Technical Article

Controller Considerations for Small Assembly Robots

May 18, 2020 by Damond Goodwin

This article provides an overview of controllers for small assembly robots, and the specific considerations to make in regards to safety and efficiency.

An Overview of Controllers

Robot cells often require a controller to run parts of the cell not generally in the robot’s scope of work. A controller, usually a type of PLC, is used to control axillary doors, clamps, safety, or anything else that may be needed for a fully autonomous cell. 

Ladder logic programming is similar to wiring diagrams in order to keep the conversion from wiring diagram to functioning program simple. Ladder logic programming runs sequentially, making it possible for certain tasks to be completed, through a chain of commands, in the cell. They are also very useful, and widely utilized, for the safety systems in a cell. They can be programmed to stop every task in the cell as soon as the safety parameters in the cell are broken.     

Robots generally use a form of C programming language and can be used similarly to a PLC with safety and I/O. Robots, however, are limited in the amount of I/O they can handle and their safety systems are often not capable of keeping up with a cell containing a large amount of safety or I/O.  

On a smaller, simple cell, the robot may have enough capability to control the entire set up and an external PLC is not necessary to control the cell properly. In this case, a  robotic cell system without a PLC is often much cheaper.  

When the cell does require a PLC, either a larger cell or a cell with more complex external systems, there are many different types to choose from. PLC’s can be remarkably simple controllers capable of smaller amounts of I/O and safety or they can control huge amounts of external systems, motors, conveyors, safety, palletizing, etc.  

The price of a PLC is directly related to its capabilities. Brand names are also a determining factor in the cost of a controller. Certain brands cost more because they may offer more support for their controller, a longer warranty, or more durability in a manufacturing environment. 

 

Which Controller Should you Choose?

When it comes to varieties of small cell controllers, the sky's the limit. Small, simple controllers that work to monitor and complete simple tasks can be found priced at only a few hundred dollars. On the other end of the spectrum, there are PLC’s that can run several tasks at once, take care of the safety system, and run a large number of inputs and outputs.  

 

Various robot controllers from KUKA. Image used courtesy of KUKA

 

The complexity of the small cell is a good indicator of whether a simple controller can be used, or if a more complex PLC is necessary. If the manufacturer is planning on upgrading the cell in the future, or if there is any reason it might need more inputs and outputs, it is best that a controller with more capacity than anticipated, be used. An oversized controller allows for more flexibility within the cell long term, accommodating any changes, such as the addition of sensors and I/O to the system.  

Different brands of PLC’s require different software programs, something to be considered when choosing the right controller. For example, Allen-Bradley PLC’s are programmed using software unique to Allen-Bradley controllers. 

Allen-Bradley, and other high-end PLC manufacturers, require licensing and specially trained programmers in order to use their software. Less expensive PLC’s, like one from Automation Direct, are far more accessible and offer free software programs. However, a cheaper PLC may not offer an extensive technical support system like the more expensive brands. Price often, but not always, dictates the durability of the PLC also. If the cell operates in a harsh environment, it may be best to choose a more expensive controller for the sake of durability due to the physical demand imposed on the electronic components. 

 

Safety and Other Considerations

All robotic cells need a safety system regardless of size or environment in order to protect operators and other employees in the plant. In some instances, the robot controller is capable of maintaining the safety systems in the cell, however, if cell safety requires much more than a pair of light curtains, the cell will require a stand-alone safety system.  

 

A diagram showing how robot controller interacts with other controllers. Image used courtesy of Ericsson

 

This generally consists of a safety-rated relay that is connected to a PLC or a PLC with a safety system integrated into it. When considering safety system options, it is best to compare the price difference between a PLC in tandem with safety relays and a PLC with safety integrated because the cost difference all depends on the specifications of the cell. 

Most plants prefer certain brands for their automation needs, to ensure uniformity throughout their automation. Brand preference reduces costs by also encouraging uniformity among technicians and maintenance programming, which will most likely offset the savings of choosing cheaper PLC. If dissatisfied with the current control system, consider reasons why and then search for a PLC that better suits the needs of the cell.  

There are many things to consider when looking at a controller to accompany a small robotic cell. Safety, complexity, product support, and brand familiarity should all be taken into consideration before choosing a specific controller.

Oftentimes a controller with a cheap purchase price may end up costing more than the difference in the price of downtime for maintenance and lack of support for the controller. If there is a lack of experience with the programming of a different brand the learning curve may be nonsensical.  Some robotic cells may not even require a separate controller if simple enough, but, if not, a controller can add efficiency and safety where necessary.