FIELD OF THE INVENTION

This invention relates to an emergency AC motor braking system. More particularly, although not exclusively, the invention relates to a bandsaw including a hazard detector and a control circuit capable of rapidly stopping the AC motor when the hazard detector detects a hazard.

BACKGROUND OF THE INVENTION

Bandsaws are used extensively in the meat processing industry and if unprotected pose a significant risk of injury to users. Safety devices have been provided to stop a bandsaw when a hazard is detected. These have typically included an electromechanical disc brake with the motor controlled by mechanical contact switches. Due to the slow switching time of mechanical contact switches; the inertia of the motor rotor, freewheel and flywheel; and the stopping time possible with a friction brake, it typically takes a number of seconds for the bandsaw to stop; by which time serious injury may be inflicted upon an operator.

Systems that directly stop the bandsaw blade by clamping it can be costly, difficult to retrofit and can damage the bandsaw blade.

It would be advantageous if there was provided a relatively low cost technique that could rapidly stop the bandsaw in a manner that did not damage the bandsaw blade. SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a bandsaw including: a. a bandsaw blade; b. an AC electric motor driving the bandsaw blade via a coupling; c. a hazard detector which produces a hazard detection signal when a hazardous situation is detected; and d. a control circuit which drives the AC electric motor during normal operation and upon receiving a hazard detection signal causes the electric motor to be brought to a rapid stop by rapid DC injection braking.

In an embodiment the control circuit includes semiconductor DC injection braking switches which connect a DC source across one or more windings of the AC electric motor.

In an embodiment the AC electric motor is a three phase AC electric motor and the DC injection braking semiconductor switches connect a DC source across two or more windings of the AC electric motor.

In an embodiment the AC electric motor is a three phase AC electric motor and the DC injection braking semiconductor switches connect a DC source across one winding of the AC electric motor.

In an embodiment the semiconductor switches are thyristors.

In an embodiment the DC source comprises at least one charged capacitor. In an embodiment the capacitor is greater than 10 mF or 10,000pF.

In an embodiment the DC voltage applied by the capacitor is equal to the rectified line voltage for the AC electric motor.

In an embodiment the DC voltage applied by the capacitor is greater than a rectified line voltage for the AC electric motor.

In an embodiment the DC voltage applied by the capacitor is greater than 900 volts.

In an embodiment the control circuit includes motor control semiconductor switches which supply power from a power supply to the AC motor during normal operation and disconnect the AC motor from the power supply upon receiving a hazard detection signal.

In an embodiment the motor control semiconductor switches are IGBT switches. In an embodiment the power supply is a three phase AC power supply and wherein the control circuit includes a phase detector for detecting a phase of two or more lines of the AC power supply at or about the time of receiving the hazard detection signal. In an embodiment the control circuit controls a sequence for injecting the DC voltage into the windings of the AC motor based on the detected phase of the line(s).

In an embodiment the hazard condition is an operator's hand being within a precluded region. In an embodiment the vision system develops a hazard detection signal when a selected colour is present in the precluded region.

In an embodiment the coupling is a flywheel made of a lightweight material, such as aluminium or a suitable composite material (e.g. a nylon or carbon fibre composite).

According to another exemplary embodiment there is provided an emergency AC motor braking system including: a. a hazard detector which produces a hazard detection signal when a hazardous situation is detected; and

b. a control circuit which drives the AC electric motor during normal operation and upon receiving a hazard detection signal causes the electric motor to be brought to a sudden stop by rapid DC injection braking.

In accordance with yet another aspect there is provided a control circuit for a bandsaw including a bandsaw blade and an AC electric motor driving the bandsaw blade via a coupling, the control circuit comprising:

a circuit which drives the AC electric motor during normal operation and upon receiving a hazard detection signal causes the electric motor to be brought to a rapid stop by injecting a DC voltage into two or more windings of the AC electric motor, the DV voltage being supplied by one or more capacitors charged at a voltage greater than a rectified line voltage for the AC line supplying power to the AC electric motor.

In accordance with a still further aspect there is provided a wheel for a bandsaw, including:

a body formed of a light weight material; a band which extends at least partially around a circumference of the body and, in use, operable to prevent a bandsaw blade from contacting the body. The bandsaw may, for example, be the bandsaw as described in accordance with the first aspect.

In an embodiment the band is formed of stainless steel.

