Servo System Application Tips

01 April 2007

Successful electric servo motion applications require attention to detail, from the system down to specific components. Some helpful tips and rules-of-thumb guidance can ease the way.

Electric servo systems provide the most advanced and precise motion control available for industrial applications. Servos excel in two distinct working modes: rapid point-to-point load positioning, and smooth, accurate trajectory control between points, as in surface contouring.

One characteristic of a maturing technology like servo motion is the body of preferred design approaches and application guidance it builds, which can offer end-users valuable ‘tips’ for implementing their specific projects. These tips range from rules-of-thumb (for inertia loads, proper grounding/shielding, and motor cooling) to sophisticated servo tuning algorithms. Several tips focus on the latter area because servo motion systems must be well-tuned to realise their full precision and dynamic performance.

Some application ‘art’ goes along with the science of making a successful servo motion system. A logical way to start is to look at the overall system.

Purchase as a complete system
Buying the motion controller, drive, and servo motor designed to work together—as a system—will eliminate
numerous problems, including wiring, configuration, and communication issues, according to George Ellis at Danaher Motion. One complication with separately sourced components is connecting leads, which might have different sequences at the motor, drive terminals, and feedback devices (encoder, resolver, Hall sensor, etc.).

‘Swap two wires and the motor might seize up or spin out of control,’ says Mr. Ellis. ‘Similarly, get one of many configuration parameters in the drive or controller wrong, and poor system performance may result for no obvious reason.’ Compatibility of set-up and tuning software also favours getting both motor and drive from one vendor.

Tuning is crucial
‘Tuning is the process of setting several gains—typically between three and five—to get fast, stable response without excessive noise,’ continues Mr. Ellis. However, tuning servo systems can be confusing, due more to unfamiliar principles than complexity.

‘Avoid a hit-or-miss tuning method, where gains are adjusted up and down in hope of obtaining good response,’ he suggests. Without a tuning plan, gain setting can get out of control. ‘The servo vendor can provide tuning procedures for its products appropriate to the user, without the need for advanced knowledge in control theory,’ he adds.

John Mazurkiewicz, of Baldor Electric, notes that today’s servo controls allow manual or autotuning. Seasoned engineers typically tune manually, although it takes longer to perform.

For autotuning, he advises first a noload trial, then tune under load, once the user becomes familiar with the
control, load behaviour, and load location. ‘Controls typically autotune current and velocity loops, however,
Baldor’s controls tune the position loop—making setup even easier. Full autotuning is usually easily accomplished in ten minutes,’ he says.

Beyond default parameters
A wide range of servo tuning techniques exists. However, machine tool builders often load only default parameters into their servo controls without tuning, notes Paul Webster of GE Fanuc. The machine may run well and nothing more is done. ‘Default servo settings are established to give basic performance in general cases. Just doing basic servo tuning can bring significant increase in machine performance,’ he says.

Beyond the default stage, initial parameter settings for high precision (and high speed) are available via
‘oneshot function,’ said to be easily set in Fanuc’s Series 0i-C low-end controllers, or by Servo Guide PC-based software on more advanced series Fanuc CNCs. The next important tuning step to improve servo performance is setting of velocity gain and resonance elimination filters using a Bode diagram, he explains.
Tuning software, such as Servo Guide, aids the process.

The Bode diagram evaluates servo system stability by analysing frequency response of a control loop relative to its gain magnitude (measured in dB) and phase angle (deg.). Important results to look for include wide as possible bandwidth (near constant gain vs. frequency), with values under 10 dB, and gain margin frequency roll-off under -20 dB, says Mr. Webster. Filter tuning is effective for resonance elimination when
positive frequency spikes are removed on the Bode plot and frequency roll-off is held under -10 dB.

To obtain full performance from servo systems, GE Fanuc provides advanced tuning functions on its CNCs.
Noteworthy is a dynamic learning function that automatically reduces quadrant protrusion (or lost motion) in
CNC machining applications. Quadrant protrusion (QP) consists of backlash—due to physical clearances between machine parts—and system ‘springiness,’ also known as wind-up. The two distinct lostmotion
components combine to act as system delay, producing a QP effect on the circle graph (see diagram).

