Job Scheduling Concepts

Once the shop floor is defined, its work schedule can be created. There are numerous steps for creating the shop floor work schedule. The first step in understanding how to schedule jobs centers around work centers. Work centers define how jobs are scheduled and when they run.

The following sections discuss some basic relationships about scheduling jobs. These relationships are presented here to facilitate the use of the SCHEDULE windows. Once understood, job scheduling within your system is straightforward.

Shifts
A machine follows one shift schedule -- the one belonging to its work center. It is important to design your work centers with this in mind. Each work center has a set of annual shift schedules. Annual shift schedules are divided into 52 weeks. Weeks start on Monday and end on Sunday.

Schedules for each week of the year can be set from one of twelve weekly schedules. Each weekly schedule defines the day-to-day shift changes for up to six different day shifts. Two idle periods per day are available for times when the machines should not be in operation. During an idle period, the machine's DCM ignores all inputs. Only idle time is accumulated. If no shift schedule is assigned to a machine, the machine is considered to be up 24 hours a day.

These shift schedules are important because they are used by your system to create historical production records. The production records are the raw data for the monitoring windows. A new historical production record is created for each machine at each shift or job change.

Jobs
A job is the basic unit which defines production monitoring on a machine. Every monitored machine in the system must have a job assigned to it. Scheduling a job is a two step process. Every job within your system starts as a work order, which is step 1. A work order is a job which has not yet been scheduled on a machine. It consists of parameters which define how to monitor a job's performance and progress. These parameters are based upon the type of product produced and the machines on which the product will run. They, in essence, describe the job and set the standard for monitoring it. They define the line between the normal operating mode and various exception states for a machine. All monitoring is based upon these values. Examples of job parameters are job name, product, material, tool, standard cycle, pieces required, required date, and process limits.

Care should be taken when defining job parameters. For instance, if the Standard Cycle parameter is set to 30 seconds and the Cycle Tolerance is set to 10 percent, a machine would be considered Slow if its cycle time was currently 33 seconds. This same machine would be considered down if the down time threshold parameter was set to less than 33 seconds.

Job parameters also determine the expected run time of a job. The pieces required, standard setup, and the standard rate factor are used when computing the completion date of a job. If 10,000 pieces are required and the standard rate is 50 pieces/hour, then the runtime of a job would be 200 hours. Add this to the standard setup time and you have the expected runtime of a job. The schedule now and schedule factory parameters also affect the calculated completion date. These fields determine if the shift schedule should be followed when computing the completion date.

Once a work order is created, it can be scheduled as a job -- the second step in job scheduling. Each machine has a currently-running job and a queue of jobs waiting to run. The scheduling person must know which jobs can be run on which machines. The main objective is to arrange the job queues so that as many jobs as possible complete on or before their required completion date.

Once started, a job cannot be removed or stopped. It can only be replaced by another job. If a current job is not complete when it is replaced with a new job, it can be suspended. A new work order is created. The "Pieces Required" parameter on this work order is set to equal the pieces remaining to complete the job. The comment parameter will indicate that this work order was created from a suspended job. The work order can then be scheduled on a new or the same machine at a future date.

A job is considered completed when the pieces required for a job is equal to the net pieces produced. When a job is completed, it may be set to an idle state if there is no new job in the machine's queue. This "job idle state" is exactly the same as a shift idle period for a single machine. The machine's DCM ignores all inputs from the machine. To put a job in the idle state, set the job parameters "Schedule Now" and "Schedule Factory"
both to No.

Additional Considerations:
Estimated Completion dates for a job are only as accurate as the job's parameters and shift schedule. The parameters "Standard Rate" and "Standard Setup Time: are especially important for accurate estimates.

To facilitate the creation of a work order, a product standard can be created. Product standards are templates which contain the same parameters as a work order. A library of standards can be created and maintained within your system. When a work order needs to be created, a product standard can be loaded in and used as a base for the work order. "Pieces Required" and "Required Completion Date" are the most common parameters which are set when using a standard to define a new job.

A maintenance (non-production) job can be created within your system. This is accomplished by setting the job parameters "Pieces Required" and "Standard Rate" both equal to zero. The "Standard Setup" parameter can then be used to define the duration of the maintenance job.