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Hewlett-Packard improves the performance of new inkjet handling systems using WITNESS simulation software


HP recently developed a new flexible assembly concept in which standard components can be quickly reconfigured to handle new products and volume changes. In the concept’s early phases, HP encountered problems specifying modules because the system complexity made it impossible to evaluate its performance using conventional linear calculations. Engineers turned to WITNESS to determine the performance required for each module to meet requirements. The ability to evaluate various scenarios before tooling was ordered made it possible to build a better system that could be more easily adapted to future demands.

HP is a leading provider of computing, Internet and intranet solutions, communications products and measurement solutions. HP had revenue of $48.7 billion in 2000. HP’s two-million square foot plant located in Corvallis, Oregon produces inkjet printers among other products. As the popularity of inkjet printers has skyrocketed in recent years, HP engineers have scrambled to develop new methods to increase their capacity for producing supplies. This challenge has been complicated by the short development cycle in the business, which makes it necessary to adapt existing equipment to handle new products. To overcome these challenges, HP engineers have developed a standard tooling system for building inkjet cartridges based on modular subsystems that can be adjusted to handle different models of cartridges and can be added or subtracted to increase or reduce production capacity for a line.

Multi-lane highways

Multi-lane conveyor highways are a critical part of the standard tooling concept. Multilane highways provide the flexibility to use varying numbers of assembly modules whose cycle time can vary by substantial amounts. For example, a three-lane highway might be serviced by three modules to assemble the lid and body, two modules to attach the flex circuit and one module to fill the cartridge with ink. Modules typically perform multiple synchronous operations. Each module can be outfitted with different end effectors and reprogrammed to handle different cartridge models. This approach makes it possible to increase or reduce the number of certain modules to change production capacity of a particular line—without modification to the handling system. If production needs to be increased, modules can be added on an individual basis. Likewise, if production needs to be reduced, modules can be removed and redeployed on other lines.

Carts of subassemblies are delivered to the line on automatic guided vehicles. Tray handlers are used to insert move carts of subassemblies onto the highway and remove them from the highway. Raw materials enter the highway at various locations. Single, dual and quadruple pick and place units move individual subassemblies between the multilane highway and the individual modules. This approach provides the flexibility to route subassemblies so that every subassembly sees module A, for example, while only one out of three subassemblies sees module B1; the others are directed to either module B2 or B3. In-process inspection is incorporated to avoid putting additional processing costs into parts that don’t meet HP’s demanding requirements. Gates are included to pass reject parts in order to avoid having grippers pick up an empty slot.

Design challenge

The design of one of these systems is an extremely challenging task. First of all, the production rate of the various modules needs to be balanced. Second, future requirements need to be considered. Sufficient space is usually left between modules to accommodate projected production increases while still leaving room for maintenance access. In addition, engineers must carefully evaluate handling capacity, both in regards to current production volume and also for projected increases. The space between the modules affects the buffer capacity between various stations. Changing the production rate also impacts the expected number of rejected parts, which affects the gating. Specifying all these aspects both to meet current production needs as well as future scenarios while minimising the cost of the system is an enormously complex task.

Capacity planning for conventional assembly systems has been spreadsheet based. This forced users to make many simplifying assumptions, such as the linearity of workflow, that don’t hold up in the real world. Using this approach, it would be very difficult to determine the specifications for individual assembly or handling modules. The only accurate method of evaluating the performance of the individual modules in relation to the overall system would be to experiment with different equipment configurations and control programs after the system had been built. This is a very time consuming process that typically requires five or six people to stand around for days watching different areas of the line looking for problems. When the inevitable problems are discovered, engineers are faced with the choice of either performing expensive modifications or settling for sub-optimal performance.

Discrete event simulation

To avoid these problems, HP engineers decided to use simulation to model the dynamics of time-based material flow prior to specifying the modules and handling equipment for the new lines. HP engineers evaluated a variety of simulation tools and settled on WITNESS.

HP engineers created a library of equipment templates that greatly simplify the creation of a simulation model. Templates define the components of inkjet assembly systems including assembly modules, multilane highways, pick and place units, reject gates, etc. A template consists of the definition of the equipment, operating characteristics such as speed and defect rate, downtime characteristics such as mean time to failure and mean time to repair, how much time is required for start up, and how many people are required to operate it. When an engineer wants to simulate a particular function within a plant, he or she simply pulls the appropriate equipment templates from the library and links them together graphically to create a simulation model. Templates can be easily changed. If the purpose of a simulation is to observe the effect of a faster module, for instance, the operating characteristics of the module can be increased to simulate the new device.


HP engineers have modelled several of the new standardised assembly systems. The model makes it possible for the engineers to graphically view the performance of a given configuration system in a minimal amount of time. This process highlights problem areas and is used in discussions with design engineering and management to demonstrate and evaluate these problems. The models are also used to graphically view the impact of changes in system configuration to determine its ability to be upgraded to meet various production increases or new product scenarios. The result in every case has been design changes that substantially improve the performance of the system. In addition, engineering knows exactly what will be required to adapt the system for future needs. This confidence in many cases reduces the cost of the system by eliminating the need to overdesign the system to allow for uncertainty.

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