Angle digitiser

Claim 1

A system for producing a digital representation of the value of an angle 8 from two first analogue quantities respectively representative of A sin e and A cos θ, where A is an amplitude factor, comprising first means for deriving from the two first analogue quantities a plurality of n second analogue quantities which are functions of 8 such that in each of n contiguous portions of a total angular range for θ of 2 π radians, each portion extending over 2π/n radians, the value of a respective third analogue quantity which is derivable from two of the second quantities and which is independent of A lies within a predetermined range of values, second means, comprising comparison means, for deriving from the second analogue quantities a first digital quantity representative of which one of the n portions the value of θ lies in, third means, comprising selection means controlled by said first digital quantity and analogue-to-digital conversion means, for deriving a second digital quantity representative of the value of the respective third analogue quantity whose value lies within said predetermined range of values and thereby being representative of the value of e within said one portion, and fourth means for deriving from the first and second digital quantities a digital representation of a, characterised in that the third means comprise amplifier means operable to produce a plurality of n fourth analogue quantities respectively substantially representative of the logarithms of the second analogue quantities over at least a range of values thereof, and further comprise addition and/or subtraction means for deriving from the fourth analogue quantities each of the third analogue quantities the value of each of which lies within said predetermined range of values in at least one respective portion of the angular range for e of 2 π radians, the third analogue quantities being representative of the logarithms, over at least a range of values thereof, of quotients of respective pairs of the second analogue quantities.

Claim 2

A system for producing a digital representation of the value of an angle 8 from two first analogue quantities respectively representative of A sin e and A cos θ, where A is an amplitude factor, comprising first means for deriving from the two first analogue quantities a plurality of n second analogue quantities which are functions of 8 such that in each of n contiguous portions of a total angular range for θ of 2 π radians, each portion extending over 2π/n radians, the value of a respective third analogue quantity which is derivable from two of the second quantities and which is independent of A lies within a predetermined range of values, second means, comprising comparison means, for deriving from the second analogue quantities a first digital quantity representative of which one of the n portions the value of θ lies in, third means, comprising selection means controlled by said first digital quantity and analogue-to-digital conversion means, for deriving a second digital quantity representative of the value of the respective third analogue quantity whose value lies within said predetermined range of values and thereby being representative of the value of e within said one portion, and fourth means for deriving from the first and second digital quantities a digital representation of a, characterised in that the third means comprise amplifier means operable to produce a plurality of n fourth analogue quantities respectively substantially representative of the logarithms of the second analogue quantities over at least a range of values thereof, and further comprise addition and/or subtraction means for deriving from the fourth analogue quantities each of the third analogue quantities the value of each of which lies within said predetermined range of values in at least one respective portion of the angular range for e of 2 π radians, the third analogue quantities being representative of the logarithms, over at least a range of values thereof, of quotients of respective pairs of the second analogue quantities.

Claim 5

A system as claimed in any one of Claims 1 to 3 wherein the third analogue quantities are related to e by expressions of the form t log [tan (θ ± mπ/n)] where m is an integer.

Claim 6

A system as claimed in any preceding claim wherein the value of each of the third analogue quantities lies within said predetermined range of values in two respective portions of the angular range for θ of 2 π radians, said value varying with θ in opposite respective senses in the two portions.