Reaction force compensator

Reaction force compensation means 70 (FIG. 1) for countering the reaction torque of a heavy body, such as rotatable yoke assembly 20, when accelerated relative to flimsily supported body 21 about axis 22, comprises a reaction member 71 rotatable about the, or a parallel, axis 22 and a reaction drive motor 74 coupled to rotate the reaction member by way of velocity step-down gearing 75. In response to acceleration of the body 21 the reaction drive motor 74 accelerates member 71 in the opposite direction producing an equal and opposite reaction torque on the structure. The reaction member is made with a higher moment of inertia about axis 22 than the body and is accelerated at a correspondingly lower rate to give the balancing reaction torque while requiring a smaller mechanical power input than might be expected. The power required of the drive motor is minimized by optimizing the gearing ratio having regard to the moments of inertia of the reaction member and drive motor.

This invention relates to reaction force compensation and in particular to 
means for substantially eliminating the reaction force exerted by a body 
on a supporting structure when accelerated relative thereto. 
It is well known for a machine structure to support a drive element such as 
a motor fixed to the structure and having a movable armature coupled to 
move a load. When a force is applied by the motor to the load a reaction 
force equal in magnitude is exerted by the motion in the opposite sense in 
the support structure. 
Steps usually have to be taken to prevent consequential movements of the 
support structure. Often this is readily achieved by virtue of the weight 
of the structure and frictional engagement with a machine bed or by normal 
fixtures. However, it is sometimes required to permit limited movement of 
the structure, such as when using resilient mountings to damp vibrations 
within the structure, or where the structure is carried by support means 
which is required to fulfil other criteria it may be unsuited to carry 
both the weight of the structure acting vertically and a reaction force 
possibly acting in a different direction. 
Whilst the invention to be disclosed herein is applicable in the widest 
sense to bodies able to move rectilinearly or rotationally the following 
description of the invention and the background thereto will in general be 
restricted to rotation about an axis. 
In a machine structure containing a body continuously rotating at a 
constant speed the angular momentum of the body is constant. Any reaction 
forces, torques, on the support structure are due to changes in the 
angular momentum of the body, perhaps due to rotary imbalance, and can be 
countered by effecting corresponding changes in the angular momentum of a 
balance shaft rotated with the body. Because the magnitudes of such 
changes are usually much less than the magnitude of the body momentum the 
balance shaft may have a much smaller angular momentum requiring little 
mechanical power to maintain its rotation. 
Other sources of change of angular momentum which give rise to reaction 
torques are friction and variation in the body inertia by changing load. 
Often a feedback system is employed to measure the reaction torque and 
cause the application of an opposing compensation torque. Such systems are 
by their nature complex to implement, depending upon the nature of the 
system, but again in general provide relatively small reaction torques to 
compensate for relatively small changes in body momentum. 
One example of such an arrangement is shown in UK Patent Application No. 
2085637 where a turntable rotating at a nominally constant velocity and 
possibly coupling motor vibrations to a plinth in the form of reaction 
torques is coupled to an identical turntable rotating in an opposite 
direction. The turntables have equal moments of inertia and the speeds are 
coupled to provide cancelling reaction torques. 
Where the body is massive and speed variations are large and/or rapid, for 
example, if the body is accelerated at a high rate from rest for each 
movement, then it will be seen that such conventional techniques of 
duplicating the body by a contra-rotating reaction member would require a 
large input of mechanical power purely to reproduce the body motion in the 
reaction member. 
Design techniques have evolved to enable energy required for the 
contra-rotating reaction member to be supplied over a much longer period 
of time, thereby reducing the input power requirements, by rotating the 
reaction member as a flywheel and using braking thereof to change its 
momentum. It will be appreciated that such a technique still requires 
additional design and construction to accommodate a continuously rotating 
flywheel and is limited to rotary motion and motion in one direction 
without further complexity in the way of transmission gearing. 
It is an object of the invention to provide for a structure supporting a 
body accelerated between constant velocities reaction force compensation 
means which enables adoption of a simple construction with a lower power 
requirement than hitherto. 
