# Switch-Off Policies in Job Shop Controlled by Workload Control Concept
## 4 Switch off Policies  
A shutdown policy is characterized by two decisions: what is the condition for shutting down and the condition for turning on the machine? Figure 1 shows the activities to implement a switch-off policy; when a machine loads a part, the state runs until the end of the machining time. So, the machine is \"on service\" and can either load another part or shut down depending on the policy being enforced. If the machine shuts down, when the policy decides to turn it on, it is considered a warm-up period to transition to the \"on service\" state. The shutdown policy must consider when to shut down when the machine is in the \"in service\" state or when to power back on when the machine is in the out-of-service state.  
The approaches proposed in the literature often evaluate the level of the upstream and downstream buffer. In the case of a job shop system with the dynamic routing of the jobs, the upstream buffer can be evaluated, while the downstream buffer cannot be used because this buffer is not related exclusively to a specific machine as in the flow lines. Then, the information that can support the switch-off policies for each machine is the upstream buffer level and the workload computation derived from the workload control mechanism. The combination of this information is proposed for computing a modified workload that supports the switch-off policy. The modified workload is calculated as the  
Figure 1: Switch-off control.  
combination of the workload and the items in the queue of the workstations. The modified workload is computed as shown in Expression (3); this expression concerns the weighted sum of two normalized values. The first is the workload of the machine m related to the workload norm, and the second is the number of parts in the queue related to the work in the process of the manufacturing system. The workload norm is the maximum value possible for the workload of the machines, and the WIP is the maximum number of parts in the manufacturing system.  
\[WLmod_{m}=\alpha*\frac{WL_{m}}{WLnorm}+\beta*\frac{Queue_{m}}{WIP} \tag{3}\]  
where WIP is the Work In Process of the jobs released in the manufacturing system and  
\[\alpha+\beta=1 \tag{4}\]  
The modified workload, as shown in Equation (3), assumes values between 0 and 1. Combining the upstream buffer level, workload, and modified workload can obtain five switch-off policies, as shown in Table 1.  
Policy 1 is used as the benchmark because it turns off the machines when the queue is empty and turns on as soon as an item arrives in the queue, and this is the upstream policy widely used in the literature [12].  
Policy 2 uses the workload computed for the workload control to switch off the machine while the machine turns on when an item arrives in the queue. Then, the first threshold (threshold\({}_{1}\)) should be defined. Policy 3 differs from policy 2 in terms of the switch-on condition that considers the workload modified, as computed in Equation (3) with another threshold (threshold\({}_{2}\)) used to define.  
The modified workload is used to switch on and off the machines for policy 4 with the relative thresholds (Threshold\({}_{3}\) and Threshold\({}_{4}\)). Finally, policy 5 considers the workload computation to switch off/on with another two thresholds (Threshold\({}_{5}\) and Threshold\({}_{6}\)).