Programmable, linear collective control system for a helicopter

A helicopter programmable collective control system includes a motorized collective control stick 2 used in conjunction with a multi-axis control stick 30, with the multi-axis control stick 30 as the default collective controller. A collective control signal processor reads collective input the pilot is giving through one of the two control sticks 2, 30 and provides a motor drive signal 44 (which is dampened to prevent over control) to position the collective control stick 3. The collective control stick position is used to drive the rotor collective pitch actuators 16.

DESCRIPTION 
1. Technical Field 
This invention relates to a collective control system for a helicopter and 
more particularly a programmable collective control system with tactile 
feedback and with a linear, motorized stick. 
2. Background Art 
Traditional helicopter flight control systems utilized a cyclic stick for 
pitch and roll, pedals for yaw and a collective stick for lift. Commonly 
owned U.S. Pat. No. 4,420,808 (Diamond et al, 1983) discloses a single, 
multi-axis sidearm control stick that provides control signals in each of 
the pitch, roll, yaw and collective/lift axes and therefore allows a 
helicopter to be flown using only one hand. The multi-axis stick detects 
the amount of force applied in different axes by the pilot and provides 
signals which are a function of those forces. The stick has limited 
displacement, which is necessary to eliminate coupling between axes due to 
hand motion. 
Collective axis tasks, such as nap-of-the-earth (NOE) flight operations, 
create difficulty for pilots when using only a multi-axis control stick, 
because they cannot accurately perceive the extent of collective (lift) 
input without diverting attention to the cockpit display. 
Commonly owned U.S. Pat. No. 4,696,445 (Wright et al, 1987) discloses a 
displacement-type collective control stick used in conjunction with a 
force-type multi-axis sidearm control stick to solve this problem. In 
Wright et al, the collective stick "tracks" the multi-axis stick using a 
motor/clutch/damper/spring arrangement so that a pilot determines the 
collective pitch of the rotor by the position of the collective stick and 
can change rotor pitch using either the multi-axis stick or the collective 
stick. The signal from the multi-axis stick controls rotor collective 
pitch until the pilot engages the collective stick. The pilot engages the 
collective stick in two ways: pushing the stick against its trim position 
or pressing a trim release switch on the stick. A clutch mechanism 
subsequently disengages the stick drive motor and allows the pilot to move 
the stick, the displacement of which is then used as the collective pitch 
command signal. 
Prior collective sticks, such as disclosed in Wright, are pivotally 
attached to the floor of the aircraft and provide a control signal 
indicative of angular displacement. The sensitivity of collective control 
is dependent on the length of the stick. Subsequently, prior systems had 
long collective control sticks requiring significant pilot motion to 
control the aircraft which added to pilot fatigue. 
Wright et al provides rotor collective pitch tactile feedback to the pilot 
because the collective stick "tracks" the multi-axis stick. The Wright et 
al stick does not, however, provide the pilot with tactile feedback as to 
the rotor load requirement for collective stick commands. In other words, 
the pilot is not provided with feedback as to the amount of stress he is 
putting on the rotor due to the command he is giving through the 
collective stick. For example, a pilot can move the stick its full length 
of travel very quickly, causing the helicopter to stall momentarily due to 
the sudden demand for a large change in energy state. A stall is 
discomforting to the pilot and is particularly undesirable if he is 
reacting to an emergency situation which demands short time response, such 
as discovery that an adversary is about to fire a weapon at him. 
Damping systems are disclosed in the prior art for helping to prevent pilot 
errors as described above. For instance, U.S. Pat. No. 4,545,322 (Yang 
1985) describes a mechanical device for artificially creating drag on a 
control stick. U.S. Pat. No. 4,477,043 (Repperger 1984) discloses a 
mechanical damping system which is controlled by using a motor or actuator 
to adjust spring tension U S. Pat. No. 4,236,685 (Kissel 1980) discloses 
an aircraft steering mechanism with active force feedback. Kissel 
describes using discreet components to read various dynamic inputs from 
aircraft flight conditions, interpret those inputs and increase the 
tension on the flight control stick using a "pitch feel" unit. 
