WO2013164585A1

WO2013164585A1 - A servomechanism

Info

Publication number: WO2013164585A1
Application number: PCT/GB2013/051083
Authority: WO
Inventors: Richard Anthony
Original assignee: University of Greenwich
Current assignee: University of Greenwich
Priority date: 2012-04-30
Filing date: 2013-04-26
Publication date: 2013-11-07
Anticipated expiration: 2014-10-30
Also published as: GB201207460D0

Description

A Servomechanism

The invention relates to servomechanisms (servos), and to devices comprising servos. A servo is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism. The term "servo" correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed or other parameters. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, fluidic or magnetic principles.

A common type of servo provides positional control. In this type of servo, control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted (target) values causes an "error signal" that is amplified and converted and used to drive the system in the direction necessary to reduce or eliminate the error. Typically these servos give a rotational (angular) output, but servos giving linear output are also widely known and use a lead screw or linear motor to give linear motion.

To achieve precision positional control, a servo usually comprises a motor, a gearbox, a positional feedback system and a control circuit. The gearbox usually provides an overall ratio of about 1000 : 1 to 5000 : 1 and enables very small, but fast running motors on about 4-6 volts to generate very high torque, but at a much lower speed. Positional feedback is achieved by adjusting the width of the control pulse sent to the servo (pulse width modulation). A control circuit then converts the pulse-width into a "position" value, and uses the positional feedback to move the motor to the correct position and hold it there, as and when necessary.

Positioning servos were first used in military fire-control and marine navigation equipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote-control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-fly wire systems in aircraft which use servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus. In industrial machines, servos are used to perform complex motion.

However, currently known positional control servos suffer from a number of problems. Firstly, they only hold their position whilst power is being continually applied, and the amount of power required to maintain the servo's position is directly related to the torque exerted by the mechanical apparatus. For example, in a robotic arm application of a servo, the more weight to be manipulated (multiplied by the distance from the joint at which the weight acts, because torque is the product of mass and distance), the more power must be used to hold the servo at the required position. Clearly, therefore, servos are a considerable drain on energy. Secondly, if either the power or control signal, which must be continuously generated, is removed from the system, the servo can rotate until it hits its end-stop. In many applications, movement of the servo upon loss of power can have significant safety implications. Thirdly, when a servo is mechanically stressed, such that it is pulled way from its target position, its internal control system works to regain this position. This can inject mechanical "bouncing" or oscillation into the system, which is clearly

undesirable.

There is, therefore, a need to provide an improved servo. According to a first aspect, there is provided a latchable electro-mechanical servomechanism.

The inventors have demonstrated that the latchable servomechanism (i.e. servo) according to the first aspect exhibits several significant advantages over known servos. As described in the Examples, by latching and fixing the servo into position once it is in the desired position and stationary, the latchable servo is able to (i) hold its position when power is turned off, (ii) hold its position if the control signal is removed/lost, (iii) has increased mechanical stability, (iv) has a reduced power consumption, and (v) can directly replace existing servos. Power can be completely removed whilst the servo is locked, thus considerably reducing total power consumption. This is important from an environmental point of view, and is especially important for battery-powered equipment, and is ideal for applications where the servo position is changed relatively infrequently, such as a security camera pan/tilt mechanism (e.g. a CCTV) and is required to stay in a fixed position until commanded to change. This feature can also be purposely exploited as a fail-safe depending on the design of the

mechanism/linkage connected to the servomechanism, and on the specific requirements of the application.

Whilst the new servo is locked by the latch, no mechanical movement of the servo output shaft is possible. This is ideal for applications where, between purposeful movements, it is desirable to hold the mechanism in a rigid position, e.g. a robotic arm, robotic production equipment or a security camera pan/tilt system exposed to high winds. Moreover, because the new servo design suffers from less mechanical stress in the system, potentially cheaper designs can be manufactured. Furthermore, advantageously, the servo of the present invention can replace an existing servo and can be switched between latching and non- latching modes, as desired. In a non-latching mode, the servo operates in exactly the same way as the original unmodified device with no additional latency. In latching mode, the servo is automatically locked in position once the target position has been reached, and remains locked until a change in target position is detected by its control system, whereupon the servo is automatically unlocked by its control system. The servo is automatically locked in position after each position change. Either mode can be used indefinitely. To switch between modes, a pair of command signals is sent to a servo control circuit.

