{"id":38011,"date":"2023-07-21T11:23:09","date_gmt":"2023-07-21T10:23:09","guid":{"rendered":"https:\/\/photonics.laser2000.co.uk\/?p=38011"},"modified":"2023-07-21T11:23:09","modified_gmt":"2023-07-21T10:23:09","slug":"enabling-peak-servo-performance-with-thermal-protection","status":"publish","type":"post","link":"https:\/\/stagingphotonics.laser2000.co.uk\/?p=38011","title":{"rendered":"Enabling Peak Servo Performance with Thermal Protection"},"content":{"rendered":"\n

Motion control systems with brushless DC, voice coil or three-phase direct drive actuators use a position servo controller that dynamically changes the current in each motor phase. These actuators are almost always driven with the minimum current required to maintain their trajectory. We can take advantage of this phenomenon by occasionally requesting brief bursts of relatively high current which can improve the system’s overall performance\u2014for instance, to keep up with a more aggressive trajectory or to overcome stronger disturbances. Luckily, in many applications only brief bursts are required to significantly boost performance. As an example, the figure below shows the typical motor phase currents in Zaber’s X-LDA stage<\/strong><\/a> during a move with very high acceleration.<\/p>\n\n\n\n

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\"Typical
Figure 1: Typical phase currents in Zaber’s X-LDA stage during a very aggressive trajectory, with just over 4g (40 m\/s\u00b2) of acceleration. Note that the X-LDA’s continuous current limit of 2.5 A is exceeded during acceleration and deceleration, but not when moving at constant velocity.<\/figcaption><\/figure>\n\n\n\n
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In order to maintain safe motor temperatures during these bursts of relatively high current, actuators that use a servo controller have a specified continuous (RMS) current limit that can be sustained indefinitely and an instantaneous (“overdrive”) current limit that may only be used for brief durations. How do we know when it is safe to use overdrive currents above the continuous limit? Simply put, we need to intentionally balance these bursts with sufficient periods of cooling in between. The most common strategy for doing this is to monitor the motor temperature, either by measuring it directly or by estimating it computationally.<\/p>\n\n\n\n

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Common thermal protection methods<\/h3>\n\n\n\n
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Motor temperature sensors<\/h5>\n\n\n\n

Temperature sensors that are integrated into the motor windings provide a direct reading of the motor temperature. In situations where heat builds up slowly in the motor windings and can be assumed to be uniformly distributed in the motor, integrated sensors provide the most accurate data. However, the time constant of thermal conduction across all phases in a motor winding is often longer than the brief bursts of overdrive currents. Hence, an embedded temperature sensor may not respond quickly enough to protect against localized hot spots created by high current loads in imperfectly uniform motor winding wires.<\/p>\n\n\n\n

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Thermal models<\/h5>\n\n\n\n

Thermal models attempt to estimate the thermal state of a system based on observable non-temperature variables. In most actuators, the primary sources of heat generation are resistive losses in the motor windings and Eddy currents resulting from alternating current in the windings. Both of these are related to motor current, so the temperature of a motor is usually estimated using the history of current driven into the motor and the motor’s thermal properties. Thermal model methods that require knowing the motor’s thermal properties are very common, but their main disadvantage is that thermal data is not readily available for most motors.<\/p>\n\n\n\n

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I\u00b2t<\/h5>\n\n\n\n

The industry standard I\u00b2t algorithm is a specific type of thermal model that is simplified to not require any explicit thermal parameters. In its most basic formulation, I\u00b2t simply defines an excess thermal energy pool that an actuator is allowed to use in addition to the thermal energy generated by the actuator’s continuous current. The name I\u00b2t is derived from the fact that the heat generated in a motor is proportional to the current squared. The algorithm simply monitors the square of current over time, and integrates the difference between instantaneous currents and the continuous current limit:<\/p>\n\n\n\n

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(1)<\/p>\n<\/div>\n<\/div>\n\n\n\n

In addition to motor current limits, the only parameter required for an I\u00b2t algorithm is the maximum allowed I\u00b2t integral, which essentially defines an amount of excess thermal energy allowed in the motor. When the I\u00b2t integral surpasses this pre-defined threshold, motor current is typically throttled back to the continuously permitted limit to prevent further heating.<\/p>\n\n\n\n

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Thermal protection in Zaber products<\/h5>\n\n\n\n

All of Zaber’s linear motor and voice coil products use the I\u00b2t algorithm for ensuring overdrive currents are used safely. As an additional level of protection, linear motor products also have a thermal switch integrated into the motor. The integrated thermal sensor helps protect against slow temperature rises in situations with sub-optimal heat sinking or from elevated ambient temperatures. The I\u00b2t algorithm mainly helps protect against relatively fast surges of heat generated by bursts of peak power.<\/p>\n\n\n\n

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Overdrive limits in Zaber’s ASCII Protocol<\/h5>\n\n\n\n

When powering linear motor and voice coil positioners, Zaber’s motor drivers maintain concurrent and completely independent I\u00b2t models for both the driver and the motor. The axis driver and motor have different current limits and I\u00b2t integral (“excess energy”) thresholds.<\/p>\n\n\n\n

The motor’s current limits are described by the following three settings:<\/p>\n\n\n\n