According to the present invention a structure supporting a body capable of 
undergoing acceleration about an axis of the structure includes reaction 
force compensation means comprising a reaction member supported by the 
structure for rotation about said axis having a moment of inertia greater 
than the body, and reaction drive means, responsive to acceleration of the 
body to accelerate the reaction member about said axis simultaneously with 
the body in the opposite direction thereto by way of velocity step-down 
coupling means and at a lower acceleration rate, arranged to produce a 
rate of change of angular momentum in the compensation means such that the 
torsional reaction forces of the body and compensation means on the 
structure are equal and opposite.

In such a radiation path steering system the path may be that of a beam of 
radiation transmitted from the system and/or the axis of a field of view, 
or sightline, by which radiation is received. For convenience the 
following description relates to a sighting system and steering of a 
sightline. 
Referring to FIG. 1 a steerable sighting system 10 is carried by a platform 
P which may be a moving vehicle. The system is arranged to direct a 
sightline axis S about a first, nominally vertical, azimuth axis to define 
a sightline azimuth direction and about a second, nominally horizontal, 
elevation axis to define the sightline elevation. The system comprises a 
reflector structure 11, including a plane reflector 12, pivotable about 
the elevation axis 13 in a yoke 14 by a direct drive elevation motor 15 
carried by the yoke and the reflector structure 11. The yoke and reflector 
structure also carry the relatively movable parts of an angular position 
sensing transducer 16, which provides signals giving a measure of the 
angular orientation of the reflector about the elevation axis, and of a 
tachometer 16' which provides signals representing the angular velocity of 
the motor. The yoke 14 has a yoke shaft 19 extending therefrom at right 
angles to the elevation axis 13. This body formed by the yoke 14, shaft 19 
and the reflector structure 11 carried thereby may conveniently be called 
the yoke assembly 20. 
The body, or yoke assembly, is supported in a structure comprising a 
support member 21 mounted on an annular base member 17 by a plurality of 
legs 18 disposed around the periphery thereof. The support member 21 
carries the yoke assembly suspended therefrom by way of the shaft 19 which 
is supported for rotation about the nominally vertical azimuth axis 22 by 
bearings 23. 
The legs 18 have a minimal thickness in the azimuth direction, that is, in 
a direction transverse to the radial sightline direction, to minimise 
obscuration. A plurality of low obscuration bracing elements, such as rods 
or wires (not shown) extend diagonally between the support member and/or 
base member and/or legs to provide torsional stiffness to the support 
member with respect to the platform P about the azimuth axis. 
The yoke assembly is driven in rotation about the axis 22 by a motor 24. 
The motor is direct drive, the stator being carried by the support member 
21 and rotor by, and coaxially with, the shaft 19. The support member 21 
and yoke assembly also carry the relatively movable components of an angle 
resolver 25 and a rotation rate sensing tachometer 25'. 
The sighting system may conform in essence to the arrangement shown in 
British patent specifications Nos. 1,491,117 or 1,559,218 in which the 
reflector structure 11 has a limited rotation about the elevation axis 13, 
the sightline S extending vertically along the azimuth axis 22 being 
deflected always to one side of the elevation axis by the reflector such 
that the azimuth direction of the sightline is determined by rotation of 
the yoke assembly through a 360.degree. field. 
Motor supply and signal connections between the support plate and the yoke 
assembly reflector structure may be made by slip ring means permitting the 
yoke assembly unlimited rotation or the rotation may be limited to the 
order of 360.degree. enabling said electrial connections to be made by way 
of cables 26 extending along an axial passage 27 of the yoke shaft 19, said 
cables being clamped to the yoke adjacent one end of the passage and by 
clamping means 28 to the support member adjacent the other end. The cables 
are arranged to be unstressed when the yoke assembly is at a datum point in 
respect of rotation about the azimuth axis. It will be appreciated that the 
total extent of rotation of the yoke assembly about the azimuth axis 22 is 
limited by cables 26 but a distance of the order of 10-15 cms. between 
clamping points is sufficient to permit rotation in excess of 
.+-.180.degree. with only tolerable torsional forces applied to the 
cables. 
