Position dependent rate dampening in any active hand controller

A control system for an active hand controller, for example, uses a control stick connected to and controlled by a motor. Electronics are provided to control the motor to eliminate oscillations due to motor torque and high gain due to breakout at the control stick when the control stick is at about its null position. Both hardware as well as software implementations can provide position dependent dampening to the control sticks such that when the control stick is located about a null position, a higher rate of dampening is provided than when the control stick is located outside the null position, when a lower rate of dampening is provided. The system provides a stable active hand controller control stick without degraded force and feel characteristics of the system.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to application Ser. No. 07/957,216 which is now 
U.S. Pat. No. 5,291,113, and application Ser. No. 07/957,278 is now U.S. 
Pat. No. 5,264,768. 
BACKGROUND OF THE INVENTION 
This invention is related to an active hand control system of the type 
wherein manual control input devices such as control sticks employed in a 
servo-coupled control system are provided with an electrically generated 
and controlled simulated variable rate feel. In particular, the invention 
relates to a system having a control stick such as is employed in 
aircraft, which is servo-coupled to the control system of the aircraft 
through electronics and a motor mechanism. 
The electronics and motor mechanism of these types of systems provide a 
simulated feel to the control stick when in operation which is similar to 
that of a purely mechanical system. Yet more specifically, in these types 
of systems when the control stick thereof is positioned near a null or 
center position, it begins to oscillate due to the high gain in the torque 
versus position curve used to achieve breakout forces by the motor which 
is driving and is in turn driven by the control stick, as well as 
oscillations caused by gear backlash at connection of the control stick to 
the motor. The system in accordance with the invention eliminates such 
oscillations at about the null position for the control stick without 
degrading the force and feel characteristics of the control stick. 
Servo-control technology is well developed as applied in the field of 
robotics. In particular, electrical motor and servo-control systems have 
been developed and employed in the past in the design of robotic hand 
controllers which are capable of reflecting forces experienced at the 
robotic end, back to, for example, a human operator. 
One example of the type of control system to which the present invention is 
directed is disclosed in U.S. Pat. No. 4,150,803 which teaches a control 
stick for an operator having an electro-simulated variable rate feel. The 
simulated feel of the device of U.S. Pat. No. 4,150,803 is provided by a 
system which generates for the operator at the control stick the proper 
force and feel characteristics when used to command a boom employed during 
refueling operations between aircraft in flight. 
These types of controllers generally involve the use of a control stick 
which is actuatable in at least two mutually perpendicular planes to 
provide both vertical and lateral control of the device being controlled. 
An example of such a control stick is shown in U.S. Pat. No. 3,270,260, in 
which the control stick of the device has electrical pickup means in plate 
form, for example, capacitance bridge circuits arranged in a symmetrical 
arrangement such that a force exerted on the stick having a symmetrical 
arrangement of circuits unbalances them and produces a signal which is 
proportional to the force being exerted on the stick. The signals 
generated by the electrical pickups are used to generate output signals 
through appropriate electronics, with the electronics being used to 
command a control actuator to effect control of the device being 
controlled. 
Typically, in these systems the control stick is mechanically coupled at 
its axis to at least one motor which applies either a resisting force on 
the control stick or, response to a signal produced by sensors that detect 
forces applied to the device surfaces being controlled, drives the control 
stick and in turn, drives the device being controlled to alleviate forces 
generated by the device under control as felt by the operator. 
These types of controllers are particularly desirable for use in the 
operation of modern day aircraft, in particular, in the form of control 
sticks or yokes. In operation, the devices as used in the cockpit of 
aircraft are typically designed to exhibit some desired force versus 
displacement characteristics to the user whereby the magnitude of the 
control stick displacement is proportional to the force applied. The 
controller produces as its output an electrical signal corresponding to 
the control stick position, and the signal is used to control the aircraft 
through the action of various motors and mechanical means, in a manner 
which is well known to those of ordinary skill in the art and is 
conventional. Thus, in use such systems provide an electronically 
controlled manual input control stick having force and feel 
characteristics like those of purely mechanically linked systems. These 
applications in aircraft are typically referred to as "fly by wire" 
applications. Examples of presently existing applications of this 
technology are the systems employed in the Airbus A300 Transport Aircraft, 
the General Dynamics F16 Fighter aircraft and the NASA Space Shuttle. 
In the past, in order to enhance or improve the force and feel 
characteristics of such simulated feel control sticks, controller 
electronics were employed in combination with sensors for detecting the 
position of the control stick. A signal was generated and processed by 
controller electronics to drive a motor which in turn drove the control 
stick to simulate the feel of a mechanical system. Such controller 
electronics also included input from the system being controlled such as, 
for example, an auto pilot system reflecting any external forces acting on 
the flight control surfaces of the aircraft which would, as a result of 
feedback and input from the surfaces, be input into the controller 
electronics to be reflected through the motor connected to the control 
stick at the user. 
