EtherCAT Servo Drives

EtherCAT logo - servo drives technologyEtherCAT® is a high performance Ethernet based deterministic network protocol developed by Beckhoff. It is industry proven to deliver high-speed, deterministic control with standard 100 base-T hardware and achieve blazing fast, consistent results in demanding applications. EtherCAT is an open technology which means everybody may use, implement and benefit from the technology. As a result, EtherCAT servo drives have become an essential part of the motion control industry.

DigiFlex Performance EtherCAT Servo DrivesEtherCAT is available for all of our FlexPro™ family and many of our DigiFlex® Performance™ (DP-Family) digital servo drives, which operate in torque, velocity, and position mode and offer full tuning control of all servo loops to achieve the highest performance for your application.

PCB mount EtherCAT-enabled servo drives also utilize ADVANCED Motion Controls’ unique, proprietary high speed multi-axis communication interface and I/O expansion capabilities (‘DxM’™ and ‘DxI/O’™ Technologies). Now a single EtherCAT® node can accommodate a combination of up to 4 axes of motion with 32 digital inputs, 32 digital outputs, 4 analog inputs, and 2 analog outputs each. The result is a highly integrated high value motion control solution for single or multi-axis applications.

ADVANCED Motion Controls’ plug-in module EtherCAT servo drives were named winners in Design News’ annual Golden Mousetrap Awards in recognition of our engineering innovation and creativity in product design! ADVANCED Motion Controls latest servo drive family, FlexPro™, was the first servo drive family to be designed from the beginning with EtherCAT® in mind.

Advantages of EtherCAT®

Maximum Bandwidth Utilization

The fundamental principle of EtherCAT is pass-through reading. Unlike other protocols where the master polls each node individually, EtherCAT sends a single message to a node which is then passed through to the next node and successive nodes until all of the nodes in the network have received the message.  This works because the message contains the information and instructions for all of the nodes in the network.  Each node only reads and responds to its own data, and each node can also send information back to the Master by inserting data into the message as it passes through.  For example, it can add position, velocity and status information to the message.  The added data will be read by the Master once the message passes through all of the nodes and returns back to the Master.

When the message arrives back at the EtherCAT Master every node in the network has received new input data from the Master and returned new output data to the Master.  An EtherCAT network can be compared to a railway where each station can off-load and re-load train cars while the train remains in motion.  Without the deficiency of small payloads or messages targeted to specific nodes, an EtherCAT servo drive network can achieve maximum bandwidth utilization.

FlexPro EtherCat Servo DrivesSpeed and Real-Time

All EtherCAT packets are generated and sent by the master using full duplex transmission.  All EtherCAT motor controllers on a network can read data from and write data to the EtherCAT packet as it passes by with only a short constant delay (usually less than 500 ns) for each slave device (independent of the size of the packet).  Typically, a large number of slaves can be accommodated using only one packet, thereby optimizing bandwidth usage and decreasing interrupt rates.  One study showed a minimum cycle time of 50 micro-seconds on a gigabit network with 50 devices and 100 bytes of payload per device.  Even with this example where the node count and payload is much higher than a typical servo system, the cycle times are still faster than the position loop and current loop update rates of the servo drives.  This means the network is operating faster than the motion control loops which ensures true real-time performance.

EtherCAT is Fast Enough to Operate With Either Distributed or Centralized Control

Distributed Control

Diagram of EtherCAT servo drives in a networkWith distributed motion control the servo drives operate in velocity mode or position mode (as opposed to torque mode).  This allows the primary controller to focus on the outer control loops and lets the servo drives worry about the inner loops.  The result is less computational overhead needed for the controller to command and control motion because the servo drives share some of the responsibility by closing the velocity loop and or position loop.  In addition, the network traffic between the servo drives and the controller is also reduced since fewer commands need to be sent.