A system employing cable connection between support plate and yoke assembly 
may be arranged with the reflector 12 rotatable to a greater extent about 
the elevation axis 12, being able to be `flipped` to an equal but opposite 
inclination with respect to a normal to the azimuth axis causing the 
sightline along the azimuth axis to be deflected to the other side of the 
elevation axis and displaced in azimuth by 180.degree.. It will be 
appreciated that rotation of the yoke assembly may be limited to between 
180.degree. and 360.degree., the above feature enabling the sightline to 
be directed over the full 360.degree.. Furthermore, relationships can be 
produced to enable a demanded sightline azimuth direction to be achieved 
rapidly by combined movement of the yoke assembly in azimuth and reflector 
in elevation. This particular construction represents only another possible 
form of yoke assembly comprising the body and beyond that forms no part of 
the present invention requiring further description. However, details 
relating to its construction and operation as a sighting system are 
contained in a copending application. 
Irrespective of the yoke assembly variants discussed above it is normally a 
requirement of a sighting system that the sightline direction be altered 
rapidly, usually from rest. 
To produce such rapid motions the elevation and azimuth motors need to be 
powerful requiring the yoke assembly construction to be robust. It will be 
seen that the yoke assembly may readily have a large amount of inertia 
about the azimuth axis and the torque required from motor 24 to accelerate 
the yoke assembly causes an equivalent reaction torque in the support plate 
about the azimuth axis. 
By contrast, the legs have minimal dimensions in the direction of yoke 
assembly rotation and may be subject to defomation by the reaction torque 
transmitted through the support member 21. 
Reaction force compensation means, in this case torque reaction means, is 
shown generally at 70 in FIG. 1 and comprises a reaction member, or 
flywheel, 71 supported on the support member 21 by bearings 72 coaxially 
with the yoke assembly for rotation about the azimuth axis 22, and 
reaction drive means 73 comprising a drive motor 74 and velocity step-down 
coupling means 75. The reaction motor 74 is fixed to the support member 21 
with its rotational axis 74' parallel to, but displaced from, the azimuth 
axis 22 and the coupling means comprises a toothed pinion 76, forming a 
drive member carried by a rotatable shaft 77, of the reaction motor which 
meshes with a corresponding toothed peripheral face 78 of the reaction 
member. 
In one mode of operation described briefly hereinafter, the rapid motion is 
achieved by configuring the motor 24 as part of an acceleration servo loop, 
the yoke assembly being accelerated for part of the rotation towards the 
new sightline azimuth and decelerated for the remainder. Furthermore the 
yoke assembly is accelerated from, and brought to, rest for each 
displacement motion and subsequent motion may be in either sense. 
The reaction member 71 has a moment of inertia about axis 22 larger than 
that of the yoke assembly and is rotated at a slower rate by reacting 
motor 74 through coupling means 75. 
The coupling between the reaction motor 74 and reaction member 71 is a 
single stage step-down gearing such that the reaction motor rotates in the 
opposite sense at a higher speed than the reaction member, the ratio being 
chosen as near as is practicable to that offering maximum reaction for 
minimum energy input to the motor. 
Considering now the rotational motions of the yoke assembly and the 
reaction member, it will be appreciated that to counter the reaction 
torque produced in the support member 21 about axis 22, the reaction 
member must be accelerated about the axis 22 in the opposite rotational 
sense in synchronism therewith such that the reaction torque exerted by 
the reaction compensation means, the combination of reaction member and 
the drive motor, is equal in magnitude to the drive torque provided by the 
yoke assembly motor 24. 
Considering initially that the reaction compensation torque is provided 
solely by the reaction member, it will be appreciated that the yoke 
assembly in rotating has an angular momentum J.sub.Y 
=I.sub.Y.multidot..omega.Y, where I.sub.Y is the moment of inertia of the 
yoke assembly about the axis 22 and .omega.Y its angular velocity, and a 
drive torque T.sub.Y, given by the rate of angular momentum 
d(J.sub.Y)/dt=T.sub.Y =I.sub.Y .multidot.d.sub..omega.Y /dt. 
