WO2003042078A1

WO2003042078A1 - Conveyor unit with timer controlled delay of speed change

Info

Publication number: WO2003042078A1
Application number: PCT/DK2002/000771
Authority: WO
Inventors: Søren KAPPELGAARD
Original assignee: Crisplant A/S
Priority date: 2001-11-16
Filing date: 2002-11-15
Publication date: 2003-05-22

Abstract

The invention relates to a conveyor unit having drive means for driving an article along the conveyor unit, and having drive control means for controlling a conveyance speed of the drive means. In order provide a conveyor unit, of which the operation is controlled by means that facilitate precise positioning of the articles after a change of conveyance speed of the conveyor unit, a detector (2) is arranged along the conveyor unit for detecting the presence of an article on a given part of the unit and producing an output accordingly. Additionally, a control unit is provided for receiving said output from the detector and comprising means for changing the conveyance speed of the drive means by means of the drive control means. Furthermore, an execution of said change is performed depending on whether a signal is received from an external control unit via a communication network within a predetermined period from the time said output is received from the detector.

Description

CONVEYOR UNIT WITH TIMER CONTROLLED DELAY OF SPEED CHANGE

The present invention relates to a conveyor unit with drive means, such as a conveyor belt, drive cylinders or drive wheels to drive an article to be conveyed by the conveyor unit. The invention further relates to an installation comprising a plurality of conveyor units and to a method of operating a conveyor unit.

BACKGROUND FOR THE INVENTION

In conveyors comprising a plurality of successive conveyor units, it may be desirable to change the conveyance speed of the drive means of the conveyor units when an article is present thereon, in particular to be able to stop the article on one of the conveyor units if an adjacent conveyor unit is not ready to receive the article, e.g. because drive means of the adjacent conveyor unit are stopped. The operation of the conveyor units of the conveyor is normally controlled by an external control unit, such as a PLC unit controlling the operation of most or all of the conveyor units.

Two prior art conveyor control systems are shown in Figs. 1 and 2, of which Fig. 1 shows a system in which the photoelectric cell (PEC) 2' detects an article, in the form of a tote supporting a separate item, when the article passes the PEC 2'. Acceleration or deceleration of the item is started once the PEC 2' has detected the article. The signal from the PEC is sent to a real-time control system via a communication network, and, in the event the article is to be decelerated to a complete stop, a stop signal is immediately returned to the conveyor unit via the communication network. This requires a reliable real- time control system in which a fieldbus time and a scan time of the control system are constant and very small compared to the time it takes the article to be conveyed along the conveyor unit. In the conveyor unit shown in Fig. 2, a PEC 2 is positioned midway between an upstream end and a downstream end of the conveyor unit. Thus, the conveyor unit may be used for conveying articles in two directions. A delay timer in the real-time control system is employed to cause a delay before acceleration or deceleration is in fact started. The delay time is defined by the sum of the fieldbus time and the scan time of the control system and a delay period of the timer. Thus, the delay time may vary if the fieldbus time and/or the scan time are not constant. Typically, the variation of the delay time is in the range of 50 ms.

In known conveyors, a PLC is controlling the conveyor units in real-time in response to a signal from a photoelectric cell comprised in or near the conveyor. Thus, a precise, and preferably short response time from the PLC is crucial for keeping track of the positioning of the article when the conveyance speed is changed, in particular when the article is stopped on a conveyor unit.

As an example, US 5,711,410 discloses a system and a method that detects articles moving along a conveyor and which controls deceleration of the conveyor in order to position the article as desired. A control system accounts for varying distances between articles on the conveyor and varying distances between objects and the desired position by calculating a delay period during which the conveyor is allowed to continue to accelerate before it is decelerated. It will thus be appreciated that the pre-characterising portion of claim 1 is based on US 5,711,410.

US 3,908,815 discloses a speed control for labelling machines for bottles, in which a labeller is operated at a speed such that the bottles are moved through it somewhat faster than they are delivered to it by a preceding machine. In order to compensate for a gap that may develop in the line of bottles moving toward feed screws it may be desirable to slow the labeller down to permit the bottles to catch up. Thus, a control circuit is provided which is responsive to a detector so as to cause speed change means to change from low speed to high speed and vice versa at appropriate times.

However, it has been found that the systems and methods of US 5,711,410 and US

3,908,815 suffer from the shortcoming that it is difficult or impossible to arrive at a precise response time from the controller and thus a precise positioning of the articles due to unforeseeably varying delay times caused by the electronic control circuits.

