How do I synchronize conveyor speeds?

Variable frequency drive opens many options.

A Control Design reader writes: I'm adding a conveyor section to the end of an outfeed conveyor that changes speed due to production demand, and I plan to install a variable frequency drive (VFD). With some products, both conveyors must match speed. What are my options for synchronizing the speed of the new downstream conveyor to the existing conveyor?

Also, depending on the product run, the downstream conveyor will need to run faster to singulate the product, and the distance between product will need to be programmable (+/-0.75 in). What do you recommend? Join the discussion in the comments section below. 

Also read: Drives integrate, gain intelligence

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  • There are a few different ways to synchronize the speed of the two conveyors. The easiest might be to pass the speed reference from the VFD on the out feed conveyor to the VFD on the new downstream conveyor. If the downstream conveyor needs to be operated at a different speed (faster) most VFD's have a trim scale that would allow you to program +/-% of line speed. If the gap between products needs to be precious and programmable you will most likely require a PLC, and other control products. Greg Semrow - Product Line Manager - ABB Low Voltage Drives

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  • First - you will need to decide if you need similar speed or synchronized speed. Similar speed is akin to setting two VFDs at 60 Hz - they should be running at the same speed, but they will be a few RPM off due to slip. Synchronized speed is where the second conveyor turns 10 degrees when the first conveyor turns 10 degrees (electronic line shaft). If you need synchronized speed (either 1:1 or 1:1.23), you will need an encoder on the outfeed conveyor and a closed loop drive that supports electronic gearing with adjustable ratio for you different product requirements. You will need to supply a product signal to the drive to choose the desired ratio. If you need similar speed, you will need a basic VFD that supports different presets and use the same command signaling the outfeed VFD uses. You will need to supply a product signal to the drive to choose the desired ratio (same speed, 3% faster, etc). If possible, alter your machine PLC to incorporate the signaling of your conveyor so the user only needs to go to one location to change the product.

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  • There are a couple ways this can be accomplished, but the common principle is that there needs to be a signal that relates the speed of the existing outfeed conveyor to the VFD on the new conveyor. First, I will assume that the existing outfeed conveyor already has a VFD installed since the reader says it is already variable speed based on production. Next, you will need a signal to reference the speed of the new conveyor. If there is an available analog output signal from the existing VFD that can be used to reference its speed, then that signal can be used as the reference speed input into the new VFD. Another option would be to install an encoder on the existing conveyor, which can be used as an input speed reference to the new VFD. The first option would typically be less expense, while the second option could be easier since you would not need to interface to the existing conveyor control system. A distributor can typically provide you with all of the components you will need (VFD, Encoder, enclosure, safety, etc) and can ensure you the components will work well together, provide the correct performance requirements and are sized properly for the motor on the new conveyor. John Uzzolino Business Development Manager, Mechatronic Solutions Lenze Americas

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  • Answer provided by: Hurley Gill, Senior Applications/System Engineer at Kollmorgen The required distance between product on the downstream conveyor would most likely be best accomplished with PM servomotor technology for its programmable acceleration and deceleration capability. A typical VFD controller will control velocity (open or closed loop); but positon-loop closure with this technology is much less common. PM servo controllers are commonly used to close a velocity and position loop with other available features such as electronic gearing. Product displacement is achieved as the first or next downstream conveyor picks up the individual product from the out-feed of the previous, while accelerating said conveyor and thus the individual product, faster than the product’s production speed. Depending on things like the product size, mass, coefficient-of-friction of the product to the conveyor belt, and required displacement, a second downstream conveyor maybe required. For explanation we will assume two servo controlled downstream conveyors are needed to properly separate the product. First, I would recommend mounting a 4096_LPR Master quadrature encoder for 16,384_CPR (minimum) to the machine’s out-feed (upstream conveyor or equivalent). Why not a lower resolution? Because the displacement repeatability of the downstream servo controlled conveyor(s) is affected by the resolution of the out-feed conveyor’s Master encoder (+/-). Remember, the downstream programmable servo-drive/controller cannot perform better than its Master input. In general, if the product on the machine out-feed is back-to-back there is no need for photo-eyes to measure distance between the individual products (e.g. out-feed could be a product magazine or conveyor, etc.). The consistency of back-to-back product running at a given product velocity accurately measured by the Master encoder can allow the first downstream conveyor’s programmable servo-drive/controller with predetermined programmed algorithms, to appropriately accelerate and decelerate its conveyor for a given displacement between individual products. In the event, one or more of the product types to be separated cannot meet or cannot consistently meet the required separation displacement/tolerance specification, a second downstream servo controlled conveyor can be added to fine tune the displacement between individual products before packaging. In this case photo-eyes over the first downstream conveyor can be used to measure present distance between individual products. The second downstream conveyor’s programmable servo-drive/controller would determine the present distance (from time and speed) between the individual products from its inputs and adjust the acceleration and deceleration of its conveyor by predetermined programmed algorithms. More photo-eyes over the second downstream conveyor could be utilized to determine actual displacement before packaging and/or feedback to the downstream conveyors’ programmable servo-drive/controller for dynamic adjustments of a displacement between given product types.