Preferably, the servo comprises a motor connected to an output shaft, which shaft is arranged, in use, to apply force to, and thereby control the position of, an apparatus to which it is attached. The motor may be an electric motor that is able to convert electrical energy into mechanical motion. Preferably, the motor comprises a spindle which rotates at high speed and with low torque. A gearbox then reduces the output shaft speed by some ratio whilst simultaneously increasing the torque available at the output shaft by the inverse of this ratio. Preferably, the torque at the output shaft will be at least 0.5, 1, 3, 5, 8, 10, 12, 15, 20, 25, 30, 40, 50 Kg/cm. Preferably, the rotational speed of the output shaft (measured as the time taken to rotate 60 degrees) will be at most 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.5, 2, 3 seconds.

Preferably, the motor runs on at least 1, 2, 4, 6, 8, 10, 12 or 15 Volts.

The apparatus connected to the output shaft may be, for example, a lever, gear wheel, actuating rod or other mechanism which adapts the rotational output of the servo to perform some mechanical purpose depending on the application, which could be, for example, an actuation mechanism in automotive

electromechanical equipment (e.g. seat position adjustment, heating/air- conditioning flaps adjuster, sunroof operating mechanism, throttle position control for cruise control system), actuation mechanism in camera control mechanism (e.g. pan mechanism, tilt mechanism, zoom mechanism), steering mechanism of model car, aileron of model aircraft or unmanned aerial vehicle, rudder of model boat, valve control in domestic heating system and in domestic household appliances such as washing machines, open/close mechanism in CD/DVD player drawer mechanisms, factory automation, medical device.

Preferably, the servo comprises a latch, which is arranged, in use, to move between a first, latched (locked) position, in which movement of the output shaft is prevented, and a second, unlatched (unlocked) position, in which movement of the output shaft is possible. The latch may comprise a receiving means which is operably connected to the output shaft, and which is arranged, in use, to receive an engagement means, which is capable of moving between the first and second positions.

Preferably, the servo comprises a gearbox comprising at least one gear by which the output shaft is connected to the motor. The gearbox may comprise a plurality of interconnected gears arranged in a gear train, and preferably at least two, three or more interconnected gears. The gearbox may be any conventional gear box that uses a gear train to provide speed and torque conversion from a rotating power source to another apparatus. Preferably, the gearbox provides an overall ratio of at least 50:1, 100:1, 250:1, 500:1, 1,000:1; 2,000:1, 3,000:1, 4,000:1, 5,000:1, 6,000:1, 7,000:1, or 8,000:1. When engaged with the engaging means, the receiving means may be able to prevent the gears of the gearbox from turning so that the output shaft (and thus any mechanism connected to it) is held in a fixed position. In this situation, the servo is said to be locked. The servo is automatically locked by default. When the application requires the servo to move to a new position (indicated by a change in the position control pulse), the latch mechanism is purposefully automatically disengaged by its control system so that the servo is unlocked to allow movement to the new target position; the latch is then automatically reengaged returning the servo to the locked state after the new target position has been reached. It will be appreciated that the servo can be latched when power to the servo is inhibited, for example when there is a power cut, which may be caused by a thunder storm, a short circuit, an overload of electricity mains, damage to transmission lines or a fault at a power station, a flat battery etc. Thus, the servo of the invention is particularly useful where maintaining the position of the apparatus is important and will have significant safety

advantages.

The receiving means may comprise a rotor, which is operably connected to the output shaft. Preferably, the receiving means comprises or is operably connected to a gear, and preferably a first gear in the gear train (i.e. the motor end of the gear train). For example, the rotor may be attached to the gear. The receiving means may comprise one or more recesses which are configured to receive the engagement means. The rotor may be substantially circular. The receiving means may be star-shaped comprising a series of spaced apart recesses and protrusions around its perimeter.

Preferably, a distal end of the engagement means is adapted to engage with the receiving means. Preferably, the distal end is shaped to fit into and engage with a recess of the receiving means. For example, the distal end of the engagement means may be parallel-sided, tapered or wedge-shaped. The engagement means may comprise a locking arm, which may be substantially elongate.

The latch may comprise a biasing means, which is arranged, in use, to urge the engagement means into the first position, preferably so that the distal end engages with a recess. The biasing means may be capable of causing the engagement means to engage with the receiving means, unless an applied force greater than the force exerted by the biasing means is employed to the engaging means. The biasing means may comprise a resilient member. For example, the biasing means may be a spring, preferably a helical spring, which may be disposed around the engagement means.