It has also been discovered that every pilot desires a different resistance 
or "feel" in the collective stick movement. Some pilots like to have a 
"touchy" stick while others prefer the stick to be more difficult to move. 
The prior mentioned control stick systems have required laborious 
adjustments to tailor their "feel" to pilot demands while minimizing pilot 
fatigue. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to provide an improved collective 
control system for a helicopter which provides tactile feedback as to 
collective pitch. 
Another object is to provide a collective control system which is 
programmable for different pilot requirements and flight profiles. 
Yet another object of the invention is to provide a collective control 
system which provides damping to prevent pilot induced oscillations. 
A further object of the present invention is to provide an improved 
motorized collective control stick for a helicopter which is easily 
programmable, thereby eliminating the need for traditional stick 
clutch/damper/spring arrangements. 
According to the present invention, a programmable, motorized, collective 
control stick system is used in tandem with a multi-axis control stick 
system for controlling the collective pitch of a helicopter rotor. The 
collective control stick has a force output (proportional to the amount of 
force placed on the stick) and a displacement output (proportional to the 
position of the stick). A collective system signal processor reads the 
collective control stick output signals, the force output of the 
multi-axis control stick, flight data from a flight system control 
computer, and various other data, and provides a drive signal for the 
drive motor of the collective control stick. The stick is moved by the 
motor at a rate controlled by the computer so as not to exceed the 
limitations of the helicopter. The displacement output of the collective 
control stick is used to drive electro-hydraulic actuators which control 
the collective pitch of the rotor. 
According further to the present invention, a sliding stick is mounted on a 
bearing block which rides on dual shafts contained in a collective control 
box. The bearing block is attached to a belt driven by pulleys and a 
motor. The motor is controlled by a signal processor which provides a 
motor drive signal indicative of the pilot's desire to change rotor 
collective pitch. 
The invention provides for simple adjustment to change the "feel" of the 
collective stick for different pilots' requirements. This may be done 
using hardware (potentiometers, etc.), software (pilot profile disk, etc.) 
or through the flight control computer. 
The invention has a small operational range which helps prevent pilot 
fatigue. It eliminates a clutching mechanism and provides positive drive 
of the collective stick without slippage. It is lighter, simpler in 
operation and has less parts to wear out than prior systems. 
The invention has a smaller length of travel and better accuracy than 
previous systems, allowing the stick to travel its operative length in one 
second or less. 
The invention allows the damping of the collective stick to be dynamically 
adjusted as a function of stick rate, position limits, rotor load, 
environmental conditions, mission profile, vertical acceleration, pitch 
rate and many other inputs. As a result, the invention helps prevent pilot 
induced oscillations, it ensures the engine operates within its 
acceleration curve and it improves aircraft efficiency. Also, the 
invention can be current limited to provide motor circuit protection. 
These and other objects, features and advantages of the present invention 
will become more apparent in the light of the detailed description of 
exemplary embodiments thereof as illustrated in the drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, a motorized, sliding, collective control stick 2 
has a grip 3, which is mounted on a collective control box 4, as is 
described with respect to FIGS. 3-5 hereinafter. The position of the 
collective control stick along its axis is converted to an electrical 
signal by a linear variable differential transformer 6, (LVDT, shown in 
greater detail in FIGS. 3-5 hereinafter), which is connected by a line 8 
to a signal processor 10. The position signal (COLLPOS) on line 8 is also 
connected to a controller circuit 12 for an electrohydraulic actuator 14 
which controls a primary actuator 16 that positions a swashplate assembly 
18 to determine the collective pitch of the helicopter rotor blades 20. 
The force applied by the pilot to the grip 3 is converted to an electrical 
signal (COLLFORCE) by a force transducer 22, such as a strain gauge, and 
connected by a line 28 to the signal processor 10. Also, the output of a 
select switch 26, placed on the grip 3, is connected by a line 24 to the 
signal processor 10. 
A multi-axis control stick 30, such as described in Diamond et al, provides 
a collective force output signal on a line 32 which is utilized by a 
flight control computer system 34 to provide an appropriate multi-axis 
stick collective control signal (MULTISTK) that is connected by a line 36 
to the signal processor 10. The flight control computer system 34 also 
exchanges data with the signal processor 10 on a data bus 40. 
