece (not shown) with its sander element 22. It can be seen from figure 9 that the means 6 for varying the flow rate of air through the pneumatic motor 3 are disposed in the intake duct 8. An exhaust duct is designated with reference sign 23. The shaft 2 is connected to the supporting member 21 by means of some kind of excentric set or gear mechanism (not shown) adapted for transforming the rotational movement of the shaft 2 into the random orbital oscillating movement of the supporting member 21. A device 24 for selecting the desired speed is disposed on the top side of a housing 25 of the tool 20. The selected desired speed is communicated to the control device 7, where it is used in the controlling of the rotational speed of the shaft 2. The selecting device 24 is disposed such that a user can easily reach and activate it during the intended use of the tool 20.

Claims

Apparatus (1) for controlling the rotational speed of a shaft (2) of a pneumatic motor (3) to a desired speed, comprising
- detecting means (4, 5) for detecting the current rotational speed of the shaft (2);

- variation means (6) for varying a flow rate of air through the pneumatic motor (3); and

- electronic control means (7) for automatically controlling the variation means (6) in response to the detected current rotational speed of the shaft (2) and depending on the desired speed.
Apparatus (1) according to claim 1, wherein the detecting means (4, 5) detect the rotational speed of the shaft (2) by means of magnetic induction caused by the rotating shaft (2) in at least one sensor element (5) for detecting a magnetic field.
Apparatus (1) according to claim 2, wherein the shaft (2) is equipped with at least one pair of ferromagnetic elements (10) of opposite polarisation.
Apparatus (1) according to claim 2 or 3, wherein the at least one sensor element (5) comprises a Hall sensor.
Apparatus (1) according to one of claims 1 to 4,
wherein the variation means (6) are disposed in an intake duct (8) for the supply air of the pneumatic motor (3).
Apparatus (1) according to one of claims 1 to 5,
wherein the variation means (6) comprise a solenoid valve electrically actuated by the control means (7).
Apparatus (1) according to one of claims 1 to 6,
wherein the control means (7) comprise a processor on which a computer program for performing the function of controlling the rotational speed of the shaft (2) of the pneumatic motor (3) is executed, thereby creating electrical control signals for the variation means (6).
Apparatus (1) according to one of claims 1 to 7,
wherein the apparatus (1) comprises a battery for providing it with electrical energy necessary for its operation and proper functioning.
Apparatus (1) according to one of claims 1 to 7,
wherein the control means (7) are adapted to constantly maintain the rotational speed of the shaft (2) of the pneumatic motor (3) at the desired speed, independently from the requested torque and/or the load applied to it.
Pneumatically driven tool (20) comprising a pneumatic motor (3), characterized in that the tool (20) comprises an apparatus (1) for controlling the rotational speed of a shaft (2) of the pneumatic motor (3) to a desired speed according to one of the claims 1 to 9.
Tool (20) according to claim 10, wherein the tool (20) is one, in particular a hand-held tool, used in the field of bodywork of motor vehicles or boats, and in the building industry, in particular for wood and stone working.
Tool (20) according to claim 10 or 11, wherein the tool (20) is one of a sander, in particular an orbital sander, a polisher, in particular an orbital polisher, and a grinder.
Tool (20) according to one of the claims 10 to 12, wherein the tool (20) comprises a device (24) for selecting the desired speed.
Tool (20) according to claim 13, wherein the device (24) for selecting the desired speed is a switch, in particular a multi-way switch.
Tool (20) according to one of the claims 10 to 14, wherein the tool (20) comprises means for transforming an air flow for operating the pneumatic motor (3) and/or a rotational movement of the shaft (2) into electrical energy for supplying the apparatus (1) therewith, in particular for recharging an apparatus' (1) battery.