Thus, one objective of the present invention is to provide a conveyor unit of which the operation is controlled by means that facilitates precise positioning of the articles after a change of conveyance speed of the conveyor unit.

In order to solve the above problem, the present invention provides a conveyor according to the preamble of claim 1 which is improved by the execution of the change of conveyance speed being dependent on whether a signal is received from an external control unit via a communication network within a predetermined period from the time instant at which the output is received from the detector. Thus, there is provided a conveyor unit having drive means, such as an endless belt, drive cylinders or drive wheels, connected to an electric motor, for driving an article along the conveyor unit, drive control means, e.g. comprising a frequency inverter or converter in case the motor is an AC motor, alternatively comprising a variable current supply for a DC motor, for controlling a conveyance speed of the drive means, a detector, such as a photoelectric cell (PEC) or a CCD camera, arranged along the conveyor unit for detecting the presence of an article on a given part of the unit and producing an output accordingly, and a control unit for receiving said output from the detector and comprising means for changing the conveyance speed of the drive means by means of the drive control means, the execution of said change depending on whether a signal is received from an external control unit via a communication network within a predetermined period from the time said output is received from the detector, the period being measured out (or determined) e.g. by means of a timer.

It will thus be appreciated that the control unit of the conveyor unit is provided with a timer that is started when the detector provides a signal indicative of the presence of an article to the external control unit via a communication network. The timer measures out a predetermined period, and the conveyance speed of the conveyor unit is changed depending on whether a signal is received from an external control unit.

Thereby it is obtained that the position after change of conveyance speed, e.g. after the article is stopped, may be determined very precisely regardless of the response time for the external control unit and the transmission time of the communication network.

It is further obtained that the response time from the external control unit, e.g. a PLC, is no longer crucial and may vary from operation to operation and does not have to operate in real-time control.

In a preferred embodiment, the change of the conveyance speed is executed in case a specific signal is received via the communication network within said period. This is useful e.g. in situations where the conveyance speed for a given article is to be changed whereas other articles passing the conveyor unit continue with the same speed, or in situations where the new target speed is communicated to the conveyor unit prior to the change of speed and articles are to pass the conveyor unit before the new target speed is activated.

In an alternative embodiment the change of conveyance speed is executed in case no signal is received via the communication network within said period. Thereby it is obtained that the conveyance speed is changed for all articles unless a contradictory signal is issued and received. In particular, the operation of the conveyor unit may be stopped, if the target speed is zero, which may be the case if operational problems occur in the external control unit or in the network connecting the conveyor units and the external control unit. It is preferred that the detector is positioned at a distance from the exit end of the conveyor unit so that a suitable period for the response from the external control unit is allowed for. In particular, the detector may be arranged for detecting the presence of an article midway between an upstream end and a downstream end of the conveyor unit. Thus, the conveyor unit may be used for conveying articles in both directions.

The period may be predetermined from the conveying speed of the conveyor unit by means of its control unit such that it is ensured that the articles have reached their respective target speeds prior to leaving the unit or, in certain embodiments, to be stopped at a predetermined position on the conveyor unit.

Alternatively, the period may be communicated to the control unit by means of the communication network.

The change of speed may be determined by the control unit of the conveyor unit itself, e.g. from a predetermined target conveyance speed for the conveyor unit if it is dedicated to accelerate or to decelerate articles conveyed thereon. However, the change of speed may be communicated to the control unit by means of the communication network so that a more detailed control of the operation of the entire conveyor may be performed, e.g. via the external control units.

In a particular embodiment, the conveyance speed is changed to zero so that the article is stopped on the conveyor unit.

In a preferred embodiment, the time resolution of the delay is within the range of 0.2-5 ms, such as within the range of 0.8-2 ms, and most preferably 1 ms, the delay being typically in the range of 20-2000 ms.

