Flexible digital servo drives accelerate machine control design

Advances in digital hardware technology and software innovations now allow a single digital servo drive to be configured to work in a variety of machine control architectures.

Figure 1 Flexible digital servo drives, such as Danaher Motion’s S200, enable a single drive to be configured for a variety of machine control architectures.

For machine builders, this means using just one drive from a single supplier for every application. Not only that, setting up the drive for a specific type of machine control requires simple configuration in the drive’s configuration software. There are no traditional programming languages ​​to learn, write, or debug.

In a machine control architecture, servo drives provide the link from the motors and I/O to the machine’s central controller (usually a PLC (Programmable Logic Controller) or IPC (Industrial PC)). Traditionally, servo drives provide power conversion and contain servo current and speed loops.

With the incorporation of digital technology, servo drives can now control servo position loops, have more digital and analog I/O, can communicate on bus networks, and can accept a variety of feedback types. That said, in any given application, the servo drive capabilities utilized depend primarily on the machine’s control architecture and other components previously specified by the drive.

Because it is the “heart of control,” machine designers often select a PLC or IPC control platform and software before selecting a driver. Key factors in selecting a machine controller are:

  • Ability to integrate HMI
  • Integrated I/O capabilities
  • Programming language/ability
  • execution ability
  • Connection to higher level controllers
  • Ability to close servo loop
  • Applications require centralized or decentralized control
  • end customer preference

The motor type can also be selected before driving, as the motor must be able to meet the mechanical and dynamic motion capabilities of the application. For example, if the application requires greater dynamic indexing than a rotary motor with a ball screw or bell pulley, a linear motor can be used to convert rotary motion into linear motion. Alternatively, if you need a motor that works well with the gearbox to get a good mechanical advantage, you can choose a traditional rotary servo motor.

Other key motor selection factors include:

  • Accuracy, repeatability, torque density, torque ripple
  • Application installation configuration and physical limitations
  • Feedback type: dig enc, sine encoder, resolver, Hall encoder

The servo drive must be compatible with the normally selected motor and/or controller. Based on the functions of PC or PLC controller, the servo drive will provide many of the following functions in addition to basic power conversion and current loop control:

  • Compatibility with feedback devices
  • Speed ​​loop servo control
  • Position loop servo control
  • Machine I/O control (motion-related), stroke limit switch, origin switch, etc.
  • Controller interface portion (digital, analog, bus) with command and status information flow
  • Motor brake control
  • Profile generation

Today’s high-performance servo drives are capable of much more than simply configuring to suit a machine’s control scheme and perform basic functions. They can actually improve machine performance, shorten the time it takes to get the machine up and running, and reduce overall machine costs.

Operation mode selection

Figure 2. Danaher’s S300 digital servo drive offers a wide range of inputs and outputs.

Servo drives can be used as simple current loops with power amplifiers, all the way up to units that close all servo loops, control I/O, and perform some or even all machine control. Some examples are:

Current loop only — In some applications it is necessary to close the servo speed and position loops external to the drive in the central controller. This enables extremely tight coordination of motion between two or more motors. Applications such as machine tools, robotics and electronics assembly require very tight coordination between axes to achieve required machine performance, such as smooth surface finish and micron-level positioning.

Some machine developers will want to develop their own control algorithms, while others will use commercial machine controllers such as Danaher Motion eXMP, which provides advanced motion kinematics to control multiple axes of motion. The driver will accept analog or digital current commands. For digital instructions, a motion bus such as SynqNet can be used. The update rate for each axis is 250 microseconds, with no performance degradation compared to analog interfaces. Additional motor feedback messages can be sent over the bus, completely eliminating feedback cables.

Master-slave — In a master/slave configuration, the purpose of the drive is simply to position the motor, and thus the machine, by following the master pulse sequence from the controller. Traditionally, such applications have tended to use stepper motors and are now turning to servo systems to achieve higher machine productivity.

Another example is an encoder master signal from another drive or encoder wheel where the drive is electronically connected to another part of the machine. These applications are often found in the web conversion and packaging industries.

Motion index in drive — In some applications, the drive uses an internal contour generator to store and execute motion indexes. Multiple motion profiles or tasks can be created using the driver’s software configuration environment. Additionally, in some applications a separate PLC is no longer required. And it’s not just the cost savings on the PLC itself; it also includes the cost of cabling, extra cabinet space, spare parts, and the need to learn a programming language.

Instantly switch operating modes — Some applications require switching OPMODES on the fly. Drives with this feature eliminate the need to stop the machine to switch operating modes, thereby reducing cycle times while maintaining machine process performance.

Two common examples include gear position control in electronic gear applications, or position torque control in fixture applications.

Input/output functions

I/O can be configured for a variety of application needs. For example, digital inputs can be used to initiate motion profiles, limit motion, indicate travel limits and switch operating modes, among many other functions. But rarely does every app require all of these features.

So instead of having 20 or more dedicated inputs (one for each function) or having to write application code to implement a specific function, a configurable drive has a smaller set of three to six inputs that can be customized based on Configuring is required for specific applications. The same goes for digital outputs. Applications using digital buses such as Profibus or DeviceNet benefit from flexible I/O because the controller can use the drive I/O as a remote I/O point, eliminating the need to add another dedicated remote I/O node.

Motor braking control, often required in vertical motion applications, is integrated into the drive. For digital servo drives, the brake automatically disengages when the drive is enabled (motor torque is applied) and engages when the drive is deactivated (no motor torque is applied). In addition, the synchronized timing of brake engagement and disengagement with drive activation or deactivation can be delayed or advanced in milliseconds by user settings. These adjustments calibrate the servo system based on machine load to prevent unnecessary motor movement, which can lead to reduced productivity or even machine damage.

Tuned for better performance

Today’s machines face increasing competition and must maximize productivity while minimizing production costs. To reduce costs, manufacturers sometimes modify load structures to make them lighter, but also more compliant and prone to resonance when rapid changes in speed are required. Flexible digital drives help overcome these challenges by providing advanced control schemes as well as tuned filters and observers to maintain or even improve the overall performance of the machine.

System error control

Typically, when a malfunction occurs or a machine operator presses the machine’s emergency stop button, they want to bring the machine to a complete stop as quickly as possible for safety reasons. Flexible digital servo drives can be configured to automatically slow down at a higher rate than normal. This feature at the driver level eliminates the need to develop additional code for the controller.

The flexible GUI in the digital driver provides a user interface that guides the user through setting the driver’s supply voltage, motor and feedback, machine limitations (position limits, top speed, hybrid current, etc.), and initial tuning gains. This time-saving feature allows machine builders to focus on other areas of machine development.

Give back flexibility

Machine builders can choose from a variety of motor feedbacks. Which one to choose for your application mainly depends on customer preferences, application needs and vendor preferences. For example, resolvers are rugged and well-suited for high-vibration, high-temperature applications such as material stamping machines. Sine encoders provide the highest accuracy and are ideal for use with pick and place circuit boards and component insertion machines.

The driver can interface with either, or with a potentially more cost-effective digital encoder, allowing users to optimize cost and performance for each application. In high-precision applications using rotational to linear motion conversion, the drive can be connected to a second linear position feedback device connected directly to the load.

This article was written by Carroll Wontrop, senior systems engineer at Danaher Transmissions in Wood Dale, Illinois. For more information, please contact Mr. Wontrop at 866-993-2624 or visit http://info.hotims.com/15140-328 .

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