Optical sensing system

Abstract

An optical sensing system for sensing an edge 22 of a low reflectance component 2 of a machine includes control means 4 connected to optical sensors 3a, 3b each with an emitter 30a, 30b and a receiver

Description

This invention relates generally to optical sensing systems for machines. More specifically, although not exclusively, this invention relates to an optical sensing system for sensing edges of a component of a machine, such as an exercise machine, which has a low reflectance surface. In particular, this invention concerns the detection of rotational or translational movement of a mechanical component by optical means, utilising the reflection of light emitted by an optical sensor assembly in the absence of high reflectivity from the moving component. Optical detection of position or speed has been frequently used since photoelectric devices became commonly available. Two basic methods are generally known: a beam of light may be arranged to pass through holes in the component, with a sensor directly opposite the emitter; or a beam of light may be reflected from the component, with some arrangement made to alter the reflectivity of the component in particular places. Both methods are relatively simple to implement with modern optoelectronics, requiring little or no signal processing to deliver an unambiguous off-on detection of a hole, a reflective stripe, or a dark stripe on an otherwise reflective surface. The transmissive method has a disadvantage: the sensor assembly necessarily has a transmitter and a receiver which must be physically placed facing each other, on opposite sides of the moving component. This may be complicated to arrange in some situations. A reflective arrangement is often preferable because it involves an assembly on only one side of the moving component. However, the reflective method suffers from a different disadvantage: the moving component must either be inherently reflective, or some feature must be added to it that is reflective, at the optical wavelength of interest - typically near-infrared. In challenging environments, or where the sensor is a considerable distance from the moving target, the reflective method may require sophisticated optical or electronic means to ensure reliability; or it may be completely nonviable. A hybrid arrangement is possible, whereby a stationary reflective surface is placed opposite an emitter-detector pair located adjacent to holes in a nonreflective moving component. In this instance, holes in the component are detected by reflection of the emitted light as the hole passes by the sensor. This arrangement is not commonly used in practice, since an additional assembly must be assembled opposite the sensor electronics. Where none of these configurations are applicable, other means exist for detecting movement of mechanical components - metal components may, for example, be sensed by using electromagnetic effects. However, the implementation of such methods is typically more complex than optical sensing, and each has its own particular disadvantages. It would therefore be advantageous to provide an optical sensing arrangement that is capable of detecting the speed of a component of a machine, which has a low reflectance surface such as a black surface, which may be matt, powder coated or have a low gloss surface finish. Accordingly, a first aspect of the invention provides an optical sensing system, e.g. for sensing an edge of a low reflectance component of a machine, the system comprising a pair of optical sensors and control means operatively connected to the sensors, wherein the control means is configured to receive signals from each of the optical sensors indicative of either a non-zero reflectance condition, corresponding to the presence of a surface of the component facing the optical sensors, or a zero reflectance condition, corresponding to the absence of a surface of the component facing the optical sensors. Another aspect of the invention provides a method of sensing an edge of a component, e.g. a low reflectance component of a machine, the method comprising detecting with each of a pair of optical sensors either a non-zero reflectance condition, corresponding to the presence of a surface of the component facing the optical sensors, or a zero reflectance condition, corresponding to the absence of a surface of the component facing the optical sensors. The control means may be configured to detect an edge in the component, for example based on the signals received from the optical sensors. The control means may be configured to generate a speed signal indicative of a speed of the component, which may be based the detection of two or more edges of the component, e.g. over a predetermined time period. The method may comprise detecting an edge in the component, for example based on the signals received from the optical sensors. The method may comprise generating a speed signal indicative of a speed of the component, which may be based the detection of two or more edges of the component, e.g. over a predetermined time period. The control means may be configured to receive a signal pair from the optical sensors, e.g. after each of a plurality of time intervals. Each signal pair may include a signal from each optical sensor. The control means may be configured to subtract the signals of each signal pair from one another, e.g. by means of a differential amplifier, to provide an output signal. The control means may be configured to process the output signal, for example to detect a feature thereof. The