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  • Answer provide by: Tim Park, Regional Applications Engineer - Eastern Zone, Danfoss Drives Americas With the range of adjustment synchronization outlined in the question, it would appear this is a somewhat precise synchronization requirement, but a lot of information is not provided. I’d use an encoder on the existing conveyor segment to get a line speed reference for the conveyor segment we’re adding. There may be one there that we can reasonably tap into, but a new one may need to be added. Likely, the best place to add this encoder would be on the driven motor itself. I’d utilize another encoder on the shaft of the motor on the new conveyor segment to assure optimum speed regulation. The Danfoss drive itself can be used to alter the ratio of speeds required as the product changes. Typical Danfoss solution would include either the VLT FL300 series or the VACON NXP series. Both would require optional encoder interface cards to complete the package.

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  • Users can more easily synchronize VFDs from Rockwell Automation with their integrated motion capabilities and Logix-based embedded instructions. Through integrated motion, users can easily create master/slave relationships, and a few instructions can accommodate this – from simple electronic gearing to complex electronic camming. These relationships can be turned on or off programmatically, and the ratios can be changed quickly and easily – all within ladder logic. The Allen-Bradley® PowerFlex® 527 and PowerFlex 755 drives from Rockwell Automation would work well in this case. The deciding point would be on preferences for features like power range and feedback support. For example, in this application, we can create motor synchronization with a simple Motion Axis Gear instruction where we set the slave axis to gear to the master at a ratio of 1. We would simply change the ratio value in this instruction to modify our system and execute corrective moves using Motion Axis Move commands to move the conveyor either forward or backward to create the desired spacing. Response from Franklin Ruffin, commercial engineer for drives and motion, Rockwell Automation

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  • The main question to ask is how accurate of control is needed. To get as little error as possible an encoder would be placed on the feed conveyor and this would be used as a speed reference for the conveyor. Ideally there would also be an encoder on this conveyor as well so that it can be operated in closed loop speed control. A trim function would be used to alter the speed slightly. If a high level of accuracy is not needed then an analog output from the feed conveyor can be used as a speed reference for this conveyor, and we can run the conveyor an open loop speed control or sensor less vector mode. To use these two conveyors to set gap between items on the conveyor a PLC would typically be required and would need to measure gap and adjust the speed of the second conveyor to achieve the desired gap. Adam Bainbridge Eaton

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  • Answer provided by Eaton: From: Adam Bainbridge, Technical Analyst, Variable Frequency Drives, Eaton, Watertown, WI The main question to ask is how accurate of control is needed. To get as little error as possible an encoder would be placed on the feed conveyor and this would be used as a speed reference for the conveyor. Ideally there would also be an encoder on this conveyor so that it can be operated in closed loop speed control. If a high level of accuracy is not needed then an analog output from the feed conveyor can be used as a speed reference for this conveyor, and we can run the conveyor an open loop speed control or sensor less vector mode. To use these two conveyors to set gap between items on the conveyor a PLC would typically be required and would need to measure gap and adjust the speed of the second conveyor to achieve the desired gap. From: Ben Haberkorn, Senior Engineer, Variable Frequency Drives, Eaton, Menomonee Falls, WI Tighter accuracy would require encoder on both drives. Not so high of accuracy analog signals can be used but there will be some amount of slip introduced in each motor drive combination that will cause some slight variance in speeds. When trying to do gapping this would typically require a an encoder for accurate gap spacing’s along with a PLC with known belt lengths and variables entered for the required gapping.

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  • Answer provided by Eaton: Ben Haberkorn, Senior Engineer, Variable Frequency Drives, Eaton, Menomonee Falls, WI Tighter accuracy would require encoder on both drives. Not so high of accuracy analog signals can be used but there will be some amount of slip introduced in each motor drive combination that will cause some slight variance in speeds. When trying to do gapping this would typically require a an encoder for accurate gap spacing’s along with a PLC with known belt lengths and variables entered for the required gapping.

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  • Answer provided by Dan Nigro, Group Marketing Manager - Product Marketing, Omron Automation and Safety The level of synchronization needs to be determined to understand the integration that is needed. If synchronization means that the first and second conveyor need to follow each other in speed by a few revolutions per minute (RPM), then sending a speed command to each drive independently will probably be good enough. Some drives have the capability to talk to each other, meaning you could give the command to the first drive only and the drive will communicate this speed to other drives. This simplifies logic. Parameters could then be set to adjust a deviation speed from this commanded speed, to allow gaps between products. If synchronization means a more precise speed following, less than a 1% deviation, then a drive that can do full closed loop vector control will probably be needed. An encoder will need to be mounted to the first and second conveyor so that the exact speed can be measured and adjusted within the drive control algorithm.

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  • Using an encoder in conjunction with a Master-Slave VSD configuration

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