Technical background In such an antenna device the scanning position, or scanning angle, needs to be accurately determined in or- der to be able to determine the lateral position of de- tected objects. For example, when provided in a car radar apparatus it is necessary to determine in what lane a de- tected obstacle is.

A known device, that could be used in the above an- tenna device, for detecting the scanning position at a high accuracy is a resolver sensor. A resolver sensor comprises two coils positioned at right angles in a mag- netic field. The distribution of the magnetic field on the two coils is detected and used as a measure of the scanning position. However, the construction of this re- solver sensor is complex rendering the sensor expensive.

This is particularly a drawback when applied in a car ra- dar apparatus.

Summary of the invention An object of the present invention is to provide an antenna device having a simple low cost yet accurate de- termination of the scanning position of the reflector; and a method for performing said determination.

The object is achieved by an antenna device and a method according to the enclosed claims.

In one aspect thereof, the invention relates to an antenna device comprising a scanning reflector, a fixed feeder interacting with the reflector for emitting radar radiation and a detector for detecting reflector passage of at least two different predetermined scanning posi- tions during scanning of the reflector. The antenna de-

vice further comprises timing means for determining pas- sage times at said passages, and prediction means for predicting the scanning position of the reflector at an arbitrary time on basis of said passage times and a pre- determined scanning motion of said reflector.

In another aspect thereof, the invention relates to a method for determining an scanning position for the scanning reflector of the above described antenna device.

The method comprises the steps of: -measuring passage times for the reflector, during reflector scanning, passing at least two predetermined and spaced scanning positions; -predicting the scanning position of the reflector at an arbitrary time by means of said passage times and a predetermined scanning motion of said reflector.

In accordance with the present invention, by arrang- ing for the measurement of passage times, at two or more spaced predetermined scanning positions, accurate rela- tionships between position and time are established. The scanning reflector of the antenna is operated in a prede- termined way as regards the motion thereof. Consequently, the momentary position of the reflector is theoretically known. However, due to mechanical and other deviations a degree of inaccuracy exists in practice. The invention substantially reduces that inaccuracy. By means of said theoretically known movement and the actual and precise time measurements at the predetermined, and accurately known, passage positions accurate predictions of scanning position at arbitrary times are enabled. By the expres- sion arbitrary times is to be understood other points of time than the measured passage times.

Further objects and advantages of the present inven- tion-will be discussed below by means of exemplary em- bodiments.

Brief description of the drawings Fig. 1 shows schematically, in top view an example of an antenna device of the type having a scanning re- flector; Fig. 2 shows schematically, in perspective view, an example of a scanning reflector as shown in Fig. 1 pro- vided with a detection device according to an embodiment of the invention; Fig. 3 shows a schematic diagram of circuitry for operation and control comprised in an embodiment of the present invention.

Fig. 4 shows a diagram of a typical movement of the scanning reflector.

Detailed description of embodiments As shown in Fig. 1, in a preferred embodiment an an- tenna device comprises a scanning reflector, or main re- flector 2, a fixed subreflector 4 and a fixed feeder 6 interacting with the reflectors 2 and 4 for emitting ra- dar radiation. The radar radiation originated from the feeder 6 has a vertical polarisation and is subjected to a first reflection by the subrecflector 4, reflecting the radiation towards the main reflector 2. Then, the radia- tion is subjected to a second reflection by the main re- flector 2, which additionally turns the radiation into a horizontal polarisation. Subsequently, the horizontally polarised radiation is emitted from the antenna device, since the subreflector 4 is transparent to horizontally polarised radiation.

The main reflector 2 scans reciprocatingly, i. e. it scans back and forth turning about a central axis, which is positioned at the centre of the feeder 6. This antenna device construction is attractive since, when turning the main reflector by an angle v the emitted radiation is turned by twice that angle, i. e. 2v. Further, the mass of the moving parts is small.

Further, the antenna device comprises servo means, as illustrated in Fig. 3, for the scanning operation of the main reflector 2. Said servo means may, for example, comprise a motor, a tachometer, etc., as disclosed in Swedish patent No. 9501706-7 and schematically indicated in Fig. 3.