The servo may comprise a solenoid which, when activated, is capable of causing the engagement means to move from the first position to the second position, i.e. into the unlatched state. It will be appreciated that the force applied to the engagement means by the solenoid is greater than the force exerted by the biasing means so that the servo becomes unlatched. Conversely, when the solenoid is unpowered, the biasing force of the biasing means is sufficient to urge the engagement means towards the first position so that the servo is latched. Preferably, the latch comprises guide means for guiding the engagement means towards and away from the receiving means as it moves between the first and second positions. The guide means may comprise at least one, and preferably two, guides that are disposed either side of the engagement means. Preferably, the servo comprises a positional feedback system and/or a control circuit. The positional feedback system may comprise a shaft encoder which is arranged, in use, to detect the position of the output shaft. The shaft encoder may be optical, magnetic or electrical-resistive in nature and, thus, provides a means for the control circuit to detect the current position of the output shaft, which it compares against the desired position of the shaft as indicated by the position-control signal provided to the control circuit.

Programming of the position of the latchable servo of the present invention may be achieved by adjusting the width of a position-control pulse sent to the servo in a method known as pulse width modulation. A control circuit may be arranged to convert the pulse width into a rotational/angular position value, and use the positional feedback system to move the motor to the desired position, and hold it there. The servo of the present invention includes a more-sophisticated control algorithm and control circuit than used in the prior art, to automatically detect changes in the position-control pulse width, and to control the resulting timing sequences of powering the solenoid and motor to ensure that the servo

mechanism holds its correct position whilst the latch is disengaged. Once the new position is attained, the servo is held steady by the control algorithm and control circuit until the latch has been re-engaged, whereupon the power to both motor and solenoid can be removed.

Preferably, the control circuit receives and interprets a position-control pulse sent by an external application to the servo. Preferably, the control circuit is able to distinguish between two types of control pulse: position-control pulses, which signal a desired rotational position of the servo, as with prior art; and new mode- command pulses, which signal the operational mode of the servo (latching or non-latching), and are outside the valid size range of the position-control pulses. Preferably, the control circuit manages the latching behaviour entirely

automatically, so that the external application's logic does not need any modification and, thus, the servo of the present invention can be used to directly replace a prior art servo. Preferably, the control circuit remembers the previous position-control pulse width and thus, by comparison, is able to detect a change in position-control pulse width. Preferably, the control circuit takes care of all timing requirements, such that the servo is only unlatched whilst moving from a current position to a new target position.

Pulse width modulation (PWM) means controlling the relative widths of the positive and negative parts of an output waveform, whilst keeping the pulse frequency constant. PWM, therefore, may control the amount of energy in the waveform and, as such, can be used to regulate the speed of a motor, the brightness of a light etc. The fraction of time spent in the positive part of the waveform is called the "duty cycle" - a duty cycle of ioo% means that maximum energy is provided, and a duty cycle of 0% means that minimum energy is provided to the controlled device. A square wave, for example, has a duty cycle of 50%. In a servo, the width of the pulse is used as a means by which the external application's logic can signal the target position of the servo to the servo's internal control circuit. The position-control pulse width may be in the range 500 microseconds to 2500 microseconds. The pulse width may be at least 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400 or 2500 microseconds.

Servos may have a limited rotational movement range of at least 90, 120, 150, 170, 180, 190, 270 degrees. The control circuit takes the position-control pulse width and converts this to an equivalent rotation, so, for example, the shortest position-control pulse width may translate to the minimum rotational angle and the largest position control-pulse width may translate to the maximum rotational angle.

The latchable servo may make use of a 500 position-control pulse-width cutoff (i.e. 500 is being the lowest pulse width that the control logic will translate into a rotational position), to facilitate an additional advantageous feature on- demand switching between latching and non-latching modes. A pair of special mode-command pulses have pulse widths less than 500 and thus can be distinguished from position-control pulses. These mode-command pulses do not cause the servo to rotate. Instead, a short mode-command pulse of at least 150, 160, 170, 180, 190, 200, 210, 220, 230, 240 or 250 is may be used to set the servo into the non-latching mode. In this configuration, the control circuit keeps the latch withdrawn and the servo's position is maintained by powering the motor and continuously applying the position-control pulse. If however, the power supply is removed from the servo, the latch is automatically applied by the mechanical biasing apparatus, holding the servo at its current position until power is re-applied.