feature may correspond to an edge, e.g. a physical edge, of the component. The control means may be configured to detect an edge in the component, for example by identifying a peak in the output signal. The peak may correspond to such an edge. The method may comprise receiving a signal pair from the optical sensors, e.g. after each of a plurality of time intervals. Each signal pair may include a signal from each optical sensor. The method may comprise subtracting the signals of each signal pair from one another, e.g. by means of a differential amplifier, to provide an output signal. The method may comprise processing the output signal, for example to detect a feature thereof. The feature may correspond to an edge, e.g. a physical edge, of the component. The method may comprise detecting an edge in the component, for example by identifying a peak in the output signal. The peak may correspond to such an edge. Each optical sensor may comprise an emitter and / or a receiver. The emitter and receiver of each optical sensor may comprise or be referred to as an emitter-receiver pair. Another aspect of the invention provides an optical sensing system for sensing the speed of a low reflectance component of a machine, the system comprising control means operatively connected to a pair of optical sensors each comprising an emitter and a receiver, wherein the control means is configured to: receive a signal pair from the optical sensors after each of a plurality of time intervals, each signal pair including a signal from each optical sensor indicative of either a non-zero reflectance condition, corresponding to the presence of a surface of the component facing the optical sensors, or a zero reflectance condition, corresponding to the absence of a surface of the component facing the optical sensors; subtract the signals of each signal pair from one another by means of a differential amplifier to provide an output signal; and detect an edge in the component by identifying a peak in the output signal. The control means may be configured to generate a speed signal indicative of a speed of the component based the detection of two or more edges of the component over a predetermined time period. Another aspect of the invention provides a method of sensing an edge of a component of a machine, the method comprising receiving a signal pair from a pair of optical sensors after each of a plurality of time intervals, each signal pair including a signal from each optical sensor indicative of either a non-zero reflectance condition, corresponding to the presence of a surface of the component facing the optical sensors, or a zero reflectance condition, corresponding to the absence of a surface of the component facing the optical sensors; subtracting the signals of each signal pair from one another by means of a differential amplifier to provide an output signal; and detecting an edge in the component by identifying a peak in the output signal. The method may comprise generating a speed signal indicative of a speed of the component based the detection of two or more edges of the component over a predetermined time period. Thus, the invention proposes the detection of moving holes in an essentially nonreflective moving component by means of a reflective optical sensing system. This may preclude the need for any additional optical components, such as mirrors or lenses. In the preferred implementation, the reliable detection of movement is robust to degradation in the optical properties of the component, mounting distance, and similar confounding factors, despite the relative simplicity of the system. This invention is intended for use in a particular scenario in which the moving component is black and it is impractical or undesirable to affix reflective patches to the component or to some stationary surface, but holes in the component already exist by virtue of its main function. In this scenario the optical speed sensing system must distinguish between two conditions: zero reflectance as holes pass by the optical sensors, and a very low level of reflectance as the base material passes in front of the optical sensors. It might be assumed that, since even a non-reflective surface will nevertheless reflect a minimal amount of light back to the transmitter, it should be possible to increase the transmitted power from the emitter, or increase the amplification applied to the detected signal, or both, and thereby achieve a signal-to-noise ratio sufficient for simple binary detection holes in an otherwise low-reflectivity surface. Whilst this is accurate to a point, the amplified signal becomes progressively noisy as the reflectivity of the base material tends towards zero. This is due to a combination of electronic noise and genuine variations in the reflectivity of the target surface. The use of a high-gain amplifier, low-pass filtering, and a thresholding means such as a Schmitt trigger could deliver an output which is useable in some circumstances, but phase noise or response speed may be unacceptable in more demanding applications, such as closed-loop speed control, and the system may be unacceptably sensitive to sensor placement or accumulation of dirt on the moving component. The system may comprise a circuit, which may comprise the pair of optical sensors. The circuit may be operatively connected to the control means. The circuit may comprise a differential amplifier. The optical sensors may be coupled, e.g. DC-coupled, to the differential amplifier. The circuit may comprise a bandpass filter. The bandpass filter may be