Position detection using a spaced apart array of magnetic field

Details Position detection using a spaced apart array of magnetic field Position detection using a spaced apart array of magnetic field

1998-09-25

2000-09-26

Strecker, Gerard

Electricity: measuring and testing

Magnetic

Displacement

32420717, 32420724, 336 45, 34087032, G01B 702, G01B 714, G01D 520, G08C 1906

Patent

active

061247081

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to position encoders generally. The invention has particular although not exclusive relevance to non-contact linear position encoders.
2. Related Art
Many types of non-contact linear position sensors have been proposed. In particular, EP 0182085 discloses a non-contact position sensor which employs an excitation winding and one or more pick-up windings mounted on the surface of a stationary element, and a conductive screen mounted on a movable element. A homogenous a.c. magnetic field is established in the vicinity of the pick-up winding by passing a current through a generally planar excitation winding. The pick-up winding, normally consisting of one turn, starts at one end of the stationary element and follows a sinuous path therealong until it reaches the other end, where it turns back along the support following a sinuous path to the starting end. The sinusoidal forward and return conduction paths that form the pick-up winding are substantially 180.degree. out of phase. Therefore, the pick-up winding comprises a series of alternating sense conduction loops, each arranged to enclose a similar area.
If the area enclosed by each loop of the pick-up winding is identical, and there is a homogenous excitation drive field over the length of the pick-up winding then, in the absence of the conductive screen, there will be no net output from the pick-up winding. However, when the conductive screen is provided adjacent the pick-up winding the homogenous field generated by the current flowing in the excitation winding induces eddy currents in the conductive screen. These eddy currents establish a counter-field opposing the forward homogenous field. This opposing field alters the balance between the excitation winding and the pick-up winding and a net output EMF in the pick-up winding results, the magnitude of which is dependent upon the position of the conductive screen within a period of the pick-up winding. In particular, the peak amplitude of the output signal from the pick-up winding varies in a sinusoidal manner with the position of the conductive screen along the pick-up winding.
In order to determine the position of the conductive screen within a whole period of the pick-up winding, a second pick-up winding is provided which is in spatial phase quadrature with the first pick-up winding. With this arrangement two phase quadrature signals are generated, from which the position of the conductive screen within a period of the pick-up winding can be determined, independent of the amplitudes of the signals. Additionally, if the absolute position of the conductive screen is to be determined, then either a counter must be provided for counting the number of periods that have passed from a reference point or an additional coarse position encoder must be provided.
The present applicant has proposed in International Application WO95/01095 a similar position sensor, which employs a resonant circuit instead of the conductive screen. By using a resonant circuit the output signal levels are increased and the system can be operated in a pulse-echo mode of operation, i.e. applying a short burst of excitation current to the excitation winding and then detecting and processing the signal induced in the pick-up windings, after the burst of excitation current has ended. Pulse-echo operation is possible because the resonant circuit continues to "ring" for a short period of time after the excitation current has been removed. This offers the advantage of ensuring that there is no unwanted cross-coupling between the excitation winding and the pick-up windings.
Although use of a resonant circuit in the position sensor allows a pulse-echo mode of operation, this is not essential. When the resonant circuit is resonating, its impedance is purely resistive. Consequently, the electrical phase of the output signal with respect to the drive voltage is well defined, and the desired output signal can be isolated from any unwanted cross-coupling signal by syn

REFERENCES:
patent: 3772587 (1973-11-01), Farrand et al.
patent: 4014015 (1977-03-01), Gundlach
patent: 4820961 (1989-04-01), McMullin
patent: 4893077 (1990-01-01), Auchterlonie
Electronics Letters, vol. 11, No. 1, Jan. 9, 1975, pp. 5-6, Gordon, "Digital xy position indicator using walsh functions".

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