The use of a feedback loop in which the position of the control stick is 
detected and a signal resulting therefrom is processed by controller 
electronics to result in control of a motor to which the control stick is 
connected to achieve force and feel characteristics typical of a 
mechanical system is known. In accordance with one improvement as 
disclosed in copending application no. (attorney docket no. 
A34-14307(15347-153)), which is incorporated by reference herein and which 
was concurrently filed herewith, it is recognized that the detection of 
the position of the control stick fails to fully achieve the desired force 
and feel characteristics typical of mechanical systems. It is taught in 
said copending application that a mechanical system can be more fully 
simulated by also detecting the amount of force being exerted on the 
control stick, and processing both the position as well as the force 
signal by controller electronics. This is done to generate a control 
signal for the motor to which the control stick is connected which 
reflects both force and position as part of the feedback loop. By 
reflecting force the loop, the effects of external operational 
non-linearities resulting from the electro-mechanical connections of the 
manual control stick are eliminated. 
Notwithstanding the improvement disclosed in said copending application, 
when the control stick of such a system is located about its null 
position, a fairly large motor torque causes the hand controller to 
oscillate at null unless the position signal is exactly at zero when fed 
to the feedback loop which processes such a signal through the controller 
electronics. Traditionally, the oscillations at null have been corrected 
by providing a conventional rate dampening signal manipulation which adds 
stability to the control loop. However, the impact of providing 
significant rate damping is a degraded transient response. More 
particularly, excessive rate dampening, in accordance with the prior art 
in an active hand controller, makes the hand controller feel viscous. The 
invention addresses the problems of providing such rate dampening with a 
degraded transient response. 
SUMMARY OF THE INVENTION 
In accordance with the invention, it is recognized that the amount of rate 
dampening required to stabilize a manual input control means, i.e., a 
control stick, controlled by servo-control loop at about the null 
position, exceeds that needed at other positions due to the high gain in 
the torque versus position curve of a motor to which such a control stick 
is connected to achieve breakout forces. Other causes of such oscillation 
include resultant gear backlash in operation in the interconnection 
between the motor and the manual input control means. 
In accordance with one aspect, the invention is directed to an improvement 
in an active hand controller system which has manual input control means, 
i.e., a control stick, connected to a motor. The motor serves to provide 
predetermined force and feel characteristics to the manual input control 
means to be reflected at a user. The motor is connected to control 
electronics, i.e., a feedback loop, for generating signals which control 
the motor in response to input signals generated at the manual input 
control means as well as at the system being controlled, for example, at 
its flight control surfaces. 
In accordance with the invention, selective rate dampening means is 
provided and serves to provide a signal representing a higher amount of 
rate dampening to the control electronics means for generating a control 
signal which dampens the oscillations occurring as a result of the 
interaction between the motor through the gears with the manual input 
control means when the manual input control is at about its null position. 
The selective rate dampening means provides a signal indicative of lesser 
rate dampening when the manual input control means is positioned at a 
location other than about its null position. Thereby the system's 
stability is enhanced without degrading the predetermined force and feel 
characteristics thereof. 
For purposes of this disclosure, it is noted that by "at about its null 
position" is meant the position of the manual input control means wherein 
oscillations occur as a result of, for example, the high gain in the 
torque versus position curve of the motor to which it is connected to 
achieve breakout forces as well as from oscillations caused by gear 
backlash at the connection to the motor. "About null" means typically less 
than about two (2) percent of full displacement of the control stick. 
In a yet more specific aspect, the selective rate dampening means in 
accordance with the invention comprises a switch movable between a first 
position and a second position which is controlled by a comparator 
connected to the manual input control means through position detecting 
means. The position detecting means detects the position of the manual 
input control means and generates a signal representative of the position 
for controlling the switch to be in the second position to provide a 
relatively high gain level signal to the controller electronics means when 
the manual input control means is at about its null position to increase 
rate dampening thereof. The switch is arranged to be in the first position 
to provide a relatively low gain signal to the controller electronics 
means to provide relatively lower rate dampening when the control stick is 
not at about the null position. 
In a yet still more specific aspect, the invention also includes 
differentiating means operating on the position signal which represents 
the position of the manual input control means, i.e., control stick, to 
generate a signal representative of velocity of movement of the manual 
control input control means. The differentiating means is connected in a 
manner wherein the position signal detected and the velocity signal 
generated are each supplied separately to the selective rate dampening 
means to be factored into the control of the switch between the first