operatively connected to the control means and / or to the differential amplifier. The bandpass filter may be configured to remove a DC offset and / or low-frequency and / or high frequency components, e.g. from the output signal from the differential amplifier such as to provide a filtered output signal. The method may comprise removing a DC offset and / or low-frequency and / or high frequency components from the output signal (e.g. from the differential amplifier), for example to provide a filtered signal output. The circuit may comprise a slope detector. The slope detector may be operatively connected to the bandpass filter and / or may be for sampling the signal output, e.g. from the bandpass filter. The output signal from the differential amplifier may be sampled, for example by the control means and / or by the slope detector, to compute a first derivative of the output signal and / or to identify zero-crossings in the resultant signal. The controller may be configured to apply a state-estimation technique to the signal, such as Kalman filtering. The method may comprise sampling the output signal from the differential amplifier, for example to compute a first derivative of the output signal and / or identify zero-crossings in the resultant signal. The method may comprise applying a state-estimation technique to the signal, such as Kalman filtering. Another aspect of the invention provides an exercise machine comprising a movable component. The exercise machine may comprise an optical sensing system, for example an optical sensing system as described above. The movable component may be adjacent the pair of optical sensors. The movable component may comprise one or more holes extending through the component. The control means may be configured to provide a speed signal indicative of a speed of the movable component of the exercise machine. The movable component may be rotatable or translatable. The movable component may be movable by rotation or translation. The component may be a disc. The disc may comprise a flywheel. The disc may comprise a plurality of holes. The holes may be spaced equally about a rotational axis of the disc. The holes may have substantially the same shape and / or size. The control means may be configured to provide a speed signal indicative of a rotational speed of the movable component of the exercise machine. The disc may be ferromagnetic. The disc may form part of an eddy brake assembly. The eddy brake assembly may be connected or coupled to a friction or clutch brake assembly, e.g. for selectively stopping and / or inhibiting movement of the disc. The exercise machine may comprise an actuation mechanism, which may be operable by a user. The actuation mechanism may be connected or coupled to the component. The actuation mechanism may be connected or coupled to the disc. The actuation mechanism may be connected or coupled to the eddy brake. The actuation mechanism may be connected or coupled to the eddy brake assembly by a loop and pulley assembly. The loop and pulley assembly may comprise a speed increasing loop and pulley assembly. The loop and pulley assembly may comprise a first loop and pulley assembly. The eddy brake assembly may be connected or coupled to the friction or clutch brake assembly by a loop and pulley assembly, e.g. a second loop and pulley assembly. The loop and pulley assembly or second loop and pulley assembly may comprise a second speed increasing loop and pulley assembly. Another aspect of the invention provides an exercise machine comprising an actuation mechanism operable by a user, an eddy brake assembly with a disc connected or coupled to the actuation mechanism for controlling its speed and a clutch brake assembly connected or coupled to the disc of the eddy brake assembly for selectively stopping and / or inhibiting movement thereof, wherein the actuation mechanism is coupled to the eddy brake assembly by a first speed increasing loop and pulley assembly and the eddy brake assembly is coupled to the friction or clutch brake assembly by a second speed increasing loop and pulley assembly. Thus, a movement of the actuation mechanism and / or a force applied thereto is amplified by the first loop and pulley assembly to drive the disc of the eddy brake assembly. Similarly, a rotation and / or torque of the disc of the eddy brake assembly is amplified by the second loop and pulley assembly to drive the friction or clutch brake assembly. Indeed, the first and / or second loop and pulley assembly may comprise a plurality of speed increasing stages. The system and method described is capable of identifying the position of holes in a moving part with good precision using a bare minimum of low-cost components and a straightforward computation demanding modest computing power, and in the preferred embodiment is both simple to fit and largely insensitive to placement. The cost of any machine that requires sensing of position or speed of a minimally-reflective component is therefore reduced. The loop and pulley assembly, e.g. the first and / or second loop and pulley assembly, may comprise two or more sprockets with an endless chain loop connecting the sprockets. Additionally or alternatively at least one of the loop and pulley assemblies, e.g. the first and / or second loop and pulley assembly, may comprise two or more belt or cable pulleys with an endless belt or cable loop connecting the belt or cable pulleys. The actuation mechanism may comprise a step mill mechanism. The step mill mechanism may be movable along a track. The step mill mechanism may