Method and device for robust rejection of periodic disturbances in an axis-position control loop

A method for rejecting periodic disturbances in a motor-position control structure, the periodic disturbances corresponding to at least one sinusoidal oscillation at a determinable frequency proportional to an angular speed of a motor, the control structure receiving a position reference input signal c(t) implementing a first loop for controlling a position of an axis of the motor, said control structure being an RST controller and being sampled so that numerical computations can be made in a programmable calculator to produce results to control the motor, the RST controller implementing computing means corresponding to a function 1/So(z−1) upstream from the motor and producing a control signal u(t) and a function Ro(z−1) in the first loop which receives as an input a position signal θ(t) from a position sensor on said motor axis, z being an operator advance of one sampling period, R and S being polynomial functions in the RST controller, wherein an assembly including the motor, the position sensor, the means for computing the functions 1/So (z−1) and Ro(z−1) in the first loop, is called a central controller,

• the method comprising:
• adding to the central controller two additional computing means, the first one being an estimator block E(z−1) and the second one a modifier block Q(z−1), as well a second loop,
• modeling the motor as a modeled system subjected to a load disturbance v(t), said modeled system receiving the control signal u(t) as an input and producing the position signal θ(t) as an output, the system model applying a function z−1B(z−1) to the control signal u(t) to produce a signal added to the load disturbance v(t), the resultant being subjected to a function 1/A (z−1) to produce the position signal θ(t),
• computing a load disturbance estimation signal {circumflex over (v)}(t) in the estimator block E(z−1) that receives the control signal u(t) as a first input and the position signal θ(t) as a second input, and in which the control signal u(t) is subjected to the function z−1B(z−1) to produce a signal, which is then subtracted from the result of the application of the function A(z−1) to the position signal θ(t), this subtraction giving the load disturbance estimation signal {circumflex over (v)}, and
• applying the load disturbance estimation signal {circumflex over (v)}(t) to the modifier block Q(z−1) to produce a modified signal, which is applied to a negative input of an additional adder arranged upstream from the means for computing the function 1/So(z−1) and then forming the second loop, the modifier block having a transfer function that varies as a function of the at least one determinable frequency and which is computed in real or quasi-real time under a constraint that a position sensitivity function Sθp of the control structure comprises a notch for the at least one determinable frequency, with the central controller rejecting only static disturbances.