A short mode-command pulse of at least 350, 360, 370, 380, 390, 400, 410, 420, 430, 440 of 450 is may set the servo into the latching mode (this mode is the default on power-up). The servo may only be powered during rotation events triggered by the detected position-control pulse width change. The mode- command pulses may be sent momentarily, interrupting the continuous stream of position-control pulse width modulation pulses. In a second aspect of the present invention, there is provided a device

comprising the latchable electro-mechanical servo according to the first aspect. In particular, the servo of the present invention can be used in robots, remote controlled models, toys, heater/air vent control, throttle control, vehicles, air conditioning units, camera pan/tilt mechanisms, home automation and assistive technologies. Those skilled in the art will realise that the aforementioned list is not a comprehensive list of uses for the servomechanism of the present invention, but rather an illustrative one.

It will be appreciated that there are continuing problems inherent with outdoor PTZ (pan, tilt, zoom) cameras during thunderstorms. Despite continuing improvements in PTZ software, problems with continual activation during thunderstorms have led some organisations to completely disable these types of camera during thunderstorms. This translates to continuing challenges in trying to monitor and protect facilities from crime and terrorism during inclement weather.

A new innovation to the PTZ camera is a built-in firmware program that monitors the change of pixels generated by the video chip in the camera. When the pixels change due to movement within the camera's field of view, the camera can actually focus on the pixel variation and move the camera in an attempt to centre the pixel fluctuation on the video chip. This process results in the camera following movement. The program allows the camera to establish the size of the object which is moving and distance of the movement from the camera. With this estimate the camera can adjust the camera's optical lens in and out in an attempt to stabilize the size of the pixel fluctuation as a percentage of total viewing area. Once the movement exits the camera's field of view the camera automatically returns to a pre-programmed or "parked" position until it senses pixel variations and the process starts over again. Accordingly, as can be appreciated from the above, the servo of present invention can be used in a PTZ camera of the type referred to above, with the latch employed when the camera is "parked".

Thus, in a third aspect, there is provided a camera comprising the latchable electro-mechanical servo according to the first aspect. Preferably, the camera comprises a PTZ (pan and/or tilt and/or zoom) or closed circuit television (CCTV) camera.

In a fourth aspect, there is provided a method of latching or unlatching an electro-mechanical servomechanism, the method comprising urging a latch to move between a first, latched position, in which movement of an output shaft of an electro-mechanical servomechanism is prevented, and a second, unlatched position in which movement of the output shaft is possible.

Preferably, the method comprises holding a servo in position when in the first, latched position.

All of the features described herein (including any accompanying claims, abstracts and drawings), and/or all of the steps of any method or process disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: -

Figure l is a schematic representation of a prior art servo set-up;

Figure 2 is a schematic representation of a servo set-up according to the present invention;

Figure 3 is an enlarged schematic side view of a first embodiment of an electromechanically operated latch servomechanism according to the present invention, showing a series of interconnected gears (i.e. a gear box) and a latch; Figure 4 a) is an enlarged side view of a second embodiment of the

servomechanism, showing a first gear wheel of the gear box of the servo engaged by a wedge-shaped latch bar;

Figure 4 b) is an enlarged side view of a third embodiment of the

servomechanism, showing a first gear wheel of the gear box of the servo engaged by a latch; Figure 5 is an enlarged side view of a fourth embodiment of the

servomechanism, showing a first gear wheel of the gear box of the servo engaged by a latch;

Figure 6 is an enlarged side view of a fifth embodiment of the servomechanism, showing a first gear wheel of the gear box of the servo engaged by a latch. This figure also illustrates that the force applied to the latch remains perpendicular to the latching and withdrawing action of the latch regardless of rotational force direction;

Figure 7 illustrates the timing involved in operation of the latchable

servomechanism of the present invention; and

Figure 8 illustrates the way in which the two special mode-command signals (to provide the advantage of being able to select between latched and unlatched modes of operation) are incorporated into the position-control signal sequence. The two special mode-command signals are differentiated from position-control signals by the fact that they have specific pulse widths which are outside of the range of values for position-control signals. This facilitates switching

dynamically between latched and unlatched modes of operation without disturbing the positional control of the servo. Examples

