Internal Name: Sensorless Position without Drift

What’s New:  Sensorless determination of solenoid position has been based on AC inductance measurement, which suffers from poor signal-to-noise ratio.  Integrating inductive voltage to obtain flux linkage is an alternative way to get at inductance and position, but this approach suffers from integration drift.  The new approach merges the two techniques, resulting in a low-noise flux integral that is free from drift and that can be used, in conjunction with a current measurement, for computing position. 

Status: Issued U.S.  Oct. 9, 2001

Why:  In AC motor applications it is common to track winding flux linkage over time by using AC integration of inductive voltage.  The flux integral leads to sensorless determination of both armature position (or angle) and magnetic force (or torque).  In an AC situation with a rotating armature, the average flux linkage is known to be zero, so the flux linkage integral is simply caused to drift toward zero at zero frequency.  In solenoid control, there is no such symmetry of flux varying about a zero average, so the flux integral must be initialized to a realistic absolute value and then integrated over time – a process that is highly prone to drift problems.  Solenoid flux integration and flux control can be made to work over the very short time interval of a quick transition to soft landing, but this process fails when control must be extended in time, possibly even for long enough to temporarily maintain partial valve lift and controlled gas flow reduction in an engine valve solenoid.  Overcoming this drift problem opens the realm of sensorless position determination and motion control to a broader range of applications, including DC position determination and servo control of position down to DC.

How:  This invention takes advantage of the AC information naturally present in the current ripple induced by a PWM drive signal, using this drift-free but inherently noisy data in a narrow frequency band extending down to DC.  In our system, position is computed from the low-noise ratio of winding current to winding flux linkage, where flux linkage is the time integral of inductive voltage in the winding.  The drift-free inductance information derived from the AC PWM data is used to correct the drift of the flux linkage integral, extending the validity of this integral down to DC.  Because a slow flux integral drift rate corresponds to a low required bandwidth for drift correction, this process can be tuned to minimize the effect of high frequency noise inherent in the PWM-based inductance data.

Download: System to Determine Solenoid Position and Flux without Drift  U.S. 6,300,733  Bergstrom 

Magnesense LLC  Gorham,ME (207) 839-8637

©2009 Joseph Seale