Definitions #

Various terms like throughput, productivity, machine rate are used to describe Goods-to-Person (G2P) systems. A useful framework in defining some of these terms would be to describe first the inputs and outputs of a process.

G2P picking process, by definition, includes sub-process of retrieving goods from storage to workstations and sub-process of picking goods at workstations, be it a person or piece-picking robot. We’ll use further the term Associate to describe any piece picking agent (human, robot, etc.)
Inputs for G2P picking process are:

• state of inventory at initial time zero
• a list of orders, order-lines, and quantities to pick

Outputs of the process are

• times of picking events.

Throughput capacity of a G2P system in its simplest form, is defined at Instock as a rate of bin presentations (per hour, for example) that is achievable and maximizes output of picking process for a given certain behavior and speed of piece-picking Associate. Under such definition, the overall useful output of G2P picking process is measured in fulfilled pieces and depends on throughput of G2P hardware (retrieval) AND on productivity of piece-picking Associate. Therefore, throughput can be represented as a function of process inputs and behavior of piece-picking Associate.

Lightspeed retrieval #

A useful exercise is to imagine G2P picking process where retrieval works instantly – it takes zero seconds to present the next bin with necessary inventory to Associate. In this case, rate of picking depends completely on Associate. (Assume experiments with fixed initial state of inventory and orders list.) Rate of output becomes a pure function of time it takes Associate to perform its operation. If output is measured in pieces picked per hour and time to complete an avaerage piece pick is in seconds, then output rate is just:

For example, if it takes 5 seconds to complete Associate’s pick of one piece on avaerage, productivity will be 720 pieces picked per hour.

Obviously, output pick rate goes to infinity as time for Associate’s pick goes to zero.