The inventors have developed a latchable servomechanism (18), which can be used in a wide variety of applications including small robots, remote controlled models and toys, heater and air vent controls, throttle controls in vehicles, air conditioning units, camera pan/tilt mechanisms, home automation, medical equipment, general machinery and assistive technologies. The inventors have found that because the servo (18) of the present invention is latchable, it is particularly useful where there is a need to hold a specific position for periods of several seconds or more, where there is a need for increased mechanical stability in rotational position holding, where there is a need to reduce power

consumption, e.g. battery operated equipment (because whilst latched no power is required to hold the servo's position), and where there is a need for automatic fixing of current position when there is an interruption to either the control signal or power supply to the servo (e.g. as a safety stop function). Referring to Figure 1, there is shown the layout of a traditional servo set-up (2). As can be seen, such a set-up (2) consists of a servo control circuit (4) mechanically connected to a motor (6), which can be driven in either direction, as illustrated by arrows A and B. The motor (6) is mechanically connected to a gear box (8), and an output shaft (10), which extends into a shaft encoder (12), which provides rotational positional feedback (14) to the servo control circuit (4). The input signal into the servo control circuit (4) is a pulse signal (16), the pulse width of which determines the position of the servo and output shaft (10).

Example 1: Development of Latchable Servomechanism

In order to circumvent the problems associated with existing servos (2), including high power consumption, less than desirable mechanical stability and the fact that they do not work during powers cuts, the inventors decided to investigate the possibility of developing a servomechanism that could be automatically latched/locked into a desired position, either by default (i.e.

automatically after each position change), or as and when required (by means of additional command pulses).

An apparatus (18) that fulfils these requirements was designed, one embodiment of which is shown in Figure 2. As with the traditional set-up (2) described above, the apparatus (18) of the invention also comprises a servo control circuit (4) connected to a motor (6), which can be driven in either direction (see arrows A and B), which is itself connected to a gear box (8). The gear box (8) is attached to an output shaft (10) and a shaft encoder (12), which provides rotational positional feedback (14) to the servo control circuit (4). The input signal into the servo control circuit (4) is also a pulse signal (16), the pulse width indicating the desired position of the servo. The apparatus (18) also comprises a gate (20) to turn off the power supply to the motor(6), a gate (21) to turn off the power supply to the solenoid (26) and a gate (22) to turn off the motor direction signal (23). The control circuit (4) controls these gates via control signals (37), (38), and (39) respectively.

However, as shown in Figure 2, the apparatus (18) also includes a solenoid (26) which controls a latch mechanism (24) consisting of a retractable latch bar (27), which together create a latchable servomechanism. The servo control circuit (4) also accepts short mode-command pulses, in addition to the aforementioned position-control pulse signal (16), to switch the servo between latching and non- latching modes of operation. When in the (default) latching mode, the actual latching behaviour is managed automatically by the control circuit (4).

Figure 3 shows a series of four inter-engaging gears (70, 72, 74, 76), which together form a gear train (30). The actual number of gears employed in the gear train may vary, depending on the overall gear ratio required in particular servo embodiments. As can be seen, the electromechanically operated latch mechanism (24) with latch bar (27) acts upon a motor spindle end of the first gear (70) of the gear train (30) rather than on the spindle (10) of the last gear (76) in the gear train (30) which is also the output shaft of the entire servo . This position is optimal as it takes advantage of the gearing of the gear train to increase the effective mechanical holding strength of the latch (27), which is increased by the overall ratio of the gear train (30). The motor spindle (34) of the first gear (70) rotates at a high speed with very low torque, whereas a servo output spindle (10) of the last gear (76) rotates at a lower speed with a higher torque. This means that the latch mechanism can hold a high torque applied to the output shaft of the servo mechanism, whilst only a small fraction of that torque is actually transferred onto the latch mechanism itself. In a first embodiment of the servomechanism of the present invention, the first gear wheel (28) is engaged by a wedge-shaped latch bar (27), as shown in Figure 4(a). This was adequate to demonstrate the concept, but suffered from two specific weaknesses. First, the sharp distal point (42) of the soft aluminium latch bar (27) was rounded off by the harder brass gear teeth (40) when mechanical slippage occurred during testing. Secondly, the shape of the gear teeth (40) and the pointed shape (42) of the latch bar (27) occasionally allowed "kick-out" when rotational force was applied to the servo output shaft (10) and, thus, required that a latch release spring (not shown) hold the latch bar (27) in place with more force than is desirable. An anti kick-out solution would enable a smaller latch bar (27) and spring (not shown) to be used, and a faster overall latch/unlatch action would be the result.

