Understanding Servo System Performance

Understanding Your Servo Systems

Servo performance directly affects the quality of the products a machine produces and the time it takes to produce them. In printing applications, positional inaccuracy in a servo often translates into dimensional variation of parts produced in cut-to-length applications and registration accuracy. Likewise, servo system smoothness affects the variation of coating thickness in coating machines and part finish in polishing applications. Response time affects the rate of production, with the fastest servos cutting more plastic bags, printing more labels, testing more samples and assembling more parts per hour.

Machine Designers can evaluate their servos with the following three key measures:

  • Accuracy—how close moving parts settle to the commanded position or velocity
  • Response—how fast the motion tracks the command
  • Stability—how reliably the motion tracks the command under various operating conditions

Accuracy

Accuracy is usually quantified in two ways: settled-position error and cyclical error. Settled accuracy, the positional accuracy of the servo when it’s stopped at its commanded position, is straightforward where errors in the servo system position translate to dimensional tolerance buildup. If the cut-to-length servo in a bag machine has a position error of +/-0.01 inches (0.254mm), it will probably contribute a variation of 0.01 inch to the bag length. Settled accuracy is often largely determined by your feedback device. Measuring errors in arc-seconds, sine encoders are the most accurate feedback devices, but are rather expensive. On the other hand, resolvers and digital encoders are less costly, but express position errors at a larger order of magnitude.

Cyclical errors are more complicated. When motors turn at constant speeds, position errors translate into apparent velocity ripples. These ripples repeat every motor revolution, hence the name, “cyclical.” Velocity ripples on feedback signals feed the velocity loop, creating current to compensate for those ripples. Unfortunately, that current creates actual velocity ripples, resulting in a loss of smoothness at speed and an increase in audible noise and motor heat. Cyclical errors are cured by higher-accuracy feedback devices. Sine encoders have so few cyclical errors that they often produce no measureable effects; the same cannot always be said of resolvers and digital encoders. Therefore, the key to selecting the right feedback device exists within each axis of motion.

Response

Transmission components such as lead screws, gearboxes, belts and pulleys all contribute error between motors and loads. Many of these problems can be adequately addressed by selecting high-quality transmission components. However, for machines that demand the highest accuracy, designers can look at two other solutions.

First, secondary feedback devices can be placed on the load side of the transmission. For example, a linear glass scale can be added to a screw-driven gantry to eliminate accuracy problems in the screw. Motor feedback devices can still be used to improve performance servo drives support “dual loop,” a servo configuration where both motor and load feedback are used simultaneously. The need for dual loop (as opposed to using only the load position) is created because the mechanical compliance between motor and load can severely limit servo performance when only a load-side position sensor is used.

The second solution involves “direct drive” systems, which eliminate transmissions altogether. By using these systems, motors directly drive their loads causing accuracy to be up to ten times better than that of traditional systems.

Stability

Mechanical compliance, or the flexibility between motors and their loads, limits many servo systems. Transmission components such as gear boxes and lead screws are not as rigid as they seem (at least not when viewed from the high frequencies at which servos operate). In fact, they act very much like damped springs between motors and loads. Those springs makes high servo gains hard to achieve because loads appear different to motors at low frequencies when compared to high frequencies.

Initial solutions to compliance issues are mechanical, but there are two main alternatives: using stiffer transmission components or matching the inertia of motor and load. By using stiffer transmission components like servo-quality gearboxes, lead screws and couplings, these problems can be minimized, but you can only stiffen a transmission so much before costs begin to get out of hand. By matching the inertia of motor and load, total inertia changes only by a factor of 2:1 when loads are disconnected from their motors (if the load is 10x the motor, the total inertia reduces by a factor of 11:1 at high frequency). This technique also comes with setbacks, as increasing motor size reduces acceleration and increases cost.

Some basic considerations to improve servo system performance are listed in the table below:

Improve Accuracy Improve Response Time Achieve Optimal Tuning Improve Resonance Problems
Select a more accurate feedback device Increase servo gains Select a drive with advanced tools such as dual feedback loops Make the machine stiffer, either with better transmission components
Put a secondary feedback sensor on the load side of the transmission Use advanced servo algorithms Select a drive with a capable auto-tuning algorithm Eliminate the transmission by using direct drive motors
Use direct drive motors Use feed-forward algorithms Use advanced anti-resonant filters

Estimated Reading Time: 4 minutes

Pete Stollberger

Area Vice President at Kaman Industrial Technologies
With over 20 years of experience in engineering, sales and management of industrial automation products & solutions, Pete joined Kaman's distribution segment as Vice President of Sales Process for Zeller Corporation.
Pete Stollberger
Share this post
  , , , ,


Join the Conversation