comprise a plurality of foot platforms. The foot platforms may be hinged to one another, for example to form a series of steps. The step mill mechanism may comprise an endless loop of foot platforms hinged to one another. The step mill mechanism may be configured to move the series of steps formed by the foot platforms along the track to simulate moving stairs. Alternatively, the actuation mechanism may comprise any one of a stationary bike pedal mechanism, a treadmill mechanism, an elliptical cross training mechanism, an independent stepper mechanism, a ski training mechanism, an ergometer mechanism and a rowing mechanism. For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention. For example, the exercise machine may comprise any one or more features of the optical sensing system relevant thereto or vice versa and / or the method may comprise any one or more features or steps relevant to one or more features of the optical sensing system or the exercise machine. A further aspect of the invention provides a computer program element comprising computer readable program code means for causing a processor to execute a procedure to implement one or more steps of the aforementioned method. A yet further aspect of the invention provides the computer program element embodied on a computer readable medium. A yet further aspect of the invention provides a computer readable medium having a program stored thereon, where the program is arranged to make a computer execute a procedure to implement one or more steps of the aforementioned method. A yet further aspect of the invention provides a control means comprising the aforementioned computer program element or computer readable medium. For the avoidance of doubt, the terms “may”, “and / or”, “e.g.”, “for example” and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. For purposes of this disclosure, and notwithstanding the above, it is to be understood that the control means may comprise a control system, control module, control unitor controller. Furthermore, any controller(s), control units and / or control modules described herein may each comprise a control unit or computational device having one or more electronic processors. The controller may comprise a single control unit or electronic controller or alternatively different functions of the control of the system or apparatus may be embodied in, or hosted in, different control units or controllers or control modules. As used herein, the terms “control unit” and “controller” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) or control module(s) to implement the control techniques described herein (including the method(s) described herein). The set of instructions may be embedded in one or more electronic processors, or alternatively, may be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present invention is not intended to be limited to any particular arrangement. In any event, the set of instructions described herein may be embedded in a computer-readable storage medium (e.g., a non-transitory storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors / computational device, including, without limitation: a magnetic, optical or magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information / instructions. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. Indeed, any combination of optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic drawing of an optical sensing system according to the invention; Figure 2 is a graph showing traces of the output of a differential amplifier, where the inputs to the differential amplifier are the two output signals from the two adjacent optical sensors in Figure 1, and first derivative thereof; Figure 3 is a perspective view of an exercise machine according to the invention; Figure 4 is a similar view to Figure 3 with a side panel omitted to reveal the first speed increasing loop and pulley assembly coupling the step mill mechanism to the eddy brake assembly; Figure 5 is an partial, enlarged view of Figure 4 showing the eddy brake assembly but with the endless belt omitted; and Figure 6 illustrates the opposite side of the eddy brake assembly and its coupling to the clutch brake assembly. Referring now to Figure 1, there is shown an optical sensing system 1 for sensing the speed of a low reflectance component 2 of an exercise machine 101. The system 1 includes a pair of optical sensors 3a, 3b, positioned side-by-side, and control means in the form of a printed circuit board (PCB) 4 operatively connected to the sensors 3a, 3b. Each optical sensor 3a, 3b includes an emitter 30a, 30b and a receiver 31a, 31b. The emitter and receiver 30a, 31a, 30b, 31 b of each optical sensor 3a, 3b may comprise or be referred to as an emitter-receiver pair 30a, 31a, 30b, 31b. The PCB 4 is configured to receive signals from each of the optical sensors 3a, 3b indicative of either a non-zero reflectance condition, corresponding to the presence of a surface 20 of the component 2 facing the optical sensors 3a, 3b, or a zero reflectance condition, corresponding to the absence of a surface, or a hole 21, of the component 2 facing the optical sensors 3a, 3b. The component 2 is a disc in this example, operable to rotate R about its central axis. The component 2 includes a plurality of holes 21, four in this case, each defined by a circular edge 22 of the component 2. The component 2 is also powder coated or painted with a matt black paint in this example, such that its surface 20 has a marginally reflectance. This arrangement therefore has four states: (a) both