A solution to the above problems was found by the inventors to be an especially designed extra wheel, referred to as a latch rotor (36), as shown in Figure 4(b) and positioned on the same shaft as the first gear wheel (28). In this case, the rotational force of the latch rotor (36) acts perpendicular to the latch action and, consequently, the latch holding force does not have to be proportional to the rotational force applied.

The latch rotor (36) and how it functions is shown in more detail in Figure 5. Here it can be seen that, when engaged with the latch rotor (36), the latch bar (27) is subjected to a rotational force the direction of which is indicated by the arrow marked (50), which acts perpendicular to it. Consequently, there is zero force in the direction of movement of the latch bar (27) the direction of which is indicated by the arrow marked (48), which, in turn, ensures that very low force is required to insert and withdraw the latch bar (27). A guide block (44) is disposed either side of the latch bar (27) forming a pair, and together they prevent lateral displacement of the latch (27) and ensure mechanical stability of the locking mechanism (24). The rotational "slack" of the latch rotor (36) represented by the arrow marked (52) is divided by the overall gear train ratio, typically between 1:1000 and 1 :500ο, and so mechanical rotational slack at the output shaft (10) of the servo when the latch bar (27) is applied is typically <o.o6 degrees. A pivot (46) connects the latch bar (27) to the solenoid (26).

Figure 6 shows the solenoid (26) attached by the pivot (46) to the latch bar (27). The solenoid (26) comprises a solenoid body (60) which houses an armature (62), which moves in and out of the body (60) under the resilient action of a helical spring (56) wound around the armature (62). A spring retainer (58) is disposed at the distal end of the spring (56), and fixed to the armature (62). Powering the solenoid (26) pulls the armature (62) into the solenoid body (60) against the biasing force of the spring (56), thereby pulling the latch bar (27) from the latch rotor (36). Turning off the power to the solenoid (26) allows the biasing force of the spring (56) to pull the armature (62) back out of the solenoid body (60), and urges the latch bar (27) back into engagement with the latch rotor (36). A rotational force of the latch rotor (36) which is represented by arrow (50) acts in either direction (clockwise or anti-clockwise), but is always perpendicular to the latch bar (27). This is important because it ensures a low force requirement for insertion and withdrawal of the latch bar (27) from the rotor (36). Rotational force applied to the latch bar (27) is the force at the servo output shaft (10) divided by the overall gear train ratio. - ι₅ -

Example 2 : Development of Control Pulses for Latchable Servomechanism

With the traditional servomechanism of Figure 1, the position of a conventional prior art servo (2) is controlled by a repeat position-control pulse. The width of the pulse is decoded by the servo control circuit (4), whereupon it is translated into a target position. The servo motor (6) is then powered on until the positional feedback (14) indicates that the target position of the output shaft (10) has been achieved. The servo motor (6) is continually powered to hold this position against any externally applied force at the output shaft (10), until the position-control pulse width changes indicating a new target position.

Like traditional servomechanisms (2), the position-control pulse width is typically in the range 500 microseconds to 2500 microseconds, and the servo rotational angle is typically in the range 170 degrees to 190 degrees. Thus, a pulse of 500 is may indicate fully anticlockwise, a pulse of 2500 μβ may indicate fully clockwise (or vice versa) and a pulse of 1500 μβ may indicate the centre position. The pulse is repeated at a typical rate of 18 milliseconds. The servo's position is held by electrical force (via the motor) (6).

However, the latchable servo of the present invention (18) operates by default in 'latching' mode. In this mode the control circuit automatically controls a latch mechanism so that the servo's position is held mechanically, rather than by electrical force (via the motor) (6) as in conventional prior art designs. The control circuit (4) automatically detects a change in the position-control pulse width, indicating that a new position is desired. The control circuit (4) automatically provides the control and timing sequences necessary to retract the latch bar (27) and rotate the servo output shaft (10) to the new position. The control circuit (4) then automatically re-engages the latch bar (27) with latch rotor (36) once the target position is reached, locking the servo mechanically. The latchable servo of the present invention (18) also makes use of the 500 μβ position-control pulse-width cut-off, to facilitate a pair of mode-command pulses which do not cause the servo to rotate. A short pulse of 200 μβ (detected in the range 150 to 250 μβ) sets the servo into non-latching mode; the servo reverts to unmodified behaviour, the servo's position being held by powering the motor, the latch being kept withdrawn and the position-control pulse continuously applied. A short pulse of 400 μβ (detected in the range 350 to 450 μβ) sets the servo into latching mode (the default on power-up); the servo motor is only powered during rotate events triggered by detected changes in the position- control pulse width The mode-command pulses are sent momentarily, interrupting the continuous stream of position-control pulse width modulation pulses.

