By Chris Gottlieb and Mike Schweiner, Global Product Managers, Rockwell Automation
A look at four ways that single-cable motion solutions can simplify system design.
Have complex cabling systems tied you up? OEMs today are expected to reduce system cost and inventory, while simultaneously increasing sustainability and ease of use. These tasks can be difficult with motion systems communicating through analog encoders that require multiple cables to operate effectively.
But today, digital technology has paved the way for the creation of single-cable motion solutions that bundle servo feedback and power cables into a single, all-inclusive cable. Pairing this new technology with highly engineered motors and drives, machine builders have a proven single-development platform for both servo and induction motor control. The result: less design time, smaller cabinet space, improved power density and greater energy efficiency.
How does it work?
Traditional servo drives and motors are connected by two cables—one for power and one for feedback. The encoder, located in the servo motor, acts as a position sensor sending analog signals in the form of sine waves to communicate position and location of the motor shaft. The servo drive reads the signals to determine absolute position of the servo motor. In these systems, power travels over a different cable to prevent electromagnetic interference with the feedback transmissions.
Single-cable motion solutions are now available with digital encoders. Digital information packets are less susceptible to interference from power transmissions. A digital encoder also improves motion-system diagnostics because many different packets of information can be sent over the same digital-bus system, without the need for a wire dedicated to a specific analog signal. This frees up users to collect additional data, which allows the system to run and start to use added data points to optimize performance. Additionally, a digital encoder sends absolute coordinates to the drive, removing the need for the servo drive to break down or read the message. Because the information packets are digital in nature, and due to technology and design improvements with the servo interface, cable, circuit boards and connectors, servo feedback signals in single-cable solutions are less susceptible to power-line interference than a traditional analog pulse width modulation (PWM) signal. The PWM action produces electrical noise that can interfere with feedback signals.
Benefits of single-cable designs
Single-cable motion solutions can provide a highly engineered, robust alternative to current motion systems. Without compromising system performance, OEMs can reap the following additional benefits:
Reduced system cost – Developing servo motion systems with a single-cable approach means OEMs only have to buy, design for, maintain and stock a single cable, reducing total system cost. Recent ordering practices have also made it possible for OEMs to purchase cable(s) in one-meter increments, allowing them to buy what the application demands and no more.
Ease of use – Fewer cable terminations per servo drive reduce installation time, lower the chance of commissioning and maintenance errors, and greatly reduce time spent on troubleshooting and repair. A traditional system with two cables, a servomotor and a servo drive may require 14 or more wires for feedback and power connections, in addition to wiring for overall system communications. If there is a connection error, users must individually work their way through each wire to diagnose and repair the problem. With single-cable solutions, there are now three power wires, two brake wires (if required) and two communication wires, vastly simplifying the process. Every wire you take out of the system is time saved in both setup and repair.
Fewer terminations also simplify machine maintenance, which can minimize the need to hire outside contractors—saving time, money and frustration.
From an applications standpoint, a variety of applications exist where single-cable solutions are useful. On any small machine where space is at a premium, one cable to route instead of two is really helpful, and having the cable in 1-m increments reduces coiling of cables where users may not have space. Additionally, for multiple axes, it greatly simplifies wiring, so if the user is spending ten minutes to wire up a single axis on a traditional system, with the fewer feedback connections, it will go much faster—potentially a couple of minutes. Over ten or more axes, this saves a significant amount of time.
Enhanced energy efficiency and sustainability – Single-cable motion solutions are also being developed with optimized drive-motor pairings. This goes back to how torque is created—through the interaction between a magnet and specialized copper windings housed in the servo motor. To create more torque or make a motor move faster, additional copper must be added. Conversely, sometimes a servomotor contains more copper winding than is necessary and ends up using excess energy. Just like you would not want your car to run at 15,000 rpm if you were only traveling 15 mph, you would not want a conveyor that only needs to run at 500 rpm being driven by an 8,000 rpm motor. A motion system with optimally matched motor and drive ratings uses half the energy of comparable solutions. Single-cable solutions are often sold as an optimized system out of the box. Combine this with the elimination of excess cable, and OEMs have an overall motion solution that requires less energy and fewer parts to operate.
Reduced footprint – Single-cable solutions enable a smaller servo drive, which in turn means the cabinet can be smaller. This helps reduce machine footprint and improve manufacturing facility floor usage.
Single-cable motion solutions provide OEMs with an array of benefits without compromising system scalability and performance—once again proving the old adage ‘less is more.’