In an embodiment the body is formed of aluminium. In an embodiment the body is formed of a composite material.

In an embodiment the composite material comprises a nylon or carbon fibre composite material. Reference to any prior art in this specification does not constitute an admission that such prior art forms part of the common general knowledge.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description of exemplary embodiments given below, serve to explain the principles of the invention.

Figure 1 shows a simplified drawing of the main components of a bandsaw, in accordance with an embodiment of the present invention; Figure 2 shows a portion of a lightened bandsaw flywheel implementing a stainless steel band, in accordance with an embodiment of the present invention;

Figure 3 shows a schematic diagram of a control circuit for controlling operation of the bandsaw shown in Figure 1 , in accordance with an embodiment of the present invention;

Figure 4 shows a schematic diagram of a control circuit for controlling operation of the bandsaw shown in Figure 1 , in accordance with an alternative embodiment of the present invention; and Figures 5a, 5b and 5c show various tables showing the relationship between a determined phase state and stopping time, in accordance with a particular embodiment

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments described herein are particularly suited for use with bandsaws that are used in the meat processing industry. Accordingly, embodiments are hereafter described in such a context. Figure 1 is a simplified drawing of a bandsaw including an AC electric motor 1 with its output shaft directly coupled to flywheel 2. A bandsaw blade 4 is driven by flywheel 2 about freewheel 3.

Conventional bandsaw blades used in the meat processing industry are typically formed of mild or stainless steel, for hygiene reasons. However, the flywheel 2 and/or freewheel 3 according to embodiments described herein may be formed of a light weight material such as aluminium, or a suitable composite material, such as a nylon or carbon fibre composite material. As will become evident from subsequent paragraphs, the minimized inertia achieved by the reduced flywheel and/or free wheel weight (i.e. when compared to conventional mild steel or stainless steel wheels), may greatly improve the blade stopping speed when combined with a DC voltage injection circuit, as described herein.

An issue with using wheels formed of aluminium is that they may cause "black marking" on to bandsaw blade 4. Further, wheels formed of a composite material may not be considered suitable for use with a meat processing band saw for hygiene reasons. To avoid any potential marking or food hygiene issues resulting from use of a light weight material, the flywheel 2 and/or freewheel 3 may advantageously employ a stainless steel band 31 that wraps around a circumference of the wheel, a partial view of which is shown in Figure 2. The band 31 includes a suitable grooved upper surface for receiving the blade 4 and is held in place by one or more bolts 35. According to a particular embodiment, the groove takes the same form as the groove that is typically found on a conventional wheel, for receiving the blade 4. It will be understood that the band 31 may take any suitable shape or configuration, provided that it prevents the bandsaw blade 4 from contacting the body of the lightened weight flywheel 2 or freewheel 3.

Returning to Figure 1 , the AC electric motor 1 is a three phase AC motor and control circuit 5 supplies power from a three phase supply 1 1 to electric motor 1 . According to the illustrated embodiment, the three phase supply 1 1 is a 50 Hz, 400V AC supply.

In a particular embodiment a hazardous situation (in this case a user's hand coming too close to the bandsaw blade 4) is detected by a vision system 8. The vision system 8 monitors a capture region 10 within table 7 containing a precluded region 9 (a region in which a user's hand is deemed to be too close to the blade). Whilst a variety of image processing techniques may be employed to detect a user's hand it is convenient to have the operator wear a glove that is a distinctive colour and to detect if that colour intrudes into the precluded region. It will be appreciated that a range of other image processing techniques could be employed using techniques such as shape or edge detection. When a user's hand is detected as intruding the precluded region 9 a hazard detection signal is generated and supplied to control circuit 5. An additional electromechanical and or electromagnetic brake 6 may also be activated to stop the bandsaw. It will be appreciated that other hazard detection systems may be employed such as an optical detection system having at least one light transmitter and one light receiver which supplies a hazard detection signal when an obstacle is detected in an optical path. Another example detection system comprises a touch sensing circuit, whereby a predefined coded pulse signal is fed through a user and a receiver is operable to issue a hazard signal when the predefined coded pulse signal is detected within the precluded region 9.

Referring now to Figure 3 the operation of a control circuit 5 in accordance with a first embodiment of the invention will be explained in more detail. The control circuit 5 employs a technique referred to as DC injection braking for stopping the motor. Although DC injection braking is known to be used as a motor braking technique, it is conventionally used to gradually slow and stop a motor and has not been applied as a rapid braking technique for an AC motor, and particularly for use with an AC motor for a bandsaw as described herein.

