Rapid Transit

Sep 01, 2001 9:30 PM  By

These speedy cost-savers may just be the ticket to race through picking tasks

PTL units cost about $200 per light position, and are most often used for fast-moving items

Although carousel picking is highly productive, adding more pickers can’t improve throughput

While you’re tinkering under the hood of your operations engine, consider this: In distribution centers, picking can represent 30%-40% of the total labor cost of fulfillment. It’s obvious that increasing picking speeds to Formula One levels can save time and money. But just what is a winning picking speed? How much cost can be justified?

Green light

Most direct-to-customer distribution centers are seasonal, with the bulk of the product demand occurring over a three- to four-month period rather than throughout the year. The picking system needs to run at 200 mph during peak season, but must also take the turns smoothly and efficiently. It needs lots of torque to accelerate rapidly from the slow-volume times to the 200 mph rush.

Distribution centers often incorporate several types of picking systems to handle items of different sizes and velocities. Picking zones may utilize different merchandise storage media as well as picking methodologies and technologies to provide optimal performance for each type of product.

Warehouse employees may pick individual items that need to be packed into a shipping container or full cases. Full-case picking includes boxes of more than one unit as well as single items that are already packaged in containers that can be shipped.

PIECE PICKING CASE PICKING
Construct or select container Identify first picking location
Apply shipping label Travel to the location
Identify first picking location Verify the location and/or SKU
Travel to the location Pick case
Verify the location and/or SKU Apply shipping label
Count items required Pick/label additional cases from the location
Place in shipping container
Confirm pick is complete Confirm pick is complete
Identify next action (complete, pass, resume) Identify next picking location

In a warehouse that uses paper pick tickets, about 80% of a picker’s time is spent reading the ticket, traveling to the location, and then confirming the pick. These areas represent the greatest opportunities for additional speed. You must consider the following points when you evaluate possible technologies and methodologies:

  • applicable types of product, orders, and storage media;
  • problems that may be introduced by the new technology;
  • equipment requirements;
  • software systems requirements;
  • ease of training new associates;
  • cost of additional technology; and
  • potential for labor savings.

Time trials

Several types of technologies are available for reducing the time required to read pick tickets and confirm picking operations. Of these, three are applicable for piece picking: radio frequency terminals, pick-to-light, and voice-directed picking.

Radio frequency terminals speed up picking because they present a worker with one item at a time in the best path sequence, eliminating the need to read the entire ticket to look for the next location. RF terminals enhance accuracy because they allow a picker to see only the information necessary for the current pick — no more picking the quantity for a different line item. Training time is quicker because the system tells the worker what to do and in what order.

A software system, usually a warehouse management system, is required to drive the RF terminals. Because the user sees only one pick at a time, he does not have the opportunity to rearrange the picking order, so the system must be tested to ensure that the picks are organized in the best sequence.

After the initial WMS investment, the costs for adding RF picking include handheld RF terminals (about $4,000 each) and the RF network (site survey, installation, antennas/access points, etc.). One terminal will be required for each picker on a shift. Pickers on different shifts can share equipment as long as sufficient batteries are available. Extra terminals will also be required for supervisors and to serve as spares. RF network costs vary depending on the size and configuration of the warehouse.

Handheld RF terminals may not be the best option for fast-pick areas, because removing a terminal from its holster and then returning it costs precious seconds. In storage media such as carton flow rack where the pick density is high, the cost of using a handheld terminal can add up quickly. Wearable, hands-free RF terminals present a slightly more expensive alternative. Wearable terminals cost about 10% more than handheld devices.

In the case-pick areas, workers can use shipping labels for direction rather than paper pick lists. The WMS must be set up to print the picking location, SKU, and description on the labels and organize them in pick path order. An in-line scanner on the conveyor system can then complete pick confirmation, or the cases can be scanned as they are palletized in the staging area. Both the shipping label and the product label are scanned and matched to verify that the proper item was selected.