Similarly it will be seen that for the reaction member 71, the angular 
momentum J.sub.R =I.sub.R .multidot..omega..sub.R and its rate of change, 
T.sub.R =I.sub.R .multidot.d.omega..sub.R /dt. 
As stated, the principle of operation is that the reaction torques, or 
rates of change of momentum inducing them, must balance so that 
EQU d.sub..omega.R /dt=(I.sub.Y /I.sub.R).multidot.d.sub..omega.Y /dt 
As I.sub.Y &lt;I.sub.R by choice then d.sub..omega.R /dt&lt;d.sub..omega.Y /dt in 
the same ratio, and after any time t in motion from rest .omega..sub.R 
&lt;.omega..sub.Y in the same ratio. 
Considering the mechanical power input to achieve the rotation, this may be 
expressed as (torque.multidot.angular velocity). 
For the yoke assembly this is T.sub.Y .multidot..omega..sub.Y and for the 
reaction member, T.sub.R .multidot..omega..sub.R and given the above 
conditions that T.sub.R =T.sub.Y and .omega..sub.R &lt;.omega..sub.Y it will 
be seen that the mechanical power required to accelerate the reaction 
member is .omega..sub.R /.omega..sub.Y or I.sub.Y /I.sub.R times the power 
required to rotate the yoke assembly, I.sub.Y, of course, being less than 
I.sub.R. 
It will be appreciated that the reaction motor 74 has to provide less 
mechanical power than the yoke assembly drive motor 24 and consequently 
may be made a smaller electrical component of lower electrical power 
rating. 
However it will be further appreciated that the motion of the reaction 
compensation means 70 includes rotation of the reaction motor 74 
(including the drive member 75) in the opposite direction to the reaction 
member 71 and the angular momentum of the reaction motor reduces the 
effective angular momentum of the reaction member in producing the 
reaction torque. 
Because the motor and reaction member axes 74' and 22 are parallel it does 
not matter that the angular momentum of the reaction compensation means is 
divided between the two members rather than concentrated into a single 
member as the torque is still effected by the rate of change of their 
combined angular momenta. 
It is known that where an electric motor accelerates itself and a load 
(such as the reaction member) by way of intermediate gearing of ratio n 
the motor torque T.sub.M accelerates both motor inertia I.sub.M and load 
inertia I.sub.L. The torque referred to the load is n.multidot.T.sub.M and 
the total moment of inertia (referred to the load) I=I.sub.L +n.sup.2 
.multidot.I.sub.M so that the acceleration of the load is 
EQU d.omega..sub.L /dt=T/I=n.multidot.T.sub.M /(I.sub.L +n.sup.2 
.multidot.I.sub.M) (1) 
The power delivered to the load is a maximum if the load acceleration is a 
maximum. By differentiating the expression for d.omega..sub.L /dt with 
respect to n and equating to zero, a maximum value may be established for 
n=(I.sub.L /I.sub.M).sup.0.5. 
When such a motor and load accelerate equal and opposite reaction torques 
are generated in the structure. The reaction torque due to the motor is 
given by 
EQU d(.omega..sub.M .multidot.I.sub.M)=n.multidot.I.sub.M 
.multidot.d.omega..sub.L /dt, 
referred again to the load, and the reaction torque due to the load given 
by I.sub.L d.omega..sub.L /dt. 
The total reaction torque is therefore 
EQU (I.sub.L +n.multidot.I.sub.M).multidot.d.omega..sub.L /dt (2) 
using the above expression (1) for d.omega..sub.L /dt in expression (2), 
the total reaction torque is given by 
EQU T=(I.sub.L +n.multidot.I.sub.M).multidot.n.multidot.T.sub.M /(I.sub.L 
+n.sup.2 .multidot.I.sub.M) 
Differentiating this with respect to n to obtain a maximum gives 
EQU n.sup.2 -2n-I.sub.L /I.sub.M =0 
or 
EQU n=1.+-.[(I.sub.L /I.sub.M)+1].sup.0.5. 
As the single stage of gearing introduces a direction reversal this 
produces a solution for the optimum value of 
EQU n=[(I.sub.L /I.sub.M)+1].sup.0.5 -1. 
