Various other processes are also involved from replenishment to dispatch. They are
Slotting
The ‘slotting’ of inventory is a term used for identifying the individual SKUs that should be located in each location. In picking, a very common approach is to use the Pareto principle (ranked by units sold or, more normally for picking, ranked by the number of order lines for an SKU during a set period). Another, more sophisticated, method is to consider the ‘value’ of a pick face run and to try to calculate how to make best use of the most valuable runs of shelving or racking (eg those nearest the start and finish of the pick runs). This is undertaken by calculating the ‘cube per order index’ (COI) of each SKU and then ranking these in order. The calculation is basically the ratio of an SKU’s storage space requirement at the pick slot to the frequency of pick accessions. It thus allocates the most valuable space next to the start/finish points to the most frequently picked SKUs that use up the least space. A typical calculation would therefore be:
SKU 1 COI = 1 cubic metre of pick slot space: 100 pick accessions per day = 0.01
SKU 2 COI = 1 cubic metre of pick slot space: 20 pick accessions per day = 0.05
The lower the COI in this case, the better the space is used, and therefore SKU No 1 would be
placed nearer the start/finish point. This formula may need to be adjusted for particular circumstances. For example, in some situations it may be more useful to compare pick accessions
to the linear metres of shelving required for an SKU.
The most valuable picking area is oft en referred to as the ‘golden zone’. This may be the area nearest the start and finish of the picking run, or locations at the ideal picking height (ie about waist height), or a combination of both. However, care must be taken not to cause congestion by concentrating most of the pick activity in just one small area. Other slotting approaches include location by weight (so that heavier items are placed at the bottom of dispatch loads) and location by store layout (so that items in roll-cage pallets can be easily placed on to shelves when they arrive at the stores).
Pick routes
Another factor that affects picking productivity in picker-to-goods operations is the actual route taken around the pick face. For example, pickers could go up one side of an aisle and down the other side, or they could pick from both sides of the aisle on one trip. Specific route options include
- Transversing the entire pick face in a ‘snake’ pattern, going up one aisle, down the next, etc, picking from both sides.
- Approaching all aisles from the same end, travelling up the aisle until all goods are picked and then returning to the same end.
Consideration should be given as to which pick route is appropriate for each operation
Information in order picking
Although travel time is normally the most significant element of overall picking time, the time taken for information also needs to be considered. This may comprise reading which location to go to, reading how many items to pick, confirming that the picker is at the right location and/or picking the correct goods, and advising the system of any shortages at the pick face.
This information exchange is necessary for the picker to complete the task and also to ensure that the pick is completed accurately. The design of the information exchange therefore needs to achieve high productivity while ensuring high levels of accuracy. There are numerous alternative methods available, supported by varying levels of complexity in information systems:
- Paper pick lists – These are printed by the computer system and list all of the SKUs to be picked, together with their location and the number required. The system normally lists these in the sequence of the locations to be visited as per the pick route used. The picker proceeds to pick all the items, noting any discrepancies (eg owing to shortages at the pick face) on the paper pick list.
- Pick by label – With this method, the pick list comprises a series of gummed labels on a backing sheet, printed in the sequence that the items need to be picked. The picker sticks a label onto each item and returns any unused labels to the pick station in order to record any shortages at the pick face.
- Bar codes – Bar code scanning is the most common method to confirm pick accuracy. Bar codes may be placed at each location (eg on a shelf or on a beam in the case of racking) and the picker then scans this label to confirm that he/she is at the correct location prior to commencing a pick. If the individual products have bar codes, then the picker may be required to scan each label (or to scan the label on one item per SKU picked). This provides a more precise check than location labels as it also identifies any replenishment mistakes (ie where incorrect goods have been placed in a pick location).
- Radio data terminals – These can provide online communication between designated warehouse workstations and warehouse management systems, and are therefore often used in order picking. The terminals may be truck-mounted, waist-mounted, or fitted to the wrists of the pickers. They are oft en combined with bar-code scanners. For example, a wrist-mounted radio data terminal may be attached to a bar-code scanner fitted as a ring on a finger so that pickers can move goods with both hands free.
- Pick by light – Normally, in these systems, every picking location is fitted with an LED (light-emitting diode) display panel, controlled by computer. A common application is for a plastic tote bin, representing a customer order, to be taken by conveyor to a specific zone of the warehouse. The bar code on the tote bin is read, and the appropriate LED panels illuminate, showing the quantities of items to be picked for all SKUs required for that order. Having picked the items, the picker presses a cancel button and then uses the conveyor to pass the bin to the next zone. This process continues until order completion. This method can give high pick rates and very high levels of picking accuracy.
- Put to light – This is similar to pick by light, except that it is normally used in the sortation process. For example, a picker may undertake a batch-pick and then return to an area of pigeon-hole shelving, with each pigeon-hole representing a customer order. On scanning a particular product, LED panels illuminate, showing the number of items required for each customer order.
- Radio frequency identification (RFID) – If cases or items are fitted with RFID tags, the accuracy of the pick can be confirmed by these tags being read at the time of picking (eg by using special gloves for reading tags or by placing a tag reader on the receptacle that the goods are being picked to).
- Voice technology – With this technology, the picker can hear voice instructions from the computer through a headset. The picker then selects the required items and speaks through a microphone to confirm the pick. Frequently, a check digit located at each location needs to be repeated by the picker to ensure that the goods have been picked from the correct location.
