Memorial Hermann Southwest Hospital, one of the largest and most sophisticated suburban hospitals in Houston, Texas is achieving remarkable savings in its operational costs projected at Euros 8.5 million over a 15-year period - following a switch to Swisslogs Telelift Transcar laser-guided AGV system powered by Saft rechargeable nickel-cadmium (Ni-Cd) battery modules. The fast-charging Saft STM battery modules enable the new hospital AGVs to take advantage of opportunity charging to ensure they are always maintained at a high SOC (state of charge), to provide maximum availability. This significantly increased availability of the Telelift TransCar AGVs, combined with a higher load capacity, has enabled the hospital to cut its AGV fleet from over 60 vehicles to just 16. Memorial Hermann Southwest
Built in 1977, Memorial Hermann Southwest is a 485-bed tertiary care facility offering inpatient, outpatient and obstetric services. Previously, it employed a 26-year-old automated guided vehicle (AGV) system that required 60-plus vehicles to deliver materials. The old system was inefficient at coordinating the vehicles, which also had to be manually loaded with the carts. The system employed a wire in the floor to guide vehicles, a physical feature which reduced the expansion flexibility potential of the system. This system was becoming too expensive and difficult to maintain. The alternatives were to hire people to manually push the carts or to purchase a fully automated system.
New AGV system
Memorial Hermann Southwest selected Swisslogs Telelift TransCar AGV system. This is a robotic cart transport system featuring a contour-following laser guidance system.
Each vehicle contains a microprocessor that stores a virtual map of the hospital facility to ensure accurate navigation. This is combined with dual range laser obstacle detection at
both ends of the vehicle to ensure safety and eliminate potential for facility damage. An added benefit of the contour-following laser guidance system is that, unlike other AGV systems, it requires no wall targets, embedded wires, floor tape or other building modifications.
The Memorial Hermann system replaces more than 60 delivery vehicles from the previous system with just 16 vehicles capable of moving tonnes of materials in carts including: refuse, clean and soiled linen, medical and surgical supplies, bio-hazard waste, patient meals and nourishment.
The AGVs transport carts horizontally through a 150 metre tunnel connecting the energy building and vertically via elevators to the hospitals 10-floor patient tower. In the tower, materials are delivered in a clean holding room located on one side of the elevator. Returning carts are picked up from the soiled holding room on the opposite side. The energy building is located across the street from the patient tower and has loading docks situated out of the sight of patients. This building houses the kitchen, laundry, receiving dock and a warehouse. The remote dock at this facility increases flexibility for future expansion of the facility, which recently added a new radiology and surgery pavilion, and is planning a new heart and vascular institute.
The TransCar Management system (TCMS) features a central computer that directs all AGV movement, tracking and diagnostics, and collects time and location data on all material movements. It dispatches the most opportune AGV for pending cart movements maximizing travel efficiency. The TCMS also monitors AGV battery levels and routes them to automatic charging stations when required.
Ni-Cd battery modules
The Saft batteries provide power for the TransCar traction and the guidance systems, as well as for the automatic lifting platform that raises the hospital carts off the floor with a
maximum lift capacity of 453 kg.
The lead-acid batteries often specified for AGVs can not be fast charged, and in this type of application would typically have to be replaced with a freshly charged battery every eight hours. This involves significant additional costs in spare batteries and the labour required to change them, as well as the need to take AGVs out of service. By using Ni- Cd batteries capable of deep cycling, combined with opportunity charging, Swisslog was able to maximize the availability of Memorial Hermann Southwests AGVs.
Charging routine
The AGV charging system consists of a wall-mounted power supply panel connected to charge rails embedded in the floor. Charging is carried out on an opportunity basis, per vehicle as directed by the TCMS. A safe feedback communications link requires a vehicle to be correctly positioned on the rails before the charging power can be applied. The battery discharge rate depends on the weight of the load carried by the AGV and the distance travelled. To ensure maximum availability, the aim is to maintain the battery system at an SOC (state of charge) between 50 and 80 per cent. This means that when not in service, the AGVs are directed to charge as often as possible, even if only for a few minutes. The 100 A charge current will recharge around one third of the capacity of the Saft STM battery modules in under 20 minutes.
Saft STM modules
The Saft STM battery modules installed in the Memorial Hermann Southwest AGVs have been developed specifically to meet the needs of electric vehicles for energy density (for range), power (for acceleration) and low maintenance, they feature sintered positive electrodes and plastic-bonded negative electrodes. Each AGV is equipped with four Saft STM 5-100 modules in series to create a 24 V, 100 Ah battery system. In this application they are expected to have a minimum service life of five years.
Operational results
Because the vehicles are in continuous communication with the central computer, and are able to use opportunity charging, a smaller fleet of TransCar vehicles (16 compared with 60) now do the heavy delivery work. The new TransCar vehicles also carry a heavier payload, allowing more materials to be carried per journey. Wear and tear on the building walls and elevator doors is reduced due to the new automatic interfaces. Most importantly, the new TransCar system is in operation for 18 to 20 hours a day, 7 days a week. Allowing for the labour cost of manual materials handling that would otherwise be required, Memorial Hermann Southwest anticipates that the new AGVs will pay for themselves in just over two and a half years. Over a 15-year period, the projected savings are over Euros 8.5 million, even including maintenance and battery replacement costs.
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