In the present apparatus the reaction member comprises the load such that 
I.sub.L =I.sub.R ' and the optimum value of n is given by n=[(I.sub.R' 
/I.sub.M)+1].sup.0.5 -1. 
It will be seen that this is somewhat smaller than suggested by the simple 
relationship but as the moment of inertia of the reaction member is 
increased with respect to that of the drive motor, the expression becomes 
closer to the simple relationship. It is found in practice that the 
transmitted power varies with gear ratio only slowly for a wide range of 
values of n so that value for n may be chosen which is substantially 
optimum while apparently being a long way from the theoretically optimum 
value. In any event, by the use of a suitable ratio, corresponding to, or 
near, the optimum value the energy requirements of the reaction motor 74 
can be minimised. 
It will be appreciated that the drive motor may be coupled to the reaction 
member by way of coupling means comprising intermediate gearing, the 
angular momentum of which is taken into consideration. For a single 
intermediate gear which would rotate in an opposite sense to the drive 
motor and reaction member, which now rotate in the same sense, the 
effective moment of inertia I.sub.R comprises 
EQU (I.sub.R' +n.sub.M .multidot.I.sub.M -n.sub.G .multidot.I.sub.G), 
where I.sub.G is the moment of inertia of the intermediate gear and n.sub.G 
and n.sub.M are the gear ratios of the gear and drive motor relative to the 
reaction member, and from which an optimum value of gear ratios n can be 
determined, although an optimum ratio will generally be higher than for 
the single step drive considered above. 
It will be appreciated that there are practical considerations in respect 
of the gearing ratio and possibly on the dimensions and rotational rates 
of the reaction components but a gear ratio of the same order of magnitude 
as the optimum n should be achievable. 
The coupling between the reaction motor pinion and reaction member may be 
other than meshing teeth, such as frictional contact, and engagement may 
be other than by an outer peripheral wall with the engagement plane 
parallel to the azimuth and reaction motor axes. 
The reaction motor 74 may also differ from that shown. The reaction motor 
may be formed coaxially with the yoke shaft 19 and azimuth axis 22 and by 
means of suitable reduction gearing drive the reaction member by coupling 
to, or adjacent to, the inner peripheral wall of the reaction member. 
Alternatively, or in addition, the reaction member may be rotatable about 
an axis displaced from, but parallel to, the azimuth axis 22 rather than 
coaxially with it. 
As state above one motion required of the yoke assembly in which the 
reaction compensation means employed is when, to achieve a large yoke 
assembly rotation, it is accelerated to minimise the time of rotation. 
The yoke assembly drive motor 24 is configured in a servo control loop as 
shown schematically in FIG. 2, which also shows the control arrangement 
for the reaction motor 74. 
The azimuth and elevation motors 24 and 15 essentially have identical 
control circuitry configuration and to simplify description it is confined 
to references to the azimuth motor. 
The azimuth motor 24 is a d.c. torque motor supplied with drive current by 
a motor drive means, such as an amplifier 35, under the control of an 
analog servo control circuit 36 containing conventional components having 
proportional and/or integrating and/or differentiating transfer functions 
as is well known in the art. 
The current supply from amplifier 35 to the motor is fed by way of low 
value resistance element 37, across which are connected input leads to an 
acceleration sensing device in the form of voltage sensing means 38, such 
as a differential input voltage amplifier, which produces an output signal 
on line 39 proportional to the current supplied to the motor and thus the 
torque developed by, and angular acceleration of, the motor. 
The tachometer 25' produces a signal on line 40 proportional to the 
rotation rate of the motor. 
The angle resolver 25 comprises a high accuracy synchro with dual windings 
giving 1 speed (coarse, low accuracy) and 36 speed (fine, high accuracy) 
outputs on lines 41, 41' connected to separate input channels of a 
multiplexer/analog-to-digital converter (ADC) 42. The multiplexer/ADC 
produces digitised motor angle signals on bus 43 connected to an input of 
digital processing means, conveniently a microprocessor 44, multiplexing 
signals for switching between channels being provided on a line 45 from 
the microprocessor. 
