Temperature compensated linear actuator and encoder

A linear actuator system comprising:

• a casing that contains a moveable shaft that is moved by a motor in response to a drive signal and is coupled to an encoder that determines movement of the shaft and forms part of a control loop that adjusts the drive signal so as to ensure accurate positioning of the shaft;
• a temperature sensor mounted in association with the shaft for producing a temperature signal indicative of instantaneously measured temperature of the shaft;
• and
• a temperature compensator responsive to the measured temperature for generating a negative or positive offset for correcting the drive signal so as to move the shaft to a position that is corrected for instantaneous expansion or contraction of the shaft owing to departures of the shaft's actual temperature from a known baseline temperature;
• wherein:
• the motor advances the shaft forward or backward and is configured for coupling to a driven mechanism that is external to the actuator;
• the encoder is a linear encoder that includes a reading head and scale and determines an actual instantaneous position of the shaft;
• the shaft is anchored at one end to the reading head;
• and
• the temperature compensator is responsive to a known geometry of the shaft for generating said offset;
• an output of the encoder is fed to an input of a first subtractor that produces at an output thereof an encoder difference signal;
• the output of the temperature compensator is coupled to a negative input of the first subtractor;
• the temperature compensator is responsive to the instantaneous temperature of the shaft as measured by the temperature sensor for determining a compensation signal, which when fed to the first subtractor produces a corrected encoder difference signal that takes into account the fact that the shaft has expanded or contracted from a nominal baseline length as established at a known baseline temperature according to whether the instantaneous temperature of the shaft is higher or lower than the baseline temperature;
• the corrected encoder difference signal is fed to a proportional-integral-derivative controller (PID) controller, which continuously calculates an error value ε(t) as the difference between a desired driver signal and the corrected encoder difference signal and minimizes the error over time by adjustment of a control variable u(t), corresponding to the position of the motor shaft to a new value determined by a weighted sum:
• u
• ⁡
• (
• t
• )
• =
• K
• p
• ⁢
• ɛ
• ⁡
• (
• t
• )
• +
• K
• i
• ⁢
• ∫
• 0
• t
• ⁢
• ɛ
• ⁡
• (
• t
• )
• +
• K
• d
• ⁢
• d
• ⁢
• ⁢
• ɛ
• ⁢
• (
• t
• )
• dt
• where Kp, Ki and Kd are all non-negative and denote the coefficients for the proportional, integral, and derivative terms, respectively.