Figure 7 illustrates the timing and control sequence involved with the latchable servomechanism of the present invention when operating in its default latching mode. After a change in the position-control pulse has been detected (A) the original pre-change pulse is regenerated (B) whilst the servo motor is powered up, so that it holds the pre-change position and does not rotate prematurely (C). The servo latch is retracted (D), i.e. the solenoid armature is withdrawn into the solenoid body, and after a delay to allow for the actuation lag, the new position- control pulse signal is applied (E), causing the servo to rotate to the new desired position (F). After a short delay to allow the rotation to complete, the servo latch is engaged, i.e. the solenoid armature is extended out of the solenoid body so that the latch is applied and thus the servo position is now held mechanically, the motor is now powered down and the position-control pulse removed (G). Figure 8 illustrates how the short mode-command pulses are inj ected into the position-control signal sequence. The control circuit differentiates the mode- command signals from position-control signals by their short pulse widths which are outside of the range of values for position-control signals. These mode- command signals facilitate switching dynamically between latching and non- latching modes of operation, without disturbing the positional control of the servo. The two modes have different optimality; the latching mode has the advantages of mechanical position holding (which include lower power consumption, improved mechanical stability, and safety locking if power is removed), although the latching mode does introduce a small amount of latency because the servo has to operate the latch before the servo is able to move position (the first prototype has a latency of 100 ms).

The design of the present invention advantageously incorporates a facility to dynamically switch between latching and non-latching modes as desired. This feature of switching between the two modes is provided to ensure universal applicability of the new design in all applications disclosed in the prior art; in non-latching mode the servo operates in exactly the same way as a traditional servo, i.e. with no additional latency.

To recapitulate, advantages of the latchable electromechanical servo of the invention also reside in the fact that it is able to (i) hold its position when power is turned off, (ii) hold its position if the control signal is removed/lost, (iii) has increased mechanical stability, (iv) has a reduced power consumption, and (v) can directly replace existing servos.

Claims

1. A latchable electro-mechanical servomechanism.

A servomechanism as claimed in claim 1, wherein the servomechanism comprises a motor connected to an output shaft, which shaft is arranged, in use, to apply force to, and thereby control the position of, an apparatus to which it is attached.

A servomechanism as claimed in claim 2, wherein the motor is an electric motor.

A servomechanism as claimed in either claim 2 or claim 3, wherein the motor comprises a spindle which rotates at high speed and with low torque.

A servomechanism as claimed in any preceding claim, wherein the servomechanism comprises a latch, which is arranged, in use, to move between a first latched position, in which movement of the output shaft is prevented, and a second, unlatched position, in which movement of the output shaft is possible.

A servomechanism as claimed in claim 5, wherein the latch comprises a receiving means which is operably connected to the output shaft, and which is arranged, in use, to receive an engagement means, which is capable of moving between the first and second positions.

A servomechanism as claimed in preceding claim, wherein the servo comprises a gearbox.

A servomechanism as claimed in claim 7, wherein the gearbox comprises at least one gear by which the output shaft is connected to the motor.

9. A servomechanism as claimed in either claim 7 or claim 8, wherein the gearbox comprises a plurality of interconnected gears arranged in a gear train.

10. A servomechanism as claimed in any of claims 7 to 9, wherein when engaged with the engaging means, the receiving means prevents the gears of the gearbox from turning so that the output shaft is held in a fixed position.

11. A servomechanism as claimed in any of claims 6 to 10, wherein the receiving means comprises a rotor, which is operably connected to the output shaft.

12. A servomechanism as claimed in any of claims 6 to 11, wherein the receiving means comprises or is operably connected to the at least one gear.