Pick-to-light technology (PTL) uses displays at each location that light up to show where picks are required. The picker goes to the location, selects the quantity shown on the display, and pushes a button to confirm the pick. Since the picker does not need to read anything and can see the lit location on the racks, picking efficiency can be increased by 50% or more over paper pick lists and RF terminals. PTL units cost about $200 per light position, and are most often used for the fastest-moving items. You may pay additional prices for software, interfaces, a PTL computer, and scanners.

PTL systems can either light all locations in one area at the same time or step the picker through the picking locations. Systems that illuminate all of a zone’s picks at once are usually more productive because workers can optimize their work within the zone. A further advantage is that another picker can assist when the assigned picker cannot keep up with the workload in his area.

Seasonal operations paticularly benefit from PTL systems because the PTL modules are location-based, rather than user-based, so no additional gear is needed to equip temporary workers. Employees can become productive quickly because PTL requires little training, and because it requires little knowledge of English, even workers with limited language skills can use it.

Voice-directed picking has many of the same advantages as PTL — both are hands-free, can increase picking speed by 50% or more, are easy to learn, and improve picking accuracy. Workers using voice direction wear a small computer and headset as they pick. The computer stores the pick list, which is typically downloaded from the WMS, then tells the picker the location and quantity for each pick. Voice-directed picking is sometimes called an “eyes-free” technology because the user does not need to read pick list information. He listens to spoken instructions and speaks the confirmation. Because voice technology is most efficient when short instructions are used, pickers confirm that they are at the correct location by reading a short, random code displayed at the location.

Before picking for the first time, each user must train the voice system to recognize his voice. This exercise usually takes 15-30 minutes and also helps train the picker to use the system. Voice systems are often multi-lingual, so a picker can hear commands and respond to them in his or her own language. To implement a voice system, the warehouse will need to be equipped with an RF network, which includes a site survey, antennas, access points, and installation. The terminals cost about $7,000 each, including the headsets, batteries, and charger. There may be additional costs for software and interfaces.

Pick stop

In areas where bins and racks are used, particular care must be taken to position fast-moving items to minimize picker travel time. The goal is to place the highest-velocity items closest to the exit area (conveyor or staging area) to minimize walking. However, picks must be spread out enough so that accessing the items does not create a traffic jam for the pickers. For example, the ten fastest-moving items must be distributed among the end-cap units rather than placed in one unit. The profile of this area must be monitored regularly to ensure that the fastest-velocity items are placed there and that congestion does not occur.

Carton flow racks offer several advantages over static storage media. Flow racks provide many pick faces in a small area, like bins, to minimize picker travel time, but also have the capacity to hold larger quantities of each item so you do not run out of fast-moving SKUs. Since replenishers are operating in a different aisle from pickers, bin locations can be refilled effectively during the picking shift. As with bins, care must be taken in profiling the area to minimize congestion during picking. The fastest-moving items should be placed in the easiest location to pick — the “golden zone,” or the shelves in the area between the picker’s knees and shoulders.

Picking from flow racks is typically directed by paper pick lists, wearable RF terminals, or pick-to-light modules. Because hands-free picking is important, voice-directed picking is also an option.

Horizontal carousels can deliver picking locations to the picker at turbocharged speed. Carousels should be grouped in pods of two to three units to minimize the amount of time the picker must wait for a carousel to turn to the proper location. Profile the fastest-moving items so that they are distributed suitably among the carousels in the pod. Carousel systems are most productive when pick lists are downloaded from the WMS to the carousel system and organized so that multiple orders are picked at one time. Carousels are usually equipped with pick lights that display the picking location and the total quantity required for each batch of orders. Put-to-light modules are used to show the quantity to be placed in each shipping container.

Be careful when analyzing whether carousels are applicable to your facility. Although carousel picking is highly productive — 200 picks or more per hour — the throughput of the units can’t be improved by adding more pickers. With bins and flow racks, you can add labor to meet seasonal demands, but adding carousel pickers does not get more product picked.

You must also evaluate replenishment requirements. Since carousels cannot be picked and refilled at the same time, locations must be sized to hold several days’ peak demand of each item, because picking productivity will be zero if there is no product available to pick.

Both automated storage and retrieval systems (AS/RS) and dispensers (such as A-frames) actually pick themselves. In AS/RS systems, a shuttle travels to the picking location to retrieve the items and deliver them to a station. Dispensers drop the product into containers or onto a conveyor belt for delivery to packing or staging areas. Both of these automated systems provide extremely high picking speeds, but they are expensive. They are typically used for only the fastest-moving items, and for products with similar size characteristics.

Checkered flag

Two other ways to reduce travel are to split up picking tasks into geographic zones or to pick multiple orders in one trip. These two methods may also be combined for a zone-batch-pick procedure. Order picking means that one picker picks one order completely before moving on to the next. This method may not be very speedy, depending on the warehouse layout and the number of items per order.

Zone picking assigns each worker to work in a specific area, or zone, of the warehouse. Workers pick all items for an order in their assigned zone, then pass the order (container, pick list, and so forth) on to the next zone. Splitting up the warehouse into smaller areas results in greater speed for each picker. Paper pick lists, RF, PTL, or voice may be used in either an order-pick or a zone-pick facility.

The “bucket brigade” is an enhancement of zone picking in which pickers are assigned to a position in an area, rather than to a zone. Unlike zone picking, in which the first picker sets the pace, in bucket brigades, the last picker sets the pace. When he is done picking a container, he walks back to the previous picker and takes the responsibility for completing it. The second-to-last picker then goes back to his predecessor’s location and takes over picking for him. Bucket brigades work best when the pick density is high in an area — each container requires many picks. The system is self-regulating in that pickers automatically pace themselves to the person upstream. If a picker doesn’t keep the pace, he ends up with no work to do.

Batch picking means that the picker is handling more than one order for each trip through the warehouse or his zone. It is most productive when the warehouse management system organizes the batches so that the same items are contained in many of the orders in the batch. To maintain accuracy, batch picking must be directed by RF terminals, pick-to-light, or voice.

A batch of containers can be placed on a picking cart for transport through the area, or it may be pushed along a conveyor side lane. When batch picking is combined with zone picking, the entire batch of orders may be passed from zone to zone, or a new batch can be assembled in the new zone.

Pamela Davey is a senior consultant at Cleveland-based Sedlak Management Consultants, a firm that provides planning, design, and implementation services for fulfillment centers. She may be contacted at pdavey@jasedlak.com.

Hybrid Engines

Combinations of picking methods and technologies are often used in the same facility, because picking the items in highest demand needs more speed. As the business grows, different parts of the picking system can be upgraded and expanded to handle increased volume. For example, a distribution center may begin by separating the inventory into two demand categories, fast (20% of the items, which typically represent 80% of the demand) and slow movers (80% of the items, which represent 20% of the demand), and use a paper-based system.

As higher speed and accuracy are needed, more technology can be added. Orders can be batched in the slow-moving area and picked using RF terminals. Alternatively, picking carts can be equipped with lights, a terminal, and a scanner to enable even greater speed.

When picking speed becomes a problem in the slow-moving area, carousels can be installed to boost productivity. If more speed is required in the fast-moving section, AS/RS systems or dispensers are options.

Piece picking is one of the most labor-intensive operations in distribution centers. Reducing travel time can greatly increase picking speed. Methods to cut picker travel include choosing the proper type of storage media, profiling items regularly, and using batch and/or zone picking. Technology, such as RF terminals, pick-to-light, and voice-directed picking, speeds up operations further by reducing the amount of time required to locate items, verify them, and confirm the pick.

Type of Item Storage Medium Pick to Technology Method
REGULAR Slow movers Bins or decked racks Carts Paper Zone pick
Fast movers Flow racks Conveyor belt Paper Zone pick
PREMIUM Slow movers Bins or decked racks Carts RF or PTL Batch-zone pick
Fast movers Flow racks Conveyor belt PTL or voice Zone pick