In order to be able to determine the scanning posi- tion of the main reflector 2 at any point of time, in ac- cordance with this invention the antenna device is pro- vided with a detector for detecting reflector passage of at least two different predetermined scanning positions during scanning of the main reflector 2. The detector comprises activator means 8 and sensor means 10. The ac- tivator means 8 is arranged on the main reflector 2 and spaced from the central axis thereof, and is constituted by a fork shaped magnetic element 8 having first and sec- ond spaced protrusions 12,14. The sensor means, or sen- sor, 10 is fixedly positioned in the antenna device. More particularly, the sensor 10 is arranged below the main reflector 2 and offset from the central axis thereof so that the magnetic element 8 passes the sensor 10 during the scanning. Preferably, the sensor 10 is constituted by a Hall element 10. The Hall element 10 is connected to timing means 18 for determining passage times at said passages. The timing means is further connected to pre- diction means 20 for predicting the scanning position of the main reflector 2 at an arbitrary time, i. e. at any time but the passage times the positions of which are known. The timing means 18 comprises conventional cir- cuits such as counters, etc., needed for performing the tasks described herein. The implementation of the timing means 18 will be obvious for one skilled in the art and is, consequently, not disclosed in detail. This applies to the prediction means 20 as well. That is, the imple- mentation of the circuits of the prediction means 20 for performing the prediction calculations will be evident for one skilled in the art.

As is evident from Fig. 4, the main reflector 2 scans forth at a lower speed and substantially linearly, while it scans back at a higher speed but rather nonline- arly. Below, the substantially linear movement will be referred to as the primary sweep and the nonlinear move- ment will be referred to as the secondary sweep. Of course, in the vicinity of the turning points, denoted A and B in Fig. 4, no one of the sweeps is linear.

When the main reflector 2 is scanning, the protru- sions 12,14 passes the Hall element 10 one at a time. At every passage, the protrusion 12 or 14 activates the Hall element 10, which in turn generates a sensor signal that is input to the timing means 18. The timing means 18 de- termines two different passage times t1 and t2 respec- tively, at which the two sensor signals are received by the timing means 18 during a primary sweep. The passage times t, and t2 are then stored and used by the prediction means 20 for determining the scanning position, i. e. the scanning angle, of the main reflector 2 at an arbitrary time. The space between the protrusions 12,14 is pref- erably related to the distance covered by them during a primary sweep such that the passage positions, and conse- quently the passage times t, and t2, are well within the portion of the primary sweep assumed to be linear. Thus, the portion of the primary sweep between the passage times t, and t2 as well as a portion beyond each of the passage times ti and t2 are considered linear. Accord- ingly, the determinations of the scanning angles for ar- bitrary times within the linear portion of the primary sweep are based on the assumption of a predetermined lin- ear scanning movement. Thus, the momentary angle v at the arbitrary time t is determined by means of the following equation: v (t) = vl * (t2-t) + v2 * (t-tl) t2-t1 where v1 and v2 are the scanning angles at the passages, and, thus, at the passage times t, and t2 respectively.

In the illustrated example, the primary sweep is centred in relation to the positions of passage of the Hall element 10. In other words, the time period from the first turning point A to the first passage time t, equals the time period from the second passage time t2 to the second turning point B. However, this does not have to be the case. The primary sweep may be offset as well, e. g. in order to compensate for an inaccurate mounting of the antenna device. Such an offset is allowed due to the above described choice of relation locating the passage times well within the linear portion of the primary sweep.

Above a preferred embodiment of the present inven- tion has been described. This should be seen as merely a non-limiting example. Many modifications will be possible within the scope of the invention as defined by the claims. Below a few examples of such modifications will be given.

It is not necessary for the movement to be linear to permit a prediction of the scanning position. For exam- ple, it could be approximately sinusoidal etc. However, the movement has to be predetermined in order to enable an accurate prediction of the angle of the main reflector at an arbitrary time. Consequently, it would be possible to predict scanning positions during the above secondary sweep as well, which, by the way, is done in alternative embodiments of the present invention.

Several different detector arrangements are employ- able. In one alternative, two separate activating ele- ments are used, one at each side of the central axis of the main reflector, a separate sensor being associated with each of said activating elements. In another alter- native, two spaced sensors and an activator having merely one activating portion is used. The activator and the sensor may change places. Other types than magnetically alerted detectors can be used. However, the arrangement of the above described embodiment is preferred due to the

simple and reliable function and cost effectiveness thereof.

In other modified embodiments, more than two passage times are generated in order to further increase the ac- curacy of the prediction of the angle. However, this is to the cost of complexity. Consequently, the above de- scribed embodiment generating two passage times is pre- ferred. Due to a stable control of the movement of the main reflector, which control is achieved by the above described motor-tachometer device, the predictions are accurate enough though based on merely two passage times.

The applicability of the invention is not limited to the type of antenna described above, but it is applicable to all types of antennas having a scanning reflector.