optical sensors 3a, 3b facing a hole 21 in the component 2, such that the output from both sensors 3a, 3b is zero, (b) both sensors 3a, 3b facing the marginally reflective surface 20, such that the output from both sensors 3a, 3b is non-zero, or (c) and (d) with only one sensor 3a, 3b facing a hole. The PCB 4 includes a processor 40 and a memory 41 operatively connected to the processor 40. The memory 41 includes computer readable program code means for causing a processor 40 to interpret signals received from the optical sensors 3a, 3b. More specifically, the processor 40 is configured to detect an edge 22 in the component based on the signals received from the optical sensors 3a, 3b. In this example, the processor 40 is also configured to generate a speed signal indicative of a speed of the component 2 based the detection of two or more edges 22 of the component 2 over a predetermined time period. In this case, the speed of the component 2 corresponds to the disc rotational speed. However, the skilled person will appreciate that the processor 40 may be configured to generate a position signal indicative of a position of the component 2, or any other useful output signal. More specifically, the processor 40 is configured to receive a signal pair from the optical sensors 3a, 3b after each of a plurality of time intervals, each signal pair including a signal from each optical sensor 3a, 3b. The reflectance of the surface 20, at any given point illuminated by one of the optical sensors 3a, 3b, may be characterized in terms of a mean value and a standard deviation. Therefore, if both sensors 3a, 3b are facing the surface 20, the two outputs will have nonzero values which are similar in magnitude, but not identical; the difference between the two will be essentially random, with the randomness characterized by the aforementioned Gaussian distribution. With a wide angle of illumination and view, the inherent randomness in the surface is optically averaged and the difference in output between the sensors 3a, 3b, adjacent at almost the same area of the moving component 2, will be relatively small. Similarly, when the optical sensors 3a, 3b are both opposite a hole 21 in the moving component 2, the output from both sensors 3a, 3b will be essentially zero, assuming limited interference between each emitter-receiver pair 30a, 31a, 30b, 31b, and almost identical. Any variation in output will be due mostly to differences in the electrical and optical characteristics of the sensors 3a, 3b, and any such differences would be relatively insignificant. In this example, the PCB 4 includes a differential amplifier 42, a bandpass filter 43 and a slope detector 44 between the optical sensors 3a, 3b and the processor 40. More specifically, the optical sensors 3a, 3b are DC-coupled to the differential amplifier 42, which subtracts the signal received from one of the optical sensors 3a from the signal received from the other optical sensor 3b. The bandpass filter 43 is operatively connected to the differential amplifier 42 and removes a DC offset and / or low-frequency and / or high frequency components from the signal output from the differential amplifier 42. The slope detector 44 is operatively connected to the bandpass filter 43 for sampling the signal output from the bandpass filter 42. When both optical sensors 3a, 3b are opposite a hole 21, both signals will be almost identical with an absolute value at or near zero, and the output of the differential amplifier 42 will be close to zero. When both sensors 3a, 3b are opposite the surface 20 of the component 2, both signals will be non-zero but somewhat similar, resulting in an output signal - when the component is moving - which appears like bandwidth-limited noise. The RMS amplitude of this noise will be determined by the optical characteristics of the sensors 3a, 3b and the surface 20, and by the gain of the differential amplifier 42. In the preferred implementation, the frequency response of the differential amplifier 42 rolls off at a few hundred hertz, reducing the gain to around 1 at the Nyquist frequency of the ADC which samples the output of the analogue section of the system for digital signal processing. Placing this filter in the differential amplifier 42 discards high-frequency components of the signal, which carry no useful information and would otherwise cause aliasing during sampling. As described above, during the states of the system 1 where both optical sensors 3a, 3b are opposite the surface 20 of the moving component 2, or both sensors 3a, 3b are opposite a hole 21, the output of the differential amplifier 42 is very small or essentially negligible. However, when one sensor 3a is opposite the surface 20 and one sensor 3b is opposite a hole 21, the output of the differential amplifier 42 is significant. The magnitude of this difference is unpredictable, but its magnitude is not particularly important; the resulting output from the differential amplifier 42 will, in these two transient states, produce either a large positive or negative pulse coinciding with the edges 22 of a hole 21 as it moves past the sensors 3a, 3b. In the preferred implementation, the differential amplifier 42 is followed by a bandpass filter 43, which removes a DC offset and low-frequency components from the signal as well as any remaining high-frequency components which have no meaning. The parameters of the bandpass filter 43 are such that it will retain the fast edges that characterize the sensor response from the edges 22 of the detected hole 21, over the expected range of