The signal processor 10 utilizes the inputs on lines 8, 24, 28, 36, 40 and 
provides a motor command (MOTOR CMD) signal to a motor driver circuit 42 
on a line 44. The motor driver circuit 42 provides a drive signal on lines 
46 to a collective stick motor 48. The collective stick motor 48 positions 
the grip 3 using a pulley arrangement (shown in greater detail in FIGS. 
3-5 hereinafter). Mechanical limit switches 52, located on the collective 
control box 4, disconnect the motor drive signal 46 from the motor driver 
circuit 42 if the grip 3 reaches its travel limits. 
The signal processor performs a number of calculations in order to provide 
the motor command signal. (These calculations are described in FIGS. 2A 
and B hereinafter.) Variables in these calculations provide flexibility in 
the operation of the collective stick. For example, the gain of the signal 
processor provides the sensitivity or "feel" of the stick. Higher gain 
makes the stick more sensitive to pilot commands so that small applied 
forces result in large rotor collective pitch changes while lower gain has 
the opposite effect. The variables may be provided through adjustments to 
potentiometers 56 in external circuitry which provide signals on lines 58. 
Referring now to FIG. 2A, a motor command calculation routine is stored in 
the memory of a digital signal processor and reached through entry point 
60. A LOAD AMETER subroutine 62 loads collective control system 
variable parameters into memory for use during forthcoming calculations. 
The variable AMETERS are provided from a number of sources such as the 
analog signals on lines 58 of FIG. 1, and digital signals provided by the 
flight control computer system 34 on data bus 40. AMETERS include 
variables which are provided for each pilot's specific requirements for 
collective system responsiveness (e.g., the "feel" of the stick may be 
programmed this way) and also variables which are used to adjust for 
dynamic flight conditions. The LOAD AMETERS subroutine 62 is not shown 
in detail herein, but may be one of many subroutines known in the art 
which poles input devices and stores the output data from each device in 
memory for retrieval during forthcoming calculations. 
A step 64 defines a FORCE parameter as the COLLFORCE signal provided by 
line 28 of FIG. 1 times a gain K1. This step provides the "feel" of the 
collective control stick because different values of K1 provide a 
different responsiveness of the stick. Next, a subroutine 66 defines a 
COLLRATE parameter as a gain K2 times the differential of the COLLPOS 
parameter provided on line 8 of FIG. 1. The COLLRATE subroutine is not 
shown in detail herein but may be any one of a number of programs known in 
the art to differentiate a parameter with respect to time. Test 68 
determines whether or not FORCE is being provided. If so, a test 70 
determines its direction. If FORCE is in the forward direction, (FWD), 
then FORCE is redefined by subtracting a forward deadband (FWDDBND) from 
FORCE and multiplying this value by a forward gain parameter (FWDGAIN) in 
a subroutine 72. If the direction of FORCE is aft (AFT), then a subroutine 
74 redefines FORCE as FORCE minus an aft deadband (AFTDBAND) multiplied by 
an aft gain value (AFTGAIN). FWDDBND and AFTDBND create a deadband which 
prevents helicopter vibrations and noise from causing extraneous motor 
command signals. FWDGAIN and AFTGAIN are parameters which allow the feel 
of the collective stick 2 to be different for forward and aft movement. 
Test 76 determines if FORCE is greater than the deadband (manifested by 
the fact that FORCE will be a negative number at this point if FORCE is 
less than AFTDBND or FWDDBND). If it isn't, step 78 sets FORCE equal to 
zero to eliminate any motor command. If FORCE is greater than the 
deadband, test 80 determines if the pilot has engaged the select switch 26 
on the grip 3. 
A negative result of test 80 indicates the pilot has not engaged the select 
switch on the grip and the force applied to the grip will be faded in/out 
over time as described in steps 82-104 hereinafter. An affirmative result 
to test 80 indicates the pilot has engaged the select switch and therefore 
does not wish the force to be faded in/out but rather any input he gives 
to the collective stick will be used immediately for collective control. 
In essence, engaging the select switch makes the collective stick more 
"touchy" because the control does not undergo the fade in/out function. 