The invention also relates to an installation for conveying articles along a plurality of conveyor units, comprising at least one conveyor unit as described above. The change from one conveyance speed to another, non-zero conveyance speed may be required to accelerate or decelerate the article, in particular in conveyors which transfer the articles between adjacent conveyor units operating at equal speeds. In the installation, the means for changing the first conveyance speed of the drive means may be operable to control the first conveyance speed so that, at a transition between the conveyor unit and an adjacent conveyor unit, the first conveyance speed is substantially equal to a second conveyance speed of the adjacent conveyor unit. Thus, no relative speed occurs between the two conveyor units, whereby wear on the articles or on totes supporting the articles may be reduced. The present invention further provides a method of operating a conveyor unit having drive means for driving an article along the conveyor unit, drive control means for controlling a conveyance speed of the drive means, a detector arranged along the conveyor unit for detecting the presence of an article on a given part of the unit and producing an output accordingly, and a control unit for receiving said output from the detector and comprising means for changing the conveyance speed of the drive means by means of the drive control means, the method comprising: executing said change when a signal is received from an external control unit (9) via a communication network within a predetermined period from the time said output is received from the detector.

It should be understood that any feature and functionality described above in connection with the conveyor unit and the installation according to the present invention also applies to the method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Two prior art conveyors as well as embodiments of the present inventions are shown in the accompanying figures of which

Fig. 1 is a first prior art conveyor in which the article detector is arranged near the downstream end of the conveyor section and the operation is controlled from a remote, external real time control system,

Fig. 2 is a second prior art conveyor as in Fig. 1, in which the detector is arranged near the middle of the conveyor unit and the remote, external real time control system comprises a timer,

Fig. 3 is an embodiment of the present invention,

Fig. 4 is a more detailed, schematic view of the embodiment of Fig. 3,

Fig. 5 is a first timing diagram showing a first operational mode according to the present invention,

Fig. 6 is a second timing diagram showing a second operational mode according to the present invention, and Fig. 7 is a third timing diagram showing a first operational mode according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figs. 3 and 4 show a conveyor unit according to the present invention, having belts 1 defining a conveying surface for supporting and conveying articles thereon. The common conveyance direction is indicated by the arrows. The conveyance direction is shown opposite in the two figures. A photoelectric cell (PEC) 2 is arranged midway between upstream and the downstream ends of the respective conveyor units, the PECs thereby surveying a line transverse, such as perpendicularly, to the conveyance direction as indicated by the broken line. The position of the PEC 2' in the known art is also indicated in Fig. 4.

The photoelectric cell 2 produces a signal to a conveyor unit controller 4, 5, 6 having a frequency transformer 4 feeding the electric motor 3 that drives the conveyor belt 1, a timer 5 and a communication unit 6 connected by means of a communication line 7 to a data communication network 8 to which a PLC 9 is connected too.

A signal from the photoelectric cell 2 starts the timer 5 and is communicated to the PLC 9 by means of the communication unit 6 and the network 8. The PLC 9 sends, according to one embodiment of the control of the unit, a "go" signal back to the communication unit 6 if the following conveyor unit is ready to receive the article. If the "go" is not received before the timer period has elapsed, the timer will cause the belt 1 to stop. In a preferred embodiment, the timer period is adjusted for the speed of the conveyor belt 1. The adjusted timer period may be communicated from the frequency transformer 4 to the timer 5.

The timing diagrams of Figs. 5-7 show control sequences of correlated signals of the PLC 9 adapted to set and reset a number of input and output channels, such as speed, reference speed, timer and Delay Speed Change (DSC). As indicated, the signals, except from timer and speed, are preferably all digital signal, i.e. they may all have two states, on or off (1,0). However, all signals may be either analogous or digital signals. Different types of PLCs and motors may be used.

A first timing diagram in Fig. 5 shows a first embodiment of an operational mode of the conveyor unit according to the present invention in which the conveyance speed of the conveyor unit is lowered from the ordinary conveyance speed V to zero via a ramp function. A speed change command is received from the external control unit within the delay time measured out by the timer 5 which is started upon input from the PEC 2. The delay timer 5 and the PEC 2 input are both reset after the timer expires so that the control unit 4, 5, 6 of the conveyor unit is ready for a new operational control cycle.

A second timing diagram in Fig. 6 shows a second embodiment of an operational mode of the conveyor unit according to the present invention. A command is set from the external control unit to freeze the present conveyance speed at a as first reference speed Vi for the frequency converter 3. A new reference speed V₂ can now be set, cf. position b, for the motor. The speed is not automatically changed as a consequence of the change of the reference speed, but remains unchanged until the timer 5 expires, cf. position d. When the Delay Speed Change (DSC) input is reset at c, i.e. when the PEC 2 input is received, the DSC Timer 5 of the conveyor unit is started and measures out the predetermined period DSC time. The DSC Timer 5 expires at d and the frequency converter 3 starts ramping down the speed \f_± towards the new reference speed V₂ which optionally may be zero. The speed change is activated because no contradictory control signal is received from the external control unit 9 within the DSC time period. Alternatively, the conveyor unit may be controlled so that the speed change is only effectuated if a control signal is received from the external control unit 9 within the DSC time period. To suppress noise the DSC timer 5 is started on the negative flank of the DSC input, cf. positions c, g, and is reset after five consecutive input scans where the input has been set, cf. positions f, g.

