Abstract
An adaptive zero pressure accumulation conveyor system is disclosed, designed to manage articles of varying lengths efficiently. The system dynamically adjusts accumulation zone boundaries based on real-time article length measurements, preventing jams and optimizing conveyor density. Each zone incorporates sensors and a local controller to detect article presence and length. A central controller coordinates zone release logic, especially for articles spanning multiple physical zones. The arrangement addresses the problem of fixed-length accumulation zones which can lead to inefficiencies or blockages when article dimensions fluctuate significantly during operation.
1. Technical Field
[0001]This disclosure relates to material handling systems, specifically to roller conveyors employing zero pressure accumulation (ZPA) principles for transporting and buffering discrete articles.
2. Technical Context
[0002]Conventional zero pressure accumulation conveyor systems divide the conveyor path into fixed-length zones. Each zone is typically sized to accommodate a predetermined maximum article length. When articles of varying lengths are introduced into such a system, several inefficiencies can arise. Short articles may occupy an entire zone, leading to suboptimal density and reduced throughput. Conversely, articles longer than a single zone require special handling or may cause jamming conditions if not properly managed, often necessitating manual intervention or system stoppage. The problem addressed is the inefficiency and potential for operational disruption caused by fixed-length accumulation zones when the length of articles conveyed varies throughout operation, leading to either lost accumulation density or physical interference between articles or zones.
3. System Overview
[0003]The disclosed system provides an adaptive zero pressure accumulation conveyor that dynamically adjusts its effective accumulation zone lengths based on the actual length of articles being conveyed. This arrangement ensures that articles are accumulated without contact, regardless of their individual lengths, while maximizing conveyor density. The system comprises a series of independently controllable conveyor zones, each equipped with sensors for article detection and length measurement, and a local zone controller. A main conveyor controller coordinates the operation of these individual zone controllers, particularly for managing articles that span multiple physical zones. The system implements specific logic for zone release, handling of articles longer than a single physical zone, precise sensor arrangements at zone boundaries, and a robust recovery protocol following an emergency stop event.
4. Components
[0004]The system utilizes several key components, as depicted in FIG. 1 and FIG. 2.
[0005]
- Conveyor Frame (10): The structural support for all conveyor components, typically constructed from extruded aluminum profiles or fabricated steel sections.
- Drive Roller (12): A powered roller that propels articles along the conveyor. Each zone may have one or more drive rollers. These are often driven by a motor within the roller itself (motorized drive roller) or via a belt/chain from a separate drive unit.
- Idler Roller (14): Unpowered rollers that support articles between drive rollers, allowing them to move freely.
- Zone Drive Unit (16): A mechanism responsible for driving the rollers within a specific accumulation zone. This can be an individual motor-driven roller, a motor driving a series of rollers via belts, or a pneumatic/electric clutch-brake system controlling power transmission to a section of rollers. Each zone drive unit (16) can be independently activated or deactivated.
- Article Presence Sensor (20): Typically a photoelectric sensor or an array of sensors positioned at the entry and exit points of each physical accumulation zone. These sensors detect the leading and trailing edges of an article (A) as it passes, indicating its presence within or passage through a zone. A typical sensor (20) might be a retro-reflective type with a range of 0.1 to 2 meters.
- Article Length Sensor (22): A device or arrangement of sensors used to determine the length of an article (A). This can be a pair of discrete photoelectric sensors (20) placed at a known separation, with timing used to calculate length, or a linear array sensor providing a direct measurement. For example, two sensors (20) separated by 50 mm can provide length data when combined with conveyor speed.
- Zone Controller (30): A micro-controller or programmable logic controller (PLC) assigned to each individual physical accumulation zone. It receives input from the article presence sensors (20) and article length sensors (22) within its zone, controls its associated zone drive unit (16), and communicates with adjacent zone controllers (30) and the main conveyor controller (32).
- Main Conveyor Controller (32): A central PLC or industrial computer that oversees the operation of all zone controllers (30). It manages overall system logic, coordinates zone release, handles article tracking across multiple zones, and processes emergency stop commands from an emergency stop button (34). It typically communicates over an industrial network such as EtherNet/IP or PROFINET.