The select switch is used by the pilot under circumstances in which he 
wants instantaneous response from his commands through the collective 
stick. 
If the pilot wishes to use the collective stick without engaging the select 
switch he must apply enough force to the grip to first overcome a breakout 
level (BREAKOUT) and then maintain that force above a threshold value 
(HOLDFORCE). Test 82 determines if FORCE is greater than HOLDFORCE. If it 
is, test 84 determines if a collective flag (indicating the breakout level 
was previously exceeded) has been set. A negative result of test 84 leads 
to test 86 to determine if FORCE has exceeded the breakout level. If it 
has, the collective flag is set in step 88 and a step 90 increments a FADE 
parameter (originally initialized to zero) by an incremental value A. Test 
92 then determines if FADE is greater than a maximum value, and if so, 
step 96 sets FADE equal to the maximum value. In either case step 94 
redefines FORCE as FORCE times FADE. 
If the pilot continues to apply force to the collective stick greater than 
BREAKOUT, the fade in/out routine (steps 82-94) increases FORCE as a 
function of incremental value A (in subsequent passes through the routine 
of FIG. 2A) until FADE is the maximum value as set in step 96. The 
collective stick remains engaged as the collective pitch controller until 
the pilot relieves the pressure he is applying to the grip below HOLDFORCE 
as determined in test 82. If FORCE is below HOLDFORCE, step 98 redefines 
FADE as FADE minus an incremental value B. Test 100 then determines if 
FADE is less than zero. If not, FORCE is redefined as FORCE times the new 
FADE value in step 94. FORCE continues to be faded out (in subsequent 
passes) as a function of B until test 100 determines that FADE has been 
decremented below zero. Step 102 then redefines FADE as zero, step 104 
clears the collective flag and the routine is exited with FORCE having 
been faded out to zero. 
Referring now to FIG. 2B, test 106 determines if either the select switch 
has been engaged (yes answer to test 80) or the collective flag has been 
set in step 88. An affirmative result means the pilot has selected the 
collective stick to control rotor collective pitch and FORCE is redefined 
as FORCE minus the COLLRATE parameter determined in step 66. It has been 
found the collective stick has a tendency to "run away" from the pilot as 
he applies force to the grip. The subtraction performed in step 110 
prevents this from happening. 
Step 112 defines a damping multiplier parameter W which has a value that 
varies between zero and one. Step 114 then redefines FORCE as FORCE times 
W. W is dependent on COLLRATE and a DAMP parameter. The derivation of DAMP 
is not explicitly defined herein, but it may be programmed to be dependent 
on any number of factors including specific dynamic flight conditions 
(e.g. rotor loading), mission profile, and pilot preference. These factors 
are manifested as parameters stored in memory and may be provided by the 
flight control computer 34. For example, it is undesirable to get lift 
from the helicopter rotor during a pure rotational manuever. To prevent 
the pilot from giving a collective lift command through the collective 
control stick 2, the attitude of the helicopter might be measured by a 
gyro type transducer and read by the flight control computer 34 which 
would provide the signal processor 10 with a scaled value (on data bus 40) 
of the amount of rotation the helicopter is undergoing. The signal 
processor would incorporate that value in its computation of DAMP. As DAMP 
becomes a larger value, W becomes smaller in step 112. FORCE is then 
reduced in step 114 and the collective control stick becomes harder, if 
not impossible to move. It is evident that a variety of parameters can be 
utilized to dampen the collective control stick in this manner. 
If test 106 has determined the pilot has not engaged the collective stick, 
step 108 redefines FORCE as an error signal equal to the collective 
control signal (MULTISTK, line 36 FIG. 1) provided by the multi-axis stick 
30 minus the collective stick position signal (COLLPOS). 
Test 116 determines if FORCE is equal to zero. If so, the routine jumps to 
step 144. If not, test 118 determines the direction of FORCE. If the pilot 
is pushing the collective stick forward, test 120 determines whether the 
collective stick position signal (COLLPOS) exceeds limit HILMT. If test 
118 determines the pilot is pulling the collective stick in the aft 
direction, test 122 determines whether COLLPOS exceeds limit LOLMT. If the 
collective stick is beyond its limits in either test 120 or 122, step 124 
redefines FORCE as zero. Steps 118-124 therefore establish a position 
limit routine whereby if the forward or aft position limits are exceeded, 
the pilot is able to command the stick in the opposite direction of the 
limit that has been exceeded. 
