|
| |
| Home Sensing Accuracy | |
Providing an accurate, repeatable HOME point is difficult. Although the
Linear Actuators can provide an incremental resolution of a few nanoinches,
traditional mechanical switches for limit-of-travel/home sensing are only
accurate to a few mils at best. This means the HOME point could be in
error by 500,000 resolution elements! However, the 2000-family uses Hall-effect
switches for limit-of-travel and Home sensors. While the Hall-effect devices
are much more accurate than mechanical switches, they still exhibit inaccuracies
from the following sources:
In contrast to the stainless steel drivescrew, the actuator's housing is aluminum. The limit sensors are on the housing and the tripping magnets move with the drivescrew. Therefore, due to differential thermal expansion of these dissimilar metals, the position varies by temperature as a function of actuator travel. In the 2000-series structure, the housing-to-drivescrew joint is closest to the Reverse Limit sensor. In contrast, in the 2100/2200-series structure, the housing-to-drivescrew joint is far away from the Reverse Limit sensor. We calculated the following information from known coefficients of thermal expansion for the various materials used in the actuators. The error represents the apparent displacement of the thruster's Nose Button at both the FORWARD and REVERSE HOME sensor as the temperature changes.  | |
| Internal housing temperature increase from running motor: Since stepping motors continuously consume high current whether running fast or slow, they pump heat into the drivescrew and housing. Because it is a localized source of heat, the motor itself can produce distortions as the actuator temperature advances toward equilibrium. For example, on a 2200-1 Linear Actuator --
Initial Conditions: • Temperature: 70°F±10°F (21°C±5°C) • Actuator style: 2200-01-AM1524-41, Microstepping 8x Change 10°F room warm-up over 30 minutes Result Moved REVERSE sensor's effective position by 700µin (17.8µm). Change Holding in hand Result FORWARD LIMIT= 200µin. REVERSE LIMIT= 1500µin. 2. Hall-effect Switch Temperature Sensitivity The 2000-family Linear Actuators use the Allegro Model 3122 Hall-Effect Switch. Its operating point can vary 5 µin/°C. 3. Losing Step Synchronism under Excessive Load Typical of all stepping motors, if the external load exceeds the thrust capacity of the Linear Actuator, the motor will lose position by 4-step multiples. Under these conditions, HOME is not repeatable. 4. Microstep Inaccuracy Under Load Microstepping motor drivers vary the magnetic stator pole strengths to balance the rotor teeth somewhere between the cardinal steps. With many microstep subdivisions, the torque difference between adjacent microsteps may be smaller than system friction and load and the actuator will not move. Without external torque, a stepping motor's rotor is in a stable resting position compared to the stator. To hold a stepping motor's rotor in place against an external torque, the magnets generate a larger opposing torque as the displacement between rotor and stator becomes larger. The greater the displacement ) the greater the torque. If the external torque is higher than the motor's maximum torque, the rotor will slip to the next detent position. Similarly, to move against an external torque, the motor must generate a larger torque. This torque is a function of both the offset between the rotor and its rest state as well as the relative current differences between the poles. When microstepping, maximum available torque for a fixed current occurs at the normal full-step detents. At the electronically-created microsteps between the full-steps, the number of steps correspondingly reduces the available torque. If the external load is high enough, the motor may not even move. It may even rotate away from its expected position until the torque generated by the displacement exceeds the torque required. 5. Variable Biasing from Stray Magnetic Fields Although the external magnetic fields may not be strong enough to trip the sensor, any field will move the trip point. 6. Mechanical Vibration and Distortion Although the actuator housing is mechanically robust, you can easily experience HOME position errors while the housing flexes under an external driving mechanical vibration. | |