- Emergency Stop Button (34): A safety device that, when activated, signals the main conveyor controller (32) to immediately halt all conveyor motion.
- Accumulation Zone (Z1, Z2, Zn): A physical section of the conveyor, defined by its own zone drive unit (16) and associated sensors (20, 22), capable of independently conveying or stopping articles.
5. Operation
5.1. Start-up
[0006]Upon system initialization, all zone drive units (16) are deactivated, and all zone controllers (30) report their status as empty and ready to receive. The main conveyor controller (32) initializes its article tracking map. The first zone (Z1) is enabled to receive an article.
5.2. Steady Running
[0007]
- Article Entry and Length Measurement: An article (A) enters the first accumulation zone (Z1). The article presence sensor (20) at the entry of Z1 detects the leading edge. As the article (A) proceeds, the article length sensor (22) within Z1 measures its length. This measurement is transmitted to the zone controller (30) for Z1.
- Zone Status Determination: The zone controller (30) for Z1, upon receiving the article length, determines the required number of physical zones the article (A) will occupy. A physical zone typically has a predetermined nominal length, for instance, 500 mm. If an article is measured at 800 mm, it will require two nominal zones.
- Accumulation Logic:
- Single-Zone Article: If the article's length is less than or equal to the nominal length of a single physical zone, the zone controller (30) for Z1 behaves as a standard ZPA controller. It checks the downstream zone (Z2). If Z2 is occupied or reserved, Z1's drive unit (16) deactivates, stopping the article (A) at the entry of Z1. If Z2 is clear, Z1's drive unit (16) remains active until the article (A) clears Z1 and enters Z2, then Z1's drive unit (16) deactivates.
- Multi-Zone Article: If the article's length exceeds the nominal length of a single physical zone, the zone controller (30) for Z1 communicates with the main conveyor controller (32). The main controller (32) then requests reservation of the necessary number of downstream physical zones. For an 800 mm article in 500 mm zones, zones Z1 and Z2 would be reserved. All reserved zones are treated as a single logical accumulation unit for that article (A). The main controller (32) ensures that the first available zone after the reserved logical unit is clear before allowing the article (A) to advance. All physical zones comprising the logical unit (e.g., Z1 and Z2) must remain active until the article (A) has entirely exited the last physical zone of its logical unit (e.g., Z2).
- Zone Release Logic: An article (A) is permitted to advance from its current zone (or logical multi-zone unit) to the next if and only if the entire downstream space required for the article (A) is completely clear and not reserved by another article. For a multi-zone article (A) occupying Z1 and Z2, it will only advance when Z3 is entirely clear.
- Tracking: The main conveyor controller (32) maintains a map of all articles on the conveyor, their measured lengths, and their current physical zone occupancy or reservation status.
5.3. Fault or Interruption (Emergency Stop Recovery)
[0008]When the emergency stop button (34) is activated, the main conveyor controller (32) immediately deactivates all zone drive units (16). All articles halt in their current positions, potentially straddling zone boundaries. For recovery:
[0009]
- System State Snapshot: The main conveyor controller (32) records the exact state of all article presence sensors (20) and any partial article length readings from article length sensors (22).
- Boundary Re-evaluation: For any article (A) detected by sensors (20) to be straddling two physical zones, the main controller (32) re-evaluates its logical zone assignment. It confirms which physical zones are occupied by which article.
- Restart Sequence: To resume operation, the main controller (32) performs a controlled restart. It may briefly activate zone drive units (16) in a specific sequence to clear articles from straddled states into a single physical zone where possible, or confirm their multi-zone occupation. For example, if an article (A) is half in Z1 and half in Z2, Z2's drive unit (16) might be briefly activated to pull the article (A) fully into Z2, then Z1 cleared. Alternatively, the system may simply recognize the straddled state as the new operational condition and resume accumulation logic from there, ensuring the article (A) is tracked as occupying both Z1 and Z2 until it clears Z2.