Steps and tests 126-142 described hereinafter provide a means for keeping 
the collective stick motor current draw to within safe operating limits. 
The motor current signal MOTORCURR provided on line 54 of FIG. 1 is 
compared with a current limit parameter CURRLMT in test 126. If MOTORCURR 
exceeds CURRLMT, a FORCELMT parameter (originally initialized to MAXFORCE) 
is decremented by a value D in step 128. Test 130 determines if FORCELMT 
is negative. If FORCELMT is negative, step 132 sets it equal to 0. If not, 
test 134 determines if FORCE is greater than FORCELMT. If test 134 is 
affirmative, FORCE is set equal to FORCELMT in step 136. If FORCE was not 
greater than FORCELMT in test 134, it remains unchanged. Therefore, if the 
collective stick drive motor is pulling too much current, the motor 
command signal (manifested as FORCE in the program) is faded out as a 
function of D until the problem no longer exists. 
If step 126 subsequently determines MOTORCURR is below CURRLMT, step 138 
increments FORCELMT by value C. Step 140 then determines if FORCELMT is 
greater than MAXFORCE. MAXFORCE is the maximum value of motor drive 
command (FORCE) which will keep the motor current draw (MOTORCURR) below 
the safe operating level (CURRLMT). If FORCELMT is greater than MAXFORCE, 
step 142 sets FORCELMT equal to MAXFORCE. If not, FORCELMT remains 
unchanged and test 134 compares FORCE with FORCELMT. Therefore, steps 
138-142 fade FORCE in as a function of C once it is determined the motor 
is no longer drawing too much current. 
Step 144 defines MOTOR CMD (provided on line 44 of FIG. 1) as equal to 
FORCE and the motor command routine is exited at point 146. 
Although shown in terms of a software flowchart, the invention may also be 
implemented with dedicated digital or analog hardware. In fact, the 
invention has only been implemented as software algorithms such as those 
disclosed in U.S. Pat. Nos. 4,270,168 and 4,564,908, which are hereby 
incorporated herein by reference. The equivalence between analog, 
dedicated digital and software is illustrated (in a different context) in 
U.S. Pat. No. 4,294,162. 
Referring now to FIG. 3 the grip 3 is mounted on a bearing block 150 having 
two bores lined with bearings which receive and ride on a pair of dual 
shafts 152 that are mounted on brackets 154 attached to the collective 
control box 4. The bearing block 150 is driven along the shafts 152 by a 
toothed drive belt 156 which is attached to the bearing block with a 
bracket 158 and bolts 160. Drive belt 156 rides on three toothed pulleys 
162-166. Motor 48 drives pulley 166 with a belt-pulley arrangement as 
shown in FIG. 1 and described hereinafter. The motor 48 receives command 
signals on input leads 46. Pulleys 162-166 have toothed hubs 170, that 
engage with a drive belt 172. The core of LVDT 6 is attached to a rod 174 
which is secured to the belt 172 by clamp 176 and nut 178. The LVDT 6 is 
mounted to control box 4 by a bracket 180. 
Referring now to FIG. 4 a toothed pulley 182 is secured to the shaft of 
drive motor 48 and drives a toothed pulley 184 using a belt 186. Pulley 
184 is connectably attached to pulley 166 via a shaft (not visible) which 
is disposed within a barrel 188. The motor and pulleys described 
hereinbefore are mounted on brackets 190, 192 which are in turn mounted to 
control box 4. 
Referring now to FIGS. 4 and 5, motor 48 is shown connected to the pulley 
182 which drives the belt 186 to turn the pulley 184 that is connected to 
the pulley 166 which drives the belt 156 around the pulley 162. The linear 
differential transformer 6 is mounted on the bracket 180. Brackets 190, 
192 hold all of the above in place in control box 4. 
Thus although the invention has been shown and described with respect to 
exemplary embodiments thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions and 
additions may be made therein and thereto without departing from the 
spirit and the scope of the invention.