The timing diagram of Fig. 7 shows that, when the DSC is enabled, cf. position a, the reference speed is frozen until the timer expires, cf. position d, and a new speed reference can be passed to the inverter, cf. position b, without affecting the current speed target. If the DSC Enable is not set, the entire DSC function and the DSC timer is reset. Only when a negative flank on the DSC input is detected c, the DSC timer starts timing, and when the timer expires, cf. position d, the inverter begins ramping down the speed Vi towards the new reference speed V₂ (optional 0 Hz for stop). To suppress noise the DSC timer is started on the negative flank of the DSC input, cf. position g, and is reset after five consecutive input scans where the input has been set, cf. positions f, e and h, i, e.g. the DSC timer expires, cf. position e, when the DSC is reset, cf. position f. If the DSC input detected, cf. position h, is a short impulse, the DSC timer is reset, cf. positions g, h, before it expires. Preferably, the time resolution of the DSC delay is within the range of 0.2-5 ms, such as within the range of 0.8-2 ms, and most preferably 1 ms, the DSC delay being typically in the range 20-2000 ms. A stop command for the motor overrules the above functionality and causes an immediate stop.

Claims

1. A conveyor unit having drive means for driving an article along the conveyor unit, drive control means for controlling a conveyance speed of the drive means, a detector (2) arranged along the conveyor unit for detecting the presence of an article on a given part of the unit and producing an output accordingly, and a control unit (4, 5, 6) for receiving said output from the detector and comprising means for changing the conveyance speed of the drive means by means of the drive control means, characterised in that the execution of said change depending on whether a signal is received from an external control unit (9) via a communication network (7, 8) within a predetermined period from the time said output is received from the detector (2).

2. A conveyor unit according to claim 1, wherein said change is executed in case a specific signal is received via the communication network within said period.

3. A conveyor unit according to claim 1, wherein said change is executed in case no signal is received via the communication network within said period.

4. A conveyor unit according to any of the previous claims, wherein the detector (2) is arranged for detecting the presence of an article midway between an upstream end and a downstream end of the conveyor unit.

5. A conveyor unit according to any of the preceding claims, wherein the period is predetermined from the conveying speed of the conveyor unit.

6. A conveyor unit according to any of claims 1-4, wherein the period is communicated to the control unit by means of the communication network.

7. A conveyor unit according to any of the preceding claims, wherein the change of speed is communicated to the control unit by means of the communication network.

8. A conveyor unit according to any claims 1-6, wherein the conveyance speed is changed to zero so that the article is stopped on the conveyor unit.

9. An installation for conveying articles along a plurality of conveyor units, comprising at least one conveyor unit having drive means for driving an article along the conveyor unit, drive control means for controlling a first conveyance speed of the drive means, a detector (2) arranged along the conveyor unit for detecting the presence of an article on a given part of the unit and producing an output accordingly, and a control unit (4, 5, 6) for receiving said output from the detector and comprising means for changing the first conveyance speed of the drive means by means of the drive 5 control means, the execution of said change depending on whether a signal is received from an external control unit (9) via a communication network (7, 8) within a predetermined period from the time said output is received from the detector (2).

10. An installation according to claim 9, wherein the means for changing the first 10 conveyance speed of the drive means is operable to control the first conveyance speed so that, at a transition between the conveyor unit and an adjacent conveyor unit, the first conveyance speed is substantially equal to a second conveyance speed of the adjacent conveyor unit.

15 11. A method of operating a conveyor unit having drive means for driving an article along the conveyor unit, drive control means for controlling a conveyance speed of the drive means, a detector (2) arranged along the conveyor unit for detecting the presence of an article on a given part of the unit and producing an output accordingly, and 20 a control unit (4, 5, 6) for receiving said output from the detector and comprising means for changing the conveyance speed of the drive means by means of the drive control means, the method comprising: executing said change when a signal is received from an external control unit (9) via a communication network (7, 8) within a predetermined period from the time said 25 output is received from the detector (2).