6. Example Embodiments
[0010]Embodiment 1 In this embodiment, each physical accumulation zone (Z_n) has a single article presence sensor (20) at its entry point and a pair of article length sensors (22) located near the zone entry. The article length sensors (22) are fixed at a known separation, for example, 100 mm. The zone controller (30) measures the time difference between the activation of the first and second length sensor (22) by the article's leading edge. Combined with a calibrated conveyor speed, this time difference allows for the calculation of the article's effective length. Zone communication occurs via a high-speed serial bus, and article tracking is managed by the main conveyor controller (32).
[0011]Embodiment 2 This embodiment utilizes an array of diffuse photoelectric sensors (20) spanning the width of the conveyor at the entry to each physical zone. This array provides not only presence detection but also approximate length and width information. Article length is determined by counting activated sensors in the direction of travel over time, synchronized with the zone drive unit's (16) encoder feedback for precise distance measurement. For articles longer than a physical zone, the system employs a 'look-ahead' protocol where the zone controller (30) requests reservation of downstream zones before the article (A) even fully enters the current zone, optimizing flow.
[0012]Embodiment 3 This embodiment integrates an industrial vision system (not shown) at an upstream entry point of the conveyor system. This vision system captures images of articles and calculates their length, width, and height before they reach the accumulation zones. This pre-measured data is then transmitted to the main conveyor controller (32) and associated with a unique identifier for each article. The zone controllers (30) then retrieve this pre-measured length information as articles enter their respective zones, eliminating the need for dedicated article length sensors (22) within each accumulation zone. Article presence sensors (20) are still used for tracking article movement between zones.
7. Alternative Configurations
[0013]
- Distributed Control: Instead of a central main conveyor controller (32), a fully distributed control architecture could be implemented where zone controllers (30) communicate peer-to-peer to negotiate zone reservations and releases. A master zone controller could assume coordinator duties dynamically.
- Variable Zone Drive: Instead of simple on/off control for zone drive units (16), variable speed drives could be employed to allow for gentle article acceleration and deceleration, or to create deliberate gaps between articles for specific downstream processes.
- Sensor Redundancy: Redundant article presence sensors (20) could be implemented at critical zone boundaries to improve reliability and provide fault tolerance in sensor failure scenarios.
8. Parameter Variations
| Parameter | Range | Effect of Increase | Effect of Decrease |
|---|---|---|---|
| Physical Zone Length | 0.3 m to 2.0 m | Accommodates longer single articles, fewer zone divisions. | Better granularity for short articles, more zone controllers. |
| Conveyor Speed | 0.1 m/s to 2.0 m/s | Higher throughput, requires faster sensor response and controller processing. | Lower throughput, potentially more precise length measurement. |
| Article Length Measurement Accuracy | ±1 mm to ±10 mm | More precise zone allocation, reduced gaps, higher density. | Larger safety gaps, potential for less efficient accumulation. |
| Sensor Response Time | 0.5 ms to 20 ms | Supports higher conveyor speeds and more accurate edge detection. | Limits maximum conveyor speed, less precise edge detection. |
| Number of Physical Zones | 5 to 100+ | Greater accumulation capacity, more complex coordination. | Less accumulation capacity, simpler control architecture. |
9. Additional Implementations
[0015]
- Article Pitch Control: The system can be extended to maintain a specific pitch (gap) between articles. After an article's length is determined, the system can calculate the precise point where the next article should stop to maintain a minimum desired gap, dynamically adjusting the stop position at the zone entry.
- Weight-Based Accumulation: Integration of load cells within zones could allow for accumulation decisions based not only on length but also on article weight, useful for preventing overloading downstream equipment or for sorting operations.
- Dynamic Merging/Diverting: The adaptive zone control logic can be integrated with merging or diverting mechanisms. By knowing the exact length and position of articles, the system can precisely create gaps or reserve paths to facilitate smooth merging from multiple lines or diversion to different destinations, minimizing interference and maximizing flow.
[0016]FIG. 1 shows a block diagram of the adaptive zero pressure accumulation conveyor system.
[0017]FIG. 2 shows an elevation view of two adjacent accumulation zones with sensors.
10. Figures