13. A servomechanism as claimed in claim 12, wherein the gear is the first gear in the gear train.

14. A servomechanism as claimed in claim 13, wherein the rotor is

attached to the gear.

15. A servomechanism as claimed in any of claims 6 to 14, wherein the receiving means comprises one or more recesses which are configured to receive the engagement means.

16. A servomechanism as claimed in any of claims 11 to 15, wherein the rotor is substantially circular.

17. A servomechanism as claimed in any of claims 6 to 16, wherein the receiving means may be star-shaped comprising a series of spaced apart recesses and protrusions around its perimeter.

18. A servomechanism as claimed in any of claims 6 to 17, wherein a distal end of the engagement means is adapted to engage with the receiving means.

19. A servomechanism as claimed in claim 18, wherein the distal end is shaped to fit into and engage with a recess of the receiving means.

20. A servomechanism as claimed in claim 19, wherein the distal end of the engagement means is parallel-sided, tapered or wedge-shaped.

21. A servomechanism as claimed in any of claims 6 to 20, wherein the engagement means comprises a locking arm, which may be

substantially elongate.

A servomechanism as claimed in any of claims 8 to 21, wherein the latch comprises a biasing means, which is arranged, in use, to urge the engagement means into the first position, preferably so that the distal end engages with a recess.

A servomechanism as claimed in either claim 21 or claim 22, wherein the biasing means is capable of causing the engagement means to engage with the receiving means, unless an applied force greater than the force exerted by the biasing means is employed to the engaging means.

A servomechanism as claimed in either claim 22 or claim 23, wherein the biasing means comprises a resilient member.

A servomechanism as claimed in claim 24, wherein the biasing means is a spring.

A servomechanism as claimed in any preceding claim, wherein the servomechanism comprises a solenoid, which when activated, is capable of causing the engagement means to move from the first position to the second position.

27. A servomechanism as claimed in any of claims 5 to 26, wherein the latch comprises guide means for guiding the engagement means towards and away from the receiving means as it moves between the first and second positions.

28. A servomechanism as claimed in claim 27, wherein the guide means comprises at least one guide, preferably two guides that are disposed either side of the engagement means.

29. A servomechanism as claimed in any preceding claim, wherein the servomechanism comprises a positional feedback system and/or a control circuit, wherein the positional feedback system comprises a shaft encoder which is arranged, in use, to detect the position of the output shaft.

30. A servomechanism as claimed in claim 29, wherein the shaft encoder is optical, magnetic or electrical-resistive.

31. A servomechanism as claimed in any preceding claim, wherein

programming of the position of the latchable servomechanism is achieved by adjusting the width of a control pulse sent to the servomechanism.

32. A servomechanism as claimed in claim 31, wherein the control pulse width is in the range 500 microseconds to 2500 microseconds.

33. A servomechanism as claimed in claim 32, wherein the pulse width is at least 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400 or 2500 microseconds.

34. A servomechanism as claimed in any preceding claim, wherein the servomechanism rotational movement range is at least 90, 120, 150, 170, 180, 190, or 270 degrees.

35. A servomechanism as claimed in any preceding claim, wherein a short mode-command pulse of at least 150, 160, 170, 180, 190, 200, 210, 220, 230, 240 or 250 is used to set the servomechanism into non- latching mode.

36. A servomechanism as claimed in any preceding claim, wherein a short mode-command pulse of at least 350, 360, 370, 380, 390, 400, 410, 420, 430, 440 or 450 is is used to set the servo into latching mode.

A device comprising a servomechanism as defined in any preceding claim.

A device as claimed in claim 37, wherein the device is a robot, remote controlled model, toy, heater/air vent control, throttle control, vehicle, air conditioning unit, camera pan/tilt and/or zoom mechanism, household appliance, home automation device, factory automation device, medical device or assistive technology device.

A device as claimed in claim 38, wherein the device is pan and/or tilt and/or zoom (PTZ) camera.

A camera comprising a servomechanism as claimed in any of claims 1 to 36.

A camera as claimed in claim 40, wherein the camera is a PTZ or closed circuit television (CCTV) camera.

A method of latching or unlatching an electro-mechanical

servomechanism, the method comprising urging a latch to move between a first, latched position, in which movement of an output shaft of an electro-mechanical servomechanism is prevented, and a second, unlatched position in which movement of the output shaft is possible. 43· A method as claimed in claim 42, wherein the method comprises

holding a servo in position when in the first, latched position.