Using a digital motion network

Machine designers strive to increase throughput, decrease machine costs, and reduce maintenance costs. The migration from traditional analog motion networks to digital motion networks accomplishes those goals in 12 important ways.
By Derek Lee August 10, 2015

Figure 1: Illustration of circle parts cut from various machines operating at high speeds on analog network versus digital network shows that higher speed digital networks create a smoother cut. Courtesy: Yaskawa America Inc.Digital motion networks allow an exponential increase in the amount of data to be communicated between devices. How the increase of data impacts throughput, machine, and maintenance costs may not be initially evident. When a machine designer uses digital motion networks, 12 key benefits result. Digital machine to machine (M2M) communications is an important part of the Industrial Internet of Things (IIoT), and greater information flow is part of Industry 4.0 and digital manufacturing efforts. The benefits follow. 

1. Increase throughput

Throughput is arguably the most weighted key performance indicator for machines. One way to increase throughput is to increase the operation speed of the machine. However, an increase in speed requires maintaining positioning resolution, adequate tuning gains, and accurate latching. 

2. Maintain resolution

When using a pulse train for positioning in an analog motion network, increasing speed typically requires decreasing resolution due to pulse circuit limitations. For example, for a motor to operate at max speed at full resolution, the servo drive would need to accept more than 50 million pulses per second. The pulse circuit is limited to an input of 4 million pulses per second, so a decrease in resolution is required. This decrease in resolution presents a problem, because the parts produced at lower resolutions may no longer fall within the specification for the part.

Figure 2.1: Latching on the controller in an analog motion network relies on machine cycle time (example: 2 microseconds), resulting in an inaccurate latched position. Courtesy: Yaskawa America Inc.

Using a digital motion network, the full resolution is maintained because the controller sends the target position points at the network cyclic rate. 

3. Dynamically adjust gains

When motion devices such as servo motors run at higher speeds, the servo motor’s tuning gains may need to be adjusted to maintain repeatability of the machine’s operations. Using an analog motion network, tuning gains are typically limited to two sets on motion devices, controlled by an input to the motion device. This limit is not applicable in digital motion networks because tuning gains can be written over the network at any time. In addition, dynamic tuning is available because motion data values such as feedback position, velocity, and torque are returned to the controller every network cycle. The controller can use this data in tuning algorithms to set motion device tuning gains on-the-fly.

Figure 2.2: This figures shows an example of a latching sampling rate of 62.5 microseconds (non-high-speed latching) on the drive, resulting in an accurate latched position, made possible with digital motion networks. Courtesy: Yaskawa America Inc.

4. Increase latch response

Machines that operate on a latched input may be restricted by speed depending on the method by which latching is performed. Using an analog motion network, the latching is performed by the controller. Increasing the speed results in a wider position window that a latched position can fall into because the position latch occurs at the end of the controller cycle, typically in milliseconds. This wider position window can result in the machine producing bad parts; therefore, speed becomes limited.

Using a digital motion network, the latching occurs on the drive side, and the drive communicates the latched position to the controller though the network. Drive-sided latching mechanisms vary from manufacturer to manufacturer, but typically are capable of a latching position in the range of 10s of microseconds. A drive that specifically implements "high-speed latching," involves using an interrupt plus additional calculations and results in the latch timing resolution in the order of microseconds. This keeps the window for the latched position on a much smaller scale, allowing higher speed machine operation.

In addition, if the controller must calculate a registration distance from the latched position, running the machine at a higher speed might not leave enough time for the controller to finish the calculation. Specifically, a digital motion network includes a latch and registration combination command, so that registration is executed immediately after the latch input occurs. This execution is completely independent of the controller’s calculations. 

5. Reduce machine cost

Figure 3: Various devices and components may be removed when using a digital motion network, simplifying the design and lowering cost. Courtesy: Yaskawa America Inc.Components and devices can be removed if the same purpose of those items can be achieved through a digital motion network. Among items that can be removed include input/output (I/O) modules, wiring for I/O, and limit sensors.

6. Remove controller I/O modules

In traditional analog systems, the controller I/O module is wired to the sensors and indicators on the machine. Because devices on a digital motion network can expose their I/O connections through the network, sensors and indicators can be wired into digital motion network devices instead of the controller. Thus, the controller I/O module no longer has a purpose and can be removed. 

7. Remove I/O wiring

Machines that incorporate I/O functions like "fault reset" input and "safe stop active" output may be incorporated into machines such as a human-machine interface (HMI).

The controller communicates those signals with wires to the slave devices. This wiring can be removed when using a digital motion network because the functions that use the wires may be available over the network.

Figure 4: Digital networks allow the controller to send configuration data to the networked devices, eliminating the need for maintenance procedures to include operating additional configuration software. Courtesy: Yaskawa America Inc.8. Remove limit sensors

For motion control machines, limit sensors are another example of devices that can be removed because software limits can be enforced by the motion devices.

9. Reduce maintenance cost

By simplifying maintenance procedures, programming fault-handling routines, and monitoring device end of life, the time and necessity for machine maintenance becomes reduced. 

10. Reduce maintenance hours

Replacing failed devices with new ones may require using configuration software to set up the replacement with the same settings as the unit that failed. Steps to use the configuration software can be removed from maintenance procedures by using the controller to send the configuration settings over a digital motion network.

In addition, a controller function can be written to read device parameters on startup to determine if it has not been set up and then automatically write parameters to set it up. If this is implemented, the maintenance procedure would only require the physical device replacement, and it would not require any operation of software. 

11. Reduce emergency maintenance

Performing maintenance may be required when a device fault occurs. Using a digital motion network, the fault information can be read through the network by the controller. The controller can be programmed to perform fault-handling routines to clear the fault and avoid the causes for the fault. These routines replace the need for a maintenance operator to clear the fault and the causes of the fault.

Figure 5: Fault information is communicated from the device to the controller through a digital network. The controller can send alarm information to an HMI and also perform fault-handling routines to clear the fault. Courtesy: Yaskawa America Inc.

For example, an excessive speed fault may occur on a servo drive, and a controller routine can be written to clear the fault and operate the machine at a reduced speed. In addition, because the fault information is available over the network, it can be displayed on the HMI. This reduces the maintenance steps necessary and advanced regulatory control (ARC) flash personal protective equipment (PPE) necessary for the maintenance operator because the operator does not need to open a live machine panel to read the fault from the device’s display. 

12. Reduce scheduled maintenance

Device total run time determines the scheduling of the next machine maintenance. Maintenance may be required sooner than the product’s total life if the controller has no information about the device’s remaining life. Using a digital motion network, the remaining life of the components in the device becomes available to the controller through the network. Examples of remaining life monitors in devices include: fans, capacitors, surge prevention circuits, or dynamic break circuits. If a controller reads these monitors, the controller can be programmed to notify the maintenance manager when the devices are near the end of life. This allows longer usage, which decreases the frequency of scheduled maintenance.

– Derek Lee is a product engineer at Yaskawa America Inc. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering,

Key concepts

Digital motion networks:

  • Increase throughput
  • Reduce machine cost
  • Mitigate engineering challenges.

Consider this

Increasing speed increases throughput and creates maintenance challenges. How would you increase performance and mitigate challenges in your systems?

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