A sightline azimuth demand signal is applied on line 46 e.g. by manual 
adjustment of a potentiometer voltage or as an output of other equipment 
such as tracking radar, to an ADC 47 and thence by input bus 48 to an 
input port of the microprocessor. An outut bus 49 connected to an output 
port of the microprocessor provides processed signals to a 
digital-to-analog converter (DAC) 50, analog output signals of which are 
connected by line 51 to an input terminal 52 of the servo controller 36. 
The microprocessor is of conventional design requiring no further or more 
specific description and has stored in the storage medium thereof a 
program through which the CPU processes received signals representing the 
above discussed angular values in accordance with the predetermined 
relationships and normal servo control relationships. 
The rate and acceleration feedback signals from tacho 25' and acceleration 
sensor 38 applied to the analog servo controller may be employed in known 
ways to modify the processing of position error signals. 
The analog output of tacho 25' is also applied by way of an ADC 64 to the 
microprocessor input port and the analog outputs of motor current 
(acceleration) measuring device 38 is also applied by way of an ADC 65. 
The analog servo controller 36 whilst essentially analog in respect of the 
transfer functions applied to the respective inputs may be configured as 
to which transfer function is employed by switching means, associated with 
each transfer function, and operated in accordance with configuration 
signals received from the microprocessor on line 63. 
For instance, the sightline may be caused to track at a demanded rotation 
rate, the microprocessor 44 providing a rotation data demand signal, by 
way of DAC 50 to input 52, the servo controller deriving from this and the 
tacho signal a rate error signal which is applied to motor drive amplifier 
35. Similarly the sightline may be caused to rotate at a demanded 
acceleration rate. 
In respect of operation as an angular position feedback servo, the feedback 
signals from angle resolver 25 are compared with the demand signal in 
digital form in the microprocessor 44 to produce a position error signal 
therein which in analog form is applied to the analog servo controller not 
as a demand signal but as a position error signal which is passed by the 
controller 36 to the motor drive amplifier. 
The microprocessor 44 is programmed however such that when the displacement 
demanded of the yoke assembly is determined the magnitude is compared with 
a predetermined threshold limit. If it is below the threshold, the yoke 
assembly is positioned as described above using feedback from the angle 
resolver 25. If it is above the threshold the microprocessor establishes a 
`fast slew` mode. The microprocessor 44 produces a configuration signal to 
switch the servo controller to an accleration servo configuration and 
produces to input 52 thereof an acceleration demand signal. This may be 
preset being a known fraction of the maximum acceleration of the drive 
motor. If the maximum is not known an output in excess of possible maximum 
may be provided initially and the actual maximum received from the means 
38. The analog servo controller 36 configured as an acceleration servo 
responds to feedback signals from the measuring device 38 to drive the 
azimuth motor 24 exerting a predetermined torque towards the demanded 
position. The instantaneous angular position as provided by angle resolver 
25 is monitored and when the angular position error has been reduced by a 
preset fraction, say one half, the microprocessor causes an acceleration 
demand signal to be produced where by the drive current to the motor is 
reversed to decelerate the motor. 
The microprocessor continuously monitors the magnitude of the remaining 
rotational distance and, by the tachometer 25', the rate of rotation and 
provides at its output port, and subsequently at controller input 52, an 
acceleration demand signal which decreases at such a rate that the motor 
will be brought to rest at the demanded angle. The deceleration rate 
(d.omega./dt) is determined from the motion equation 
(d.omega./dt)=-.omega..sup.2 /(2.multidot..epsilon.) where .epsilon. is 
the angular displacement error. 
The microprocessor may be programmed with the further step of configuring 
the servo controller to the `angle` mode when the remaining distance has 
been reduced below said threshold level, thereby optimising its response 
over the final and slower part of the motion. 
The reaction motor 74 is shown in FIG. 2 receiving motor acceleration 
signal from the output of the acceleration-configured servo-controller 36. 
The motor drive amplifier 79 inverts and amplifies the signals to provide a 
reaction motor drive current in the opposite sense and proportionally lower 
than that applied to yoke assembly drive motor 24. 