speeds, while discarding as much as possible of the random output that characterizes the sensor response from the low-reflectivity surface 20. In the preferred implementation, the overall frequency response of the analogue subsystem falls to approximately OdB at one-fifth of the sampling frequency. The upper trace 45 of Figure 2 shows a typical output signal. The final processing step follows sampling of the analogue signal. A simple estimation of signal maxima can be used to define windows in time where peak detection should occur, and the first derivative of the signal - the lower trace 46 in Figure 2 - passes through zero at the exact points of inflection of these peaks. The system 1 can thereby detect the edges 22 of holes 21 in the moving surface 20 with good precision, resulting in an estimate of speed-of-motion with low phase noise. State-estimation techniques, such as Kalman filtering, may be used to further improve the end result. Turning now to Figures 3 to 6, there is shown an exercise machine 101 that incorporates the optical sensing system 1 of Figures 1 and 2 for determining the speed of a flywheel 120 of an eddy brake assembly 102. The flywheel 120 incorporates the component 2 shown in Figure 1, which is ferromagnetic in this example. The optical sensors 3a, 3b are mounted within a housing 103 mounted to a bracket 104 secured to a frame 140 of the exercise machine 101. The PCB 4 is also mounted to the frame 140 and incorporates other electronic components for controlling the exercise machine 101. The exercise machine 101 is a stair climber in this example, which includes an actuation mechanism in the form of a step mill mechanism 110 movable along a track (not shown). The step mill mechanism 110 includes a plurality of foot platforms 112 hinged to one another to form an endless loop of steps 113 in the usual way. The step mill mechanism 110 is configured to move the series of steps 113 formed by the foot platforms 112 along the track to simulate moving stairs. The step mill mechanism 110 is coupled to the flywheel 120 of the eddy brake assembly 102 by a first speed increasing loop and pulley assembly 105, which is shown most clearly in Figures 4 and 5. The first speed increasing loop and pulley assembly 105 includes a first, large sprocket 150 coupled to the step mill mechanism 110 and driven by the movement of the step mill mechanism 110 along the track. The first sprocket 150 is operatively connected to a second, small sprocket 151 by an endless chain loop 152. The second, small sprocket 151 is coupled for rotation with a first, large belt pulley 153 operatively connected to a second, small belt pulley 154 by an endless belt 155. The second, small belt pulley 154 is coupled for rotation with the flywheel 120. The eddy brake assembly 102 is coupled to a clutch brake assembly 106 by a second speed increasing loop and pulley assembly 107. More specifically, the flywheel 120 is coupled for rotation with a first, large belt pulley 170 operatively connected to a second, small belt pulley 171 by an endless belt 172. The second, small belt pulley 171 is coupled for rotation with a rotor of the clutch brake assembly 106. As such, the force exerted by a user on the step mill mechanism 110 is amplified multiple times by the first loop and pulley assembly 105 to drive the flywheel 120 of the eddy brake assembly 102. Similarly, a torque of the flywheel 120 of the eddy brake assembly 102 is amplified by the second loop and pulley assembly 107 to drive the clutch brake assembly 106. The skilled person will appreciate that this enables a finer control of the resistance provided by the eddy brake assembly 102 to the operation of the step mill mechanism 110, which provides smoother control and an improved user experience. This arrangement also enables smaller components to be used. More particularly, the skilled person will appreciate that the second speed increasing loop and pulley assembly 107 enables the use of a modest clutch brake assembly 106. This reduces the cost of the exercise machine 101, whilst providing smooth resistance control. This is particularly beneficial in the competitive fitness market. Put simply, the invention involves identifying the presence of an edge 22 of a hole 21 from the output of a pair of relatively inexpensive sensors 3a, 3b, and computing the speed from a series of timestamps (representing holes passing by the sensors 3a, 3b). This is achieved by the aforementioned processing of the two signals. The analog section subtracts one optical signal from the other, amplifies this difference, and cleans up the noise (of which a great deal may be present due to the marginally-reflecting surface). The digital processing detects the peaks in this signal which correspond to the edges 22 of holes 21, which is done by differentiating the signal and then detecting the zero-crossings. These zero-crossings correspond to the holes 22 in the moving component 21. The inventors were surprised by the precision and reliability of this measurement, despite wide variations in machine characteristics from unit to unit. It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention. For example, the optical sensing system 1 may be incorporated in other exercise equipment, such as a stationary bike, a treadmill, an elliptical cross trainer, an independent stepper, a ski trainer, an ergometer or a rowing machine. It will also be appreciated by those skilled in the art that any number of combinations of the 5 aforementioned features and / or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.

