Line Balance Optimisation
The five levels of control have been developed to optimise line balance based on the perfect flow model. Whilst it may not be possible to completely replicate the model on a production line, the five levels of control get extremely close.
The five levels of control are all steps that can be taken to modify the existing line balance and control with minimum to no investment.
- Equipment control and monitoring – automation
- Speed settings – line balance, V-curve
- Optimised conveyors – speeds intelligently controlled
- Cascade control – elimination of response times
- Automatic linked speed control
Level 1 – Equipment Evaluation: Assessment of the presence and use of automation
The first level of control is around the automation of individual pieces of equipment. The objective is to absolutely minimise the number of human interventions required for a machine to run as effectively as possible. This means that the machines should automatically stop and start in as many circumstances as possible – including the normal lack (of primary materials) and buildback (of primary materials). This ensures that each individual piece of equipment runs as often as it can to maximise overall line throughput.
When this level of control is present on the equipment the following is true:
- The operator doesn’t need to start/stop the machine during normal operation.
- The operator doesn’t need to change speed controls during normal operation – this include times where the machine goes into lack or buildback.
- The machine automatic speed control isn’t ever overridden for specific products or flavours.
- The operator doesn’t put the machine in manual control during a lack or buildback scenarios.
- The operator doesn’t believe that through manual intervention they can improve the output of the line.
- The Mean Time To Repair (MTTR) is minimised with respect to the operator being alerted to a malfunction with the machines under his supervision.
- After a defined level of line/machine inactivity, high energy consumption equipment is automatically put into a standby state.
Level 2 – Speed settings: Assessment of the machine speeds and line ‘V- curve’
As previously described to minimise the effect of minor stops of machines other than the critical machine on the critical machine, it is favoured to ensure that each machine runs progressively faster as we move away from the critical machine. It is ideal if this ‘v-curve’ is only present in a recovery situation (i.e. after a machine has had a stoppage), and then the machines return to a nominal speed for normal running. Sometimes there can be confusion around which machine should be the critical machine, this is normally down to line design and maximising the cost base of each machine. This means that when a line is designed normally it is done in a way that maximises the asset value. So there would be little point in having a two million pound filler that has its speed curtailed by a four hundred thousand pound labeller – it would make more sense having two labellers or a faster (and maybe more expensive) labeller. Another area to note, when lines are designed with on line blowing capability, the asset value of a blower and filler to achieve similar speeds are often quite similar, and therefore these will often be very closely matched.
When this level of control is implemented on the equipment the following is normally true:
- Conveyors before the critical machine are generally full and after empty.
- There is no easy manual method of changing machine speeds.
- The operator doesn’t ever manually modify machine speeds.
- For post bottleneck machines minimise instances where the nominal speed of the machine is faster than a downstream machine capability (e.g. If a labeller can run faster than a packer, don’t set it faster than the packer max speed).
- It is worth finding a balance between improving recovery time and equipment performance (no. of minor stops).
- Assuming that most machines have a greater than 10 minute MTBF (Mean time between failure) achieving full recovery in this time is a good target.
- There is a relationship between the amount of dynamic accumulation available and the recovery speed potential. If there is very little accumulation, there is little point in having a >10% speed increase.
- There is no hard and fast rule for recovery speeds, every line is individual – find what works best for each particular line.
- The only real way to ensure that modifying the line machine speed balance has a positive result is by observing or using LineView or similar.
- A simple method of determining if one machine is running faster than another is watching the position where the containers spread across the width of the conveyors. If this is moving forward, the machine in front is faster than the one behind and vice-versa.
Notes on Critical machine speed:
- As a rule, avoid controlled slow speeds on the critical machine – if the machine is a filler it can affect fill levels and torque control for capping.
- Don’t always accept the rated machine speed based on OEM advice, quite often this is conservative and if a machine is in good mechanical order it is possible to run faster.
- The effort required to achieve a 5% speed increase on the critical and the resulting increased performance can often be low compared to achieving a similar performance uplift reducing the number of minor stops across the whole line.
- A good rule of thumb is, as long as a machine is performing the required functions satisfactorily and is not self-destroying, there is room to increase its speed.
Level 3 – Optimised conveyors: Assessment of conveyor control
This area of the line control is only applicable to transport equipment (conveyors) although the control of the other machines will impact the conditions of the conveyors. The objective in this area is to maximise and optimise all conveyors and this is mainly achieved through considered positioning of sensors and dynamic speed control.
When this level of control is implemented on the equipment the following is normally true:
- During stoppage conditions conveyors will either nearly completely empty when in lack or completely fill up when in buildback. In other words there will be very little unutilised conveyoring.
- On the normally empty conveyors, the ‘in transport’ containers will only partly fill the width of the conveyors, providing plenty of space as dynamic accumulation.
- An inverter controlled conveyor will have a variable amount of dynamic accumulation, however the important part to observe is how much it has in its nominal condition (i.e. During stable running / nominal conditions how many bottles are on the conveyor?)
- For normally empty conveyors as the speed of the conveyor is increased the total number of bottles in transport is reduced, therefore increasing the dynamic accumulation.
- Airveyors often run better either full or empty, a compromise should be found between airveyor jams and good line balance, a good airveyor system can have a MTBF (time between jams) of greater than 8 hours.
- When considering the different types of container accumulation, dynamic is generally the best. An extra wide conveyor, with dynamic control is better than a static accumulation table. Static tables can introduce bottle handling issues and will have an effect on material traceability during production runs.
Level 4 – Cascade control: Assessment of machine restart times
Cascade control is about eliminating reaction times between machines. In a recovery situation (i.e. after a stoppage that puts the critical machine into buildback), good cascade control will almost eliminate the delay between downstream machines starting and the critical machine starting up. The control ensures that the critical machine stops for the minimum time period possible for each buildback and lack scenario.
Cascade control is always considered between two machines and the conveyor between them. Therefore a line of 5 machines with the critical machine being number 3 would have 4 zones to consider the benefits of cascade control.
Cascade control is not worthwhile when the following is true:
- Two machines are blocked together (i.e. one machine immediately starts/stops with another)
- A machine stops in lack or buildback with containers in it
- When the bottle conveyor is full, there is a disconnection of flow between the front and back of a conveyor section between two machines (i.e. 10 bottles removed from the front section doesn’t leave a 10 bottle space at the back of the conveyor train)
The different levels of cascade control are illustrated below. In each of these examples we are considering the same pair of machines (a filler and labeller where the filler is the critical machine) with a different level of cascade control applied, no cascade, Classic cascade, and Cascade plus.
In each example the sequence starts with the labeller stopping in fault, the conveyors between the filler and labeller fill up, the filler goes into buildback. Then the labeller restarts and the filler is controlled out of buildback and restarts itself.
No cascade (fig.1) or normal (conveyor based) control of the filler. The labeller stops, the accumulation fills until a conveyor switch is made, the filler bottle stop is activated. The capacity of the filler is emptied onto the conveyor. The labeller then restarts, after a period of time, the bottles clear the conveyors sensor and the filler bottle stop is released.
Classic cascade (fig.2): The filler stops as above using the conveyor sensor, and now on restart when the labeller reaches a set speed (normally greater than the filler), the conveyor sensor is muted allowing the bottle stop to release early.
Cascade plus (fig.3): The filler stops as per the first example, and now on restart the filler is allowed to restart almost the instant the labeller starts and ramps up in line with the labeller to ensure bottles can’t buildback into the filler discharge. In this situation it is advisable to have a one shot timer to allow this control to work once in each 2-5 minute period. (Otherwise revert to classic cascade)
In both of the cascade style control the buildback sensor is only muted whilst the labeller is either higher than the filler speed or ramping up as normal. If the labeller stops during the recovery period the conveyor sensor would come back into normal use
Some other notes of interest when considering cascade control:
- The overall performance increase from this type of control improvement is determined by each individual time gained and the frequency of occurrence.
- If a machine normally empties in a buildback or lack scenario, and has a large number of containers in it, there are often big gains to be had.
- If a line runs at 90-95% efficiency the gains will be minimal compared to a line running at 65-70% (the more lack / buildback events at the critical machine the more potential)
- Cascade control will not work directly between packers and palletisers, pack conveyors often need their own cascade control between each section
Level 5 – Linked speed control: Assessment of consistent running speeds
The objective of linked speed control is to minimise the number of machine speed changes, and during nominal conditions for all machines to be running at the same speed. The critical machine is the ‘master’ speed controller and each other machine that has the capability of speed control is a slave. In practice, what this means, is that during nominal conveyor conditions (i.e. full before and empty after the critical machine) all machines would match the speed of the critical machine. During recovery, after a stoppage, the machine speeds go to the ideal v-curve.
Machine speed consistency has many benefits including reduced machine inertia changes, reducing wear, and often increasing the machine operating window. This can also reduce the total number of stoppages on a machine driving up the mean time between failure (MTBF) and ultimately increasing line performance. It will often improve the quality of output, reducing the number of rejects and thus increasing yield.
Figure below illustrates linked speed control in a before and after scenario. It is considering a filler and labeller, with the filler being the critical machine. It traces the speed of the machines through some labeller stoppage scenarios – neither of which are long enough to cause the filler to go into buildback.
Some other notes of interest when considering linked speed control:
- Consider protecting the machine speeds (from operators) so that in automatic mode they cannot be modified.
- If manual control of speeds is absolutely necessary use a time out scenario, where if selected a machine will run at a lower speed for a pre-determined period of time before reverting to automatic mode again. This allows for scenarios like post changeover adjustments, and periods where running slow is necessary to identify a problem.
- Palletisers and cyclic machines – i.e. machines that respond to the arrival of containers/packs cannot be controlled dynamically.
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- A structured methodology to achieve perfect flow is called (5 Levels of Control, Five levels of control)