‘The feature we use to remove quadrant protrusion is called 'backlash acceleration,’ which tunes backlash
amount and adds a correction to the velocity command for lost-motion factors,’ Mr. Webster says. Dynamic
tuning occurs in successive steps—until the user or application is satisfied with the motion path accuracy. ‘Once started, the learning process runs by itself and, in a sense, the CNC learns from its initial
errors,’ he adds.

Other servo system vendors also offer some form of lost-motion correction.

Don’t forget load inertia
The ratio of load inertia to motor inertia is an important consideration. Different rules of thumb exist, with load/motor inertia ratio of 10:1 (or less) often recommended—for example, by Baldor Electric. ‘Some may suggest higher ratios, but the point is to limit the range of 'inertia mismatch,’ to allow tuning to be accomplished easier,’ says Baldor’s John Mazurkiewicz.

Bosch Rexroth Corp. similarly advises examining inertia magnitude of the ‘driven load.’ Quick speed changes and positioning become very difficult if load inertia is high compared to the motor’s rotor inertia, explains Brian Van Laar, senior applications engineer at Bosch Rexroth. ‘In some cases the load may
actually drive the motor during deceleration, causing overshoot and long settling times,’ he says. The
company recommends the following 'good standards’ for inertia mismatch: <2:1 for quick positioning, <5:1 for moderate positioning, and <10:1 for quick velocity changes.

One way to improve inertia mismatch is to increase gear ratio or ball screw pitch, if the application allows, suggests Mr. Mazurkiewicz. This has the effect of reducing load inertia magnitude as reflected at the motor. Select gear ratio, screw pitch, and motor at same time, and don’t leave motor sizing as an afterthought.

On the other hand, much higher inertia ratios can be accommodated by proper tuning. For example, Baldor
mentions the case of manually tuning a servo system with a 144:1 inertia ratio mismatch for a packaging machine. However, it took six hours of tuning to fully satisfy the customer. Reduction in response can be expected at higher inertia mismatch values.

Pay attention to that motor
Mr. Mazurkiewicz cites several common issues found in servo motor installations:

Motor not reaching speed results from insufficient voltage available. Select motors with at least 10% voltage
‘headroom’ allowance to handle low-line conditions; and verify voltage by measurements at the motor.

Insufficient torque may be due to underestimated load (motor selected was too small) or magnet demagnetisation (with ferrite magnets). ‘Demag’ is easy to check via voltmeter or scope: Measure voltage with the motor running at a test speed, then back-drive it at the same speed using another motor. If output voltage is not the same, the test motor is demagnetised. Remagnetisation must be done by the motor manufacturer, but check and correct what caused demag so it does not recur.

Overheating of servo motors comes from excessive torque demand— indicating motor undersized for the
application—or presence of current ripple (check with a scope). ‘Ripple may be caused by improper tuning or improper alignment of the feedback device,’ says Mr. Mazurkiewicz. He warns about ‘measuring’ motor temperature by the touch of a hand. The housing temperature of a properly sized brushless servo motor will be 100-125 °C.

Feedback devices
For resolvers, verify resistance with an ohmmeter, checking through motorconnector as well as connected cable. For encoders, use a 5 V supply and scope to check channels A and B for 5 V square waves measured between ±A and ±B signals. Do not coil up feedback cabling.

Some motor controls offer an
alternate way to check feedback devices. For example, Baldor Series II drives include a ‘feedback fault enable’ function to check for missing signal complements and can set a ‘fault.’

Cool motor = best performance
Internally generated heat affects all motors negatively. However, direct-drive motors (rotary and linear) also become a heat source affecting the accuracy of high-precision production machines they serve, because of their physical integration into the machine structure. As a result, direct-drive motors (DDMs) are mostly liquid cooled for optimal heat control. DDMs are often found in high-speed metal cutting machines, including laser type, wood-working machinery, and gantry stacker/destacker systems.

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