Claims

1. An optical sensing system for sensing an edge of a low reflectance component of a machine, the system comprising control means operatively connected to a pair of optical sensors each comprising an emitter and a receiver, wherein the control means is configured to:receive a signal pair from the optical sensors after each of a plurality of time intervals, each signal pair including a signal from each optical sensor indicative of either a non-zero reflectance condition, corresponding to the presence of a surface of the component facing the optical sensors, or a zero reflectance condition, corresponding to the absence of a surface of the component facing the optical sensors;subtract the signals of each signal pair from one another by means of a differential amplifier to provide an output signal; anddetect an edge in the component by identifying a peak in the output signal.

2. The optical sensing system of claim 1, wherein the control means is configured to generate a speed signal indicative of a speed of the component based the detection of a plurality of edges of the component over a predetermined time period.

3. The optical sensing system of claim 1 or claim 2 further comprising a bandpass filter operatively connected to the control means and the bandpass filter is configured to remove a DC offset and / or low-frequency and / or high frequency components from the output signal from the differential amplifier to provide a filtered output signal.

4. The optical sensing system of claim 2 or claim 3, wherein the output signal from the differential amplifier is sampled by the control means to compute a first derivative of the output signal, and identify zero-crossings in the resultant signal.

5. The optical sensing system of claim 4, wherein the control means is configured to apply a state-estimation technique to the resultant signal.

6. An exercise machine comprising the optical sensing system of any preceding claim and a movable component adjacent the pair of optical sensors and comprising oneor more holes extending through the component, wherein the control means is configured to provide a speed signal indicative of a speed of the component.

7. The exercise machine of claim 6, wherein the component is a disc with a plurality of holes spaced equally about a rotational axis of the disc and the control means is configured to provide a speed signal indicative of a rotational speed of the component.

8. The exercise machine of claim 7, wherein the disc is ferromagnetic and forms part of an eddy brake assembly.

9. The exercise machine of claim 8, wherein the eddy brake assembly is coupled to a friction or clutch brake assembly for selectively stopping and / or inhibiting movement of the disc.

10. The exercise machine of claim 9 comprising an actuation mechanism coupled to the eddy brake and operable by a user.

11. The exercise machine of claim 109, wherein the actuation mechanism is coupled to the eddy brake assembly by a first speed increasing loop and pulley assembly.

12. The exercise machine of claim 11, wherein the eddy brake assembly is coupled to the friction or clutch brake assembly by a second speed increasing loop and pulley assembly.

13. The exercise machine of any one of claims 10 to 12, wherein the actuation mechanism comprises a step mill mechanism movable along a track.

14. The exercise machine of any one of claims 10 to 12, wherein the actuation mechanism comprises one of a stationary bike pedal mechanism, a treadmill mechanism, an elliptical cross training mechanism, an independent stepper mechanism, a ski training mechanism, an ergometer mechanism and a rowing mechanism.

15. A method of sensing an edge of a component of a machine, the method comprising:receiving a signal pair from a pair of optical sensors after each of a plurality of time intervals, each signal pair including a signal from each optical sensor indicative of either a non-zero reflectance condition, corresponding to the presence of a surface of the component facing the optical sensors, or a zero reflectance condition, corresponding to the absence of a surface of the component facing the optical sensors;subtracting the signals of each signal pair from one another by means of a differential amplifier to provide an output signal; anddetecting an edge in the component by identifying a peak in the output signal.

16. The method of claim 15 comprising generating a speed signal indicative of a speed of the component based the detection of two or more edges of the component over a predetermined time period.

17. The method of claim 16 comprising removing a DC offset and / or low-frequency and / or high frequency components from the output signal to provide a filtered output signal.

18. The method of claim 16 or claim 17 comprising sampling the output signal from the differential amplifier to compute a first derivative ofthe output signal, and identify zerocrossings in the resultant signal.

19. The method of claim 18 comprising applying a state-estimation technique to the resultant signal.

20. A computer program element comprising computer readable program code means for causing a processor to execute a procedure to implement the method of any one of claims 15 to 19.

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