Figure 1: Position jitter showing servo versus floor vibration in µm.
All images courtesy of Aerotech

Eventually, automation hardware becomes obsolete. It’s hard to know exactly when to retrofit or redesign, but some indications cannot be ignored. It’s time to upgrade when:

  • The system’s tolerances and throughput no longer meet market demand
  • The tolerances of the previous generation system can only reach thousandths of an inch, and the market is requiring ten-thousandths
  • The machine’s throughput is being compromised due to higher tolerances or because of a high failure rate

In all cases, servo-drive technology will have a significant impact on the success of the new system. Servo drives provide motion where human interaction is not possible. Drive technology must be carefully selected to ensure the automation process does what’s intended. Selecting the right hardware will increase performance value and improve costs.

The goal for next-generation products is to align the market’s performance requirements and price points. Project managers must thread the needle between engineers’ desires to work with a proven and known technology and marketers’ desires to include the latest advancements. Select components wisely while mitigating the risk of using new and untested technology. All changes should increase performance, capabilities, or ease of use.

At Aerotech, we followed the same approach when designing our next- generation X-series servo drives. We built upon a technology that could run multiple motor types (brush, brushless, stepper) from the same drive with only parameter changes, supporting 20 digital and 4 analog I/O points per drive, and accepting more than one encoder per axis. We improved an already reliable product, increasing bus speed and making it immune to electrical interference. The drive has lower in-position jitter and faster encoder sampling rates.

Communication breakdown

When the network fails or glitches occur in the industrial workplace, downtime and loss of productivity are sure to follow. However, servo-drive communication hardware can reduce potential connectivity issues.

Even though wireless technologies have come a long way throughout the past 20 years, a hard-wired connection is still preferred for industrial motion control, and Ethernet connections are the gold standard. However, these copper cables also provide a path for noise. Electromagnetic interference (EMI) has always caused issues with inter-drive communications over copper connections because low-level electrical signal communication packets can become corrupted.

Grounding practices and additional line filters, capacitors, metal shielding, and inductors are traditional methods of eliminating EMI by minimizing noise spikes. In a motion control environment with large motors, amplifiers, and I/O, there are many noise sources that need to be addressed. These additional components and cabling add material costs and a significant amount of engineering and labor costs since they must be designed into the machine and installed by hand. Some noise sources are only on the factory floor and are not discovered until the machine is being commissioned.

Fiber-optics use light rather than electricity to transmit signals, making such communications immune to EMI. This increases system reliability by minimizing downtime and motion errors. A fiber-optic bus increases communication reliability and minimizes the tediousness of tracking down noise problems.

With a fiber-optic connection, the distances between drives can lengthen without increasing EMI susceptibility along the cable run. Transmission distances can grow to hundreds of meters or more, compared to less than 10m for drives connected with copper wires, allowing distributed panels throughout the machine or factory floor and eliminating the need to run all motor power and feedback cables back to a centralized location.

Keeping motor cable runs shorter lowers the cost of cable and the need to have elaborate cable trays and runs through the floor or ceiling. Drives can be closer to the moving hardware and farther away from the controlling PC. As control communication reliability increases and wiring cable placement becomes easier, machines benefit from reduced downtime and easier installation. This builds upon the proven technology of distributed I/O and applies it to motors and drives.

Figure 2. X-Series linear servo induced jitter, 10nm pk-pk

Tooltip jitter, blurry Images

Uncontrolled and unwanted motion can cause havoc in any process. A cutting tool bouncing around causing a wavy cut or a camera shaking and creating a blurry image will impact system throughput. A common way to compensate for this type of motion is to lower velocities and acceleration rates.

The level of this unwanted motion is called the noise floor (see Fig. 1, pg. 10), the ambient level of disturbance in a system. When measuring features, it’s imperative to observe this noise floor and understand how it affects measurement and how precise the measurement can be. Analyze the noise floor with and without the servos active to get a picture of natural versus servo-induced vibrations.

Jitter in the motion system, ground floor vibrations, and drive jitter contribute to the noise floor. A servo loop corrects positioning errors, an air isolation system minimizes ground floor vibrations, and a current-loop control minimizes drive-induced jitter. Aerotech’s X-drives correct two of the three vibration sources.

For a lower noise floor, increase the current-loop resolution. This provides smaller current steps and increases the servo loop rate, allowing earlier detection and compensation of vibration. Our newest drive hardware has implemented both features, reducing the noise floor by 2x to 4x compared to older drive technology. This in-position stability is critical while taking measurements or triggering an operation while the part is in situ.

Since the new servo hardware has 2x to 4x less noise, it may be possible to get away from more expensive linear amplifier-based drive hardware and use economical pulse width modulated (PWM) hardware. For applications that formerly required less efficient, costlier, and bulkier linear amplifiers, PWM amplifiers can minimize cabinet space and lower the cost of the finished machine. PWM drives generate less heat than linear drives and can operate at higher power ratings.

Figure 3. ADC noise reduction.

Analog noise, data rate bottlenecks

Most positioning systems require feedback to accurately sense system position. The control system uses this information to generate an error signal. The control loop’s determination is based on this signal so that it can compensate for this error. Incremental encoders are typically digital or analog. Digitizing a signal removes fidelity and takes a true sinusoid and turns it into a staircase. Optical encoders are still analog, but this signal gets digitized either in the encoder electronics or at the drive end. Drives that take analog feedback signals eventually digitize these signals internally. The higher the interpolation value on these analog signals, the closer the representation is to a pure sinusoid.

Analog encoder signals have their own noise floors. Filtering techniques minimize this noise. The higher the sampling of the analog encoder, the more the drive electronics can oversample and filter this signal to remove this noise. Figure 3 (pg. 14) shows the results of this increased sampling rate. Since this noise directly relates to position jitter, we can see an improvement of 100x compared to older drives.

Another benefit of higher sampling rates is increased speed. A drive can only read so many counts per second – the higher the sampling rate, the more counts per second. Encoder manufacturers come up with finer pitch scales every year that are affected by a drive’s maximum input rate. Moving from a 40µm pitch scale to a 4µm pitch scale results in 10x lower potential max. positioning speed if the drives are not also upgraded to read these scales faster. The X-Series drives have 4x the encoder input rate compared to their predecessors.

Since higher performance is always the goal of a new design, using analog encoders with the X-drives creates an environment with a robust controls package to minimize audible noise and maximize velocity stability.

Conclusion

The design phase offers the opportunity to look for partners who are willing to get the most out of your new machine. Picking the wrong servo drives could limit the overall performance of the machine. Improved communication reliability, positioning accuracy, and encoder sampling built into a proven drive technology that is reliable, robust, and performance-enhancing is the safe choice when designing your next system.

Aerotech Inc.

About the author: Matt Davis is senior applications engineer - Control Systems Group, Aerotech Inc. He can be reached at mcdavis@aerotech.com or 412.963.7459.