Network traffic can be further reduced if the drives operate in PVT mode.  In this mode the drives are sent a series of Position, Velocity and Time values.  Each PVT point defines that at a given time, the motor will be at the designated position and will be moving at the designated velocity.  The “T” elements of the PVT points can be spaced out over 10’s of milliseconds thus greatly reducing the network traffic.  The drives then interpolate between the points.  Multiple PVT points can be sent to the servo drive and stored in a buffer to be consumed at the appropriate times.  With PVT the drives handle all of the motion control tasks, and the controller just defines the motion path.  Slower networks like CANopen can greatly benefit from PVT control.  With EtherCAT, the network is fast enough to operate with either centralized or distributed control leaving it up to the system designer’s preference to pick which type of control they want.

Centralized Control

In centralized control, the controller sends torque commands to the motor drivers and closes all of the loops.  The overhead in terms of computational requirements and network traffic is much higher with centralized control than with distributed control.  Fortunately, EtherCAT is easily fast enough to operate with a centralized control scheme.  So what’s the payoff?  Why run with centralized control?  In high performance applications, system designers might want to use specialized control algorithms, filters, feedforward, or want to follow motion paths more closely than with interpolated moves.  Centralized control gives the system designer complete control of the motion control in the system.

Reduced Wiring for I/O, Feedback, and Peripherals

Before high-speed networks in motion control, the feedback and signal wires from the motors and other devices had to come all the way back to the controller.  If you look at a traditional centralized multi-axis controller don’t be surprised to find 50 to 100 terminals for all of the signal wires.  In a traditional system each servo axis needs at least 11 wires to come back to the controller:

Encoder with differential signals – A, B, I, power = 8 wires

Command signal = 2 wires

Inhibit line = 1 wires

For example: a robot arm with 3 axes of motion control can easily become a wiring and reliability nightmare.  With the integrated servo motors and drives embedded in each joint you would have to feed at least 33 wires down the arm and into the controller.  By the time the wiring bundle reaches the shoulder joint it will be at its largest and have the most wires.  The whole bundle will still need to be reliable and withstand all of the bending and twisting from the arm.  In a real robot arm, the number is actually closer to 50-75 when considering brake relays, sensors, servo drive power and other components.  Every additional wire reduces reliability and makes installation more difficult.

Luckily with high speed networks such as EtherCAT, this is no longer a problem.  The number of signal wires can be reduced to 6 for the EtherCAT signal plus logic power if needed.  With the integrated servo motors and servo drives located at each arm joint, the EtherCAT servo drives can act as the interface between all of the peripheral devices and the controller.  The devices don’t even need to be compatible with EtherCAT as long as they’re compatible with the servo drive.  Any signal that the servo drive sees can be interpreted and sent to the controller.  Including motor position, velocity, motor current, and I/O status.  Even analog sensors can be used as network I/O by using the analog inputs on the servo drives.  The drive’s A/D converter can convert the analog signal to digital then send the value to the controller over EtherCAT.

Conclusion

Because of its fast cycle rate EtherCAT can be used in centralized and distributed control schemes in real-time motion control systems.  EtherCAT is fast enough to not only take on motion control, but also carry the data from peripheral devices.  Channeling all of the data requirements through EtherCAT can increase reliability by significantly reducing the number of wires that are susceptible to failure.  EtherCAT is an open technology that everybody may use.

  • Based on standard Ethernet for 100baseT
  • Real-time down to the I/O level
  • Multiple topologies possible - line, star, tree, daisy chain, drop lines - can be used in any combination
  • Requires no special Ethernet hardware - standard Network Interface Cards (NIC) can be used
  • CANopen over EtherCAT (CoE) allows use of CANopen protocol and feature set over EtherCAT
  • Ethernet over EtherCAT (EoE) allows other Ethernet-based services and protocols on the same physical network

ADVANCED Motion Controls' Capabilities

  • 2x100 Mbit/sec max speed
  • 10 kHz update rate
  • < 1 ms jitter