The torque reaction balancing system thus far described is an `open loop` 
system in that the current applied to reaction motor 74 is a constant 
fraction of that applied to the yoke drive motor 24, being a function of 
the relative inertial masses of the reaction balancing system and the yoke 
assembly 
If desired account may be taken of mechanical or environmental 
imperfections of the arrangement by forming a closed loop control system. 
In the sighting system of FIG. 1 described above it is desired to eliminate 
any relative rotation between the annular base member 17 and the support 
member 21 about azimuth axis 22. 
A simple feedback control loop may be provided by measuring such relative 
movement and modifying the current supplied to the reaction motor 74. 
It will be appreciated that many forms of such measuring apparatus exist 
although not all may be suitable. For instance, in detecting relative 
motion displacement sensors are more attractive than rate sensors but can 
give rise to noise and loop stability problems. Furthermore care is 
required in introducing integral action into the control loop as this can 
lead to a non-zero output in a quiescent state with undesirable rotation 
of the motor. The availability of suitable sensors may further be limited 
by the need to avoid any obscuration of the sightline. In this respect the 
sensors may advantageously measure the motion of each member with respect 
to inertial space, the relative motion being derived from the individual 
measurements. Suitable forms of sensors which may be used are rate gyros 
or rate integrating gyros connected to give a rate output. 
A schematic block diagram of such a control loop is shown by the broken 
lines in FIG. 2, transducers 80 and 81 being coupled to the base member 17 
and support member 21 respectively. Their outputs are combined and to give 
a signal representing relative rotation of the members which signal then 
applied to gain and compensation circuitry at 82 before being added as an 
input to the motor drive amplifier 79 to modify the reaction motor drive 
current, received from the controller 36 of the yoke drive motor, and 
which comprises a feedforward signal for the control loop. 
In an alternative form of closed loop control, shown by the additional 
chain dotted lines in FIG. 2, an additional transducer, such as a 
tachogenerator 83 is employed to measure the rotation rate of the reaction 
member 71 and an angle transducer 84, such as an electro-optical 
displacement sensor, is used to measure the relative angular displacement 
of base member 17 and support member 21. The displacement and reaction 
member rotation rate signals are combined with each other at 85 and, via 
the gain and compensation circuitry 82, combined in turn with the 
feedforward signal from the yoke drive motor controller 36. 
It is re-iterated that as well as the reflector 12 directing received 
radiation along the azimuth axis 22 onto a detector, radiation may also be 
transmitted by way of the reflector 12. Also, in addition to optical 
radiation, that is, in the visible, infra-red or ultra-violet part of the 
spectrum, the system may employ, or be employed with, longer wavelength 
microwave radar radiation; that is, the apparatus in general terms relates 
to the directing of a radiation path axis. 
Furthermore the above described elevation and azimuth axes are not 
constrained to such orientations. For instance the axis 22 may comprise a 
nominally horizontal elevation axis. 
The radiation path axis directing system has also been described as 
responsive to angular demands received. If the platform p is a moving 
vehicle the reflector 12 may be stabilised with respect to a desired 
radiation path axis despite vehicular moments by means of gyro stabilising 
means (not shown) commonly employed with reflectors pivotable about azimuth 
and elevation axes. Such gyro-stabilisation means may be carried by the 
reflector assembly but preferably is carried by the platform p or support 
member 21 and provides signals to the servo-controlled positioning 
arrangement whereby the reflector orientation is stabilised in space, as 
well as directable to demanded radiation path axes. 
The above described structure comprising a radiation path steering system 
in general and a sighting system in particular is only exemplary of a 
structure including a rotatable body which undergoes accelerating motion 
and for which torque reaction compensation is required. 
The reaction force compensation means described above has been described 
with respect to an apparatus having a body which is usually rotated from 
rest to rest. It will be appreciated that there compensation means is 
equally applicable where the body accelerates from speeds other than zero. 
It will be understood that the rotatable yoke assembly may be any body in a 
system used for other purposes or with other reasons for requiring torque 
reaction compensation.