Adaptive Speed Control in Conveyor System DesignMonday, August 12, 2013
By: Tom Conrad
Introduction & Overview
Conveyor systems have long been the heart of the modern material handling industry. They have been implemented through various vendors and integration teams throughout the industry. The advancements in technology have allowed them to continually improve the accuracy and speed at which product can be moved. Rudimentary design philosophy for a new system requires that system to be planned to handle a customer’s current production capacity plus any near-term future increase in production capacity that the customer plans to see through growth.
Many times systems are designed to run continuously at a customer’s desired rate once the system’s start button is pushed. This works well for systems that run consistently at or near their design capacity without a lot of throughput fluctuation. More often than not, though, most companies find they have more fluctuation in their production. They then find themselves with a system that is able to handle their peak volume, but at other times it is completely underutilized.
In recent years material handling customers have also found themselves under pressure to conserve resources and reduce their carbon footprint on society. There is a large push to implement practices that are more in line with the “Go Green” movement that is happening across industry sectors. The cost of basic resources has also put more emphasis on doing more with less. A conveyor system that runs at full speed all the time, even when production capacity is at 50 percent, is generally not a ringing endorsement for energy or resource conservation. It is also more costly to run and maintain than it needs to be. This would be similar to having light switches in your house turn on every light in the house instead of each switch only controlling the room where it is located. Separating the switches allows you to put the light where you need it, when you need it.
With the associated cost to operate systems continually growing, customers find themselves looking to spend more on projects up-front to reduce their cost of ownership over the lifespan of a system. To help material handling customers solve these two main problems (and reduce their cost of ownership over time), integrators began using a combination of advancements in technology and flow control philosophy. They began to design in and retrofit systems with what was known as “Energy Conservation Logic.” This enhancement basically added product detection sensors to various points in a system, typically one on every section of transportation. Doing so allowed the movement of product to be monitored, and if no product was seen in a configured time period the transportation section would shut down. It would similarly start up if product was detected in route to a section that was shut down.
The increased use of product detection sensors gives integrators the ability to improve system diagnostics, and understand the movement of product within the system to much higher levels. From this the philosophy of “System Flow Management” was developed. The increased amount of data allows the integrators to start and stop different areas based on the perceived volume in a given section of the system. These techniques give customers the ability to start realizing some cost savings on their ownership of a system.
To take this to the next level (and provide customers with even more value), system integrators started to offer more flexible systems utilizing variable frequency drives (VFDs). Coupling the VFDs with System Flow Management, integrators are now able to offer customers the option of adding “Adaptive Speed Control.”
What is Adaptive Speed Control?
Adaptive Speed Control is the additional programing algorithms and hardware put into the design of a material handling system that allows the system to synchronize with the demands of the customer’s production systems. It utilizes the information gathered from systems flow management programming in conjunction with the additional VFD hardware to modulate the system’s throughput and speed to match production capacity up to the system’s designed maximum rates. This allows a more dynamic system, and ultimately puts more flexibility into the customer’s hands.
What does it take to implement?
In general, an integrator needs to understand the level of Adaptive Speed Control a customer wants to implement. Basically the more systems or components that need to fluctuate in speed, the more upfront cost the customer will need to bear. For adaptive speed control to be implemented a customer will need to have two things.
The first is a way to obtain the data required from a flow management algorithm that can be used to determine what speed the system should run at and when to initiate a change. This will typically involve the addition of package detection sensors to the material handling system. Algorithms are then written to constantly monitor these sensors. Although theoretically a system could be set up to constantly react to the algorithms, it is more practical to define different system levels to run at. Once these levels are set, thresholds can be set to allow the system to move from one level to another. This methodology has produced very solid and stable operating systems. It also improves the tracking reliability of systems that need to monitor product and product data within the system.
The second is a means to allow the system to react to decisions made by the flow management algorithms. This typically has required an integrator to add VFDs to the system if they were not already included. This is the part of implementing adaptive speed that adds the most upfront cost, but is also what allows the customer to realize the multiple benefits that being able to reduce the speed will bring over time.
In general integrators will give the customer the capability to enable and disable the adaptive speed control in case an override is needed. When disabled the customer can select what level they want the system to run at.
Both of these items require additional design and engineering time to accomplish during the implementation lifespan of a project. As with most things, planning for adaptive speed control in the beginning of a project will reduce the amount of time it takes to implement and test. Leaving this for the end of a project or trying to add it in after the fact requires roughly twice the effort.
