Three Approaches to Small System Motion Control
Compact, powerful systems can handle many motion control functions.
Motion control is a complex task that has often been limited to larger controllers, or more expensive specialty hardware. That’s changing as new hardware and software makes it possible to manage moving elements with small systems that have one or two axis of movement.
The changes come in a number of areas as equipment designers leverage the declining prices and increased processing speeds of microprocessor technology and take advantage of more sophisticated software. Small controllers available today have more built-in high-speed I/O with improved performance for encoders and pulse train outputs. Those inputs are handled by more powerful small controllers that can manage the large amounts of data necessary for efficient motion control. These programmable controllers also have communications connectivity so it’s easy to link them to multiple drives and other peripherals.
Packing all this power into a single small programmable controller is a major advance. It eliminates the space once required for motion control co-processor expansion modules added to traditional PLCs, or a purpose-built PC-board for motion control, while also simplifying maintenance requirements over time. Not to mention eliminating the cost of a separate specialty module.
The controllers in these small systems can also run more complex software and exchange intelligence between known servo drive and motor combinations. Among them are motion control setup wizards found within controller programming software as well as support for standard motion control instructions such as PLCOpen instructions.
As these controllers move down into more mainstream applications, product developers are also employing software that can be used by engineers or technicians who don’t have the extensive experience of their counterparts who use expensive, complex systems. Small systems such as Siemens SIMATIC S7-1200 can be configured using setup wizards that do much of the complex programming based on the parameters provided by users.
Simplifying setup is important because instructions running in the controller handle tasks such as ramping up to the desired speed, maintaining a steady velocity, then slowing down without causing shuddering or any other unwanted movement. The powerful controllers and extensive I/O needed to handle these complex tasks is now included in affordable, compact hardware like the S7-1200, which can have as much as 2 Mbytes of memory for storing motion-related data.
That’s enough power to handle simultaneous tasks like high-speed counting from an encoder and a digital or analog interface to a servo drive and motor. The S7-1200 user program can signal the drive to have the motor run in a jog mode, referenced or managed using either absolute or relative positioning.
“All the signaling data for a specified movement is passed to the drive, which knows the motor configuration and profile for the job that’s being run,” says Paul Ruland, product marketing manager for the Automation Systems Division of Siemens Industry.
There are typically three different approaches to small system motion control. The simplest is to use analog and discrete hardwired I/O signals. Analog signals can be used for velocity control, while discretes can be used for start and stop functions. This approach works well for simple applications that don’t require a lot of precision, such as an arm that sweeps parts off a conveyor.
Another alternative controller-to-drive interface uses pulse train signals to tell a stepper or servo motor, or other mechanism to move a certain number of counts. Using a pulse train signal is an efficient interface to a servo drive controlling a motor on a glue gun for applying adhesive onto a surface or to control the position of a part of a belt conveyor, for example. Pulse train systems can control starts, stops and positions as well as keeping a motor running at a specific velocity.
The third solution utilizes communications, linking a controller and servo drive via a specific motion control protocol. For the S7-1200, a motor can be positioned by sending the set point position and the set point velocity to the drive via an RS-485 serial connection using the Siemens USS protocol. Compact servo drives like the SINAMICS S110 give feedback over this communication link to the controller using various status signals, the actual position or the actual velocity. Small controller communication interfaces for motion control are constantly improving and enabling connectivity to multiple drives simultaneously over Profinet/Ethernet communications. The ability to perform motion control and data exchange over standard Ethernet cabling and TCP/IP protocol saves significant wiring and configuration cost while removing the need for a separate “motion control only” network.
Though small system motion control can perform a number of complex jobs, users need to remember that it has limitations. For example, today’s small systems aren’t powerful enough to handle operations such as interpolations, where two motors move simultaneously to move an arm diagonally. Other tasks like cutting circles or moving in a helical fashion are also difficult.
But for users who are doing comparatively simple jobs like pick and place or other applications that require only one or two axis control, small systems are a very viable solution. They can provide good precision and run systems at fairly high speeds. They’re also much simpler to set up than larger, more complex controllers.
Equally important in today’s price sensitive market, they can be installed for costs counted in hundreds of dollars. That’s a huge difference from conventional motion control systems that sell for several thousand dollars.
Small system motion control can help companies improve efficiency when they do tasks as varied as simple assemblies, cut-to-length, drilling, filling, tool changing and material positioning.
For more information on small system motion control, click here for access to Application Notes on these types of applications.
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