In more detail, control circuit 5 includes a first semiconductor switch array 12 controlled by controller 28 for connecting the windings 13, 14 and 15 of AC motor 1 to a three phase supply 27 to drive the motor 1 . The semiconductor switches of semiconductor switch array 12 may suitably be insulated-gate bipolar transistor (IGBT) switches. The use of such semiconductor switches allows fast turn off as will be explained later.

The DC voltage injection circuit for rapidly stopping the motor in emergencies will now be described. A semiconductor switch 16 supplies power to a rectifier 17 which charges capacitor 18. According to the Figure 3 embodiment, the capacitor 18 is charged to a voltage corresponding to the rectified line voltage (i.e. 600 volts). Capacitor 18 may suitably have a capacitance of greater than 10 mF and according to the illustrated embodiment is 12mF. Employing a capacitor arrangement for storing the substantial DC voltage is advantageous for a number of reasons. Firstly, it ensures the circuit does not draw excessive current from the supply and in turn trip the circuit breaker (which would result in the controller losing power and therefore be unable to stop the blade). Secondly, it provides a known energy source that can deliver a very high current for a short period.

A second semiconductor switch array is formed of a first bank of thyristors 19 and a second bank of thyristors 20 controlled by controller 28 (via sub- controller 21 ). According to the illustrated embodiment, each bank includes three silicon controlled rectifiers (SCRs) where each SCR in a bank has a corresponding SCR in the other bank having a different polarity. The controller 28 may be a suitably configured microcontroller. Upon controller 28 switching thyristor arrays 19 and 20 on, the DC voltage across capacitor 18 is applied to windings 13, 14 and 15. To simplify the circuit the voltage across capacitor 18 may be applied to only one or two windings. Applying the DC voltage across only two windings has been found to be almost as effective as applying it across three and simplifies the circuit.

A converter 22 provides a DC supply which, when semiconductor switch 24 is activated by controller 28, controls electromechanical brake 23. Electromechanical brake 23 may suitably be a disc brake connected to the flywheel 2.

Another converter 25 provides a DC supply which, when semiconductor switch 27 is activated by controller 28, controls electromagnetic brake 26.

Controller 28 receives operating inputs from user inputs, such as switches, and a hazard detection signal from vision system 8. It will be understood that the controller 28 is in fact communicable with the SSRs 16, 24 & 27, though this is not shown in the figure for ease of illustration. When a user input to start the bandsaw is received by controller 28, semiconductor switch 16 and converters 22 and 25 are enabled. A high DC voltage is then applied to electromechanical brake 23 to release it and then a lower voltage is applied to maintain it in the released position. The brake is maintained open against a biasing spring and when the DC voltage is no longer applied the spring actuates the brake.

The IGBT switch array 12 is then switched on by controller 28 and AC motor drives bandsaw blade 4. During operation vision system 8 monitors capture region 10 to detect a user's hand within the precluded region 9, and if detected generates a hazard detection signal which is supplied to controller 28. Upon receiving a hazard detection signal controller 28 turns off IGBT switches 12 to disconnect motor 1 from the AC supply 27 and switches thyristor arrays 19 and 20 to apply the DC voltage from capacitor 18 across one or more windings 13, 14 and 15. Additionally electromechanical and/or electromagnetic brakes 23 and 26 may be activated.

Once the motor is stopped semiconductor switch 16 and converters 22 and 25 are disabled, thyristor arrays 19 and 20 are turned off and electromagnetic brake 26 is turned off. The system of Figure 1 with control circuit 5 has been shown to enable a bandsaw to be stopped within 70 ms. Important factors contributing to rapid stopping include: 1 . reduced inertia due to light flywheel and freewheel;

2. use of a capacitor for storing the DC voltage to be injected into the AC supply to avoid tripping and ensure high current injection;

3. faster motor stopping due to semiconductor switch of AC supply to motor; and

4. faster motor stopping due to rapid DC injection braking via semiconductor switches.

Figure 4 depicts a control circuit 5' in accordance with an alternative embodiment of the present invention. The control circuit 5' of Figure 4 includes many of the same circuit components as for the Figure 3 control circuit, as well as a number of additional components that operate to further reduce the time taken to stop the motor 1 . The Figure 4 embodiment uses an SSR with the main motor contactor and overload, to ensure zero crossing start up for the IGBT switches 12. The main contactor and overload are also monitored by the controller, though these connections are not shown on the diagram for clarity.

More particularly, the control circuit 5' includes a phase detection circuit 32. As persons skilled in the art will appreciate, the AC line current by its nature allows for the magnetic flux in each winding to flow in a positive and negative direction. The phase detection circuit 32 advantageously interrogates each of the phases to determine whether it is conducting in either a positive or negative direction. This information is continuously communicated to the controller 28. When a hazard detection signal is received, the controller 28 determines the current flux direction status and injects the DC current into the windings of the motor 1 in a predefined sequence to ensure the fastest re-action of the winding on the rotor. If fired in a non-optimised sequence, the flux may cause the motor to increase speed which may adversely impact on the stopping time. In other words, the phase detector 32 allows the firing polarity to be controlled relative to the phase of the motor supply 1 at the time of firing the DC injection. According to the illustrated embodiment, the detector 32 comprises two current transformers and associated comparator circuits to provide phase timing pulses back to the controller 18. It will be understood by persons skilled in the art that the phase detection circuit 32 need only measure the phase across two of the lines, with the third phase able to be calculated based on the determined phase of the first and second lines. In a particular embodiment the controller 28 implements program code that inspects a look-up table storing optimised DC injection sequences for any given phase state determination. With additional reference to Figure 5a, there is shown test data for various DC injection sequences, in accordance with an embodiment. More particularly, the data in table 500 demonstrates how stopping time varies for two particular phase offsets (with the polarity of the SCRs being changed for each set). The polarity table 502 shows the polarity settings for each SCR for each of the tested sequences (noting that there are six possible combinations for applying the polarity to the motor coils, with only the SCR 19 being listed; the corresponding three SCRS 20 have the opposite polarity). With reference to Figures 5b and 5c, there is shown a table 504 and corresponding chart 506 depicting the stopping time with a fixed SCR sequence, but changing the time in the phase A waveform when the trigger is fired (i.e. serving to explain the effect of phase offset from the monitoring circuit to the trigger injection, whilst keeping the polarity applied to the motor coils constant). From this data it is possible to determine the best possible combination of polarity and phase relationship to minimise the overall stopping time of the motor. The look up table may thus be programmed and evaluated by the controller for any determined phase state (i.e. at the time of receiving the hazard signal). In addition, the control circuit 5' includes a step up transformer 33 allowing the capacitor (in this case a capacitor bank 18') to be charged with a voltage that is greater than the rectified line voltage. According to the illustrated embodiment, the step up transformer allows the capacitor bank 18' to be charged to a DC voltage of at least 900 Volts. The step up transformer 33 is controlled by semiconductor switch 16 to inhibit recharging the capacitor bank until the SCRs are fully off. According to the illustrated embodiment, the capacitor bank 18' provides a total of 12mF i.e. 12,000MF at 1000VDC maximum. Higher or lower values may be used, depending on the desired implementation.

A high voltage monitor 37 measures the high voltage stored in the capacitor bank 18', using a resistor divider network to sample the voltage safely. This is fed into an opto-isolated linear amplifier circuit, and then sampled by an analog to digital converter circuit within the controller 18. The controller utilises this data to ensure that the expected voltage is stored within the capacitor bank 18' and may output a warning if it is not.

The circuit further comprises a monitoring circuit (not shown) on the motor semiconductor switch 16 to eliminate the possibility of latent short circuits on any channel. This circuit is very similar to the HV DC monitor 37, except that the input is first fed into a bridge rectifier before the resistor divider network.

The addition of the phase detection circuit 32 and increased DC voltage stored by the capacitor bank 18' have been found to further reduce the stopping time of the wheels to less than 20 ms. There is thus provided a bandsaw capable of stopping within a very short time from detecting a potentially hazardous event (less than 20ms). The control system may be retrofitted to existing machines making it possible to upgrade existing equipment instead of replacing the whole machine. The solution can be applied to different types of AC motors including both star and delta wound motors and for use with different AC supplies (i.e. having different voltages and frequencies to that described for the illustrated embodiment). The solution also avoids damage to the bandsaw blade or other components from an emergency stop.

While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. For example, the present invention is not limited to use in bandsaws. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the applicant's general inventive concept.