What benefits does adaptive speed add to the system?
There are three main benefits that a customer can realize from an adaptive speed controlled system.
The first (and probably the most significant) is a reduction of power consumption. The addition of VFDs to a system allows a more even demand of power from a customer’s power company. Standard motor starters produce high inrush spikes of current and tie the motors’ highly inductive load directly to the company’s power grid. This is in turn reflected to the power company if there are no isolation transformers in place. These high spikes and full loads cause companies to draw a great deal of power that provides no actual benefits. They also have a negative effect on the bill a company sees from the power company.
VFDs take the inductive load and turn it into a capacitive load that is able to even out the current required by a motor. In addition, they allow acceleration and deceleration curves to be put in place that help mitigate the instantaneous spike that is generated from the starting of a motor. The VFD also allows a motor to be run at slower speeds, which lowers the overall current required by the motor. This reduction in power translates directly to a greener material handling operation as well as savings for the customer on their electric bill.
The second benefit is in maintenance costs. There will always be maintenance costs associated with a system, but how often those maintenance costs hit can be reduced by the use of adaptive speed control. The faster you run something, the faster it is going to wear out. Unfortunately for mechanics this is normally the case on an exponential curve. Therefore the ability to slow the system down to run only as fast as needed helps bearings, belts, chains, sprockets, sorters, and many other components last longer. Here’s a good example: a 25 foot belt running at 400 feet per minute will roll around 8 times in that minute. If that same belt runs at 200 feet per minute it will only roll around 4 times. That is half the movement and half the friction that the belt had to see. In addition, because this belt is run by a VFD, the splice on the belt does not see the high torque pull that a motor starter would produce at startup. Instead it sees a nice gradual acceleration to run speed.
The third benefit aids the employees’ working conditions. A system running at its full capacity tends to generate a lot of ambient noise. By lowering the speed of the system, the ambient noise in the building is also reduced. This can reduce the need for earplugs to be used during low production runs or non-peak seasons. In general it also allows better communication for plant personnel around the system.
Building the business case
Building the business case for making a change to a green conveyor system can be challenging because there is usually no historical data for a system that is already in place, so there’s nothing on which to base a comparison. If that is the case, the best strategy is to develop small test cases that can be extrapolated into larger systems. For instance, documenting the sound reduction of various pieces of equipment running at different speeds provides data that helps management better understand the effect a full adaptive speed control system would have on the building. In similar fashion, a documented case of a conveyor run using a VFD alongside a conveyor run with a motor starter could be developed and used to prove general power savings that could be seen and realized by the organization. Obtaining a case study from the manufacturer that documents the life cycle on a given part (such as a bearing) at different speeds can help illustrate how wear and tear is affected, demonstrating maintenance savings.
A lot of the understanding of these benefits come from the common sense realization of what speed inherently does to a system. By creating small case studies that can be extrapolated to larger systems, managers can help the C-level understand the long-term effects adding adaptive speed will have on the organization’s material handling processes.
Going green V conserving green
The other element that has to be considered is the cost. While many supply chain organizations have a desire to move to green (or greener) technologies, there is a limit to how much additional they will spend in order to make that move. An adaptive speed implementation can be tailored in many cases to various budget thresholds. An example of this would be that instead of having all the belts of a conveyor system adjust their speed, consider only adjusting the speed of key pieces of equipment such as high speed sorter and gapping systems. Once the organization begins to see the long-term benefits of adaptive speed, they will be far more likely to invest a little more up-front to gain them.
Adaptive speed systems allow organizations to pursue their goals of reducing energy consumption and noise levels while realizing a lower system cost of ownership with consistent operation over time. While they may carry a larger initial investment, in many cases the upfront cost can be mitigated by the sole benefit of reducing the systems power requirements. With energy costs constantly rising adaptive speed systems become more and more appealing. And as more of these systems replace older, constant-speed conveyors, the benefits will become more obvious and universally-recognized. They are the future.
Tom Conrad is the VP of Controls for Wynright and has 15 years of experience in the material handling industry with specific expertise in Sortation, Case/Pallet Conveyor, Storage Retrieval Machines, AGV’s, and Robotic palletizing. Tom can be reached at Wynright via e-mail at firstname.lastname@example.org.View all White Papers