Comparison of Time-Based Demand-Pull versus POLCA
POLCA (Paired Overlapping Loops of Cards with Authorization) is a prior art system to produce solutions to the application addressed herein, that is, demand-based shop floor control in a high mix, or high variety, production environment. POLCA has various pre-requisites to implementation:
1. a High Level Material Requirements Planning system (HL/MRP);
2. a cellular organization;
3. cells involved in the implementation have rough-cut capacity and lead-time planning ability; and
4. the HL/MRP system (or an associated scheduling system) can produce dispatch lists.
The demand-pull system described herein advantageously does not require 3 or 4 above, Once the pre-requisites for POLCA are met, the following activities, at a minimum are required to execute the system:
1. Each pair of adjacent WCs within the production environment must be defined so that POLCA cards can be created that are unique to each WC pair. And, of course, when new WC pairings are created by new products or variations on existing products, more POLCA cards must be defined.
2. The number of POLCA cards for each WC pair must be computed based on production forecasts and WC capacity. When forecasts or capacity changes, the card numbers must be adjusted to compensate.
The present demand-pull system is rules-based: no color-coded cards, no pairing of WCs, no creating work quanta, no calculating the optimum numbers of cards, and no dispatch lists (unless an organization decides to supplement prioritization with MRP-based time). It is, therefore, simplified and optimum for a software embodiment. In addition, POLCA requires considerable ongoing maintenance as explained below.
1. Work cells must be paired based on any possible combination of production sequences, and if a new product with a different workflow is introduced, these new pairings must be added to the system, and cards generated.
2. POLCA requires that physical, multi-colored cards be created that circulate with the WOs through the production facility, and the demand-pull system described herein does not require them.
3. POLCA requires periodic examination of WC pairs, capacity, and throughput projections to regulate the numbers of POLCA cards that are allowed, so management must periodically add or remove cards based on conditions in the factory. The demand-pull system's algorithm is self-regulating and automatically adjusts the WC queue policy to compensate for changes in capacity and WIP.
4. POLCA uses MRP dispatch lists to determine which work is authorized and the sequencing of the work. This returns the system to a "quasi-push" system and dispatch lists can be labor intensive and subject to inaccuracy. The demand-pull system herein assumes that if a WO has been created by MRP and accepted into production, that it is valid and should move through production as expeditiously as possible. Sequencing of work is a two-step process: 1. First the WO must be authorized (Pull-tagged) based on its priority and downstream demand; 2. Once authorized, WOs are pulled into production on a First-Authorized, First-Out (FAFO) basis: the first WO in a WC's queue to be assigned a PT is the first one to be worked on. There are over-ride provisions in the system, but the present demand-pull system works automatically based on FAFO. The authors prefer that the priority-setting method for the authorization step be First-In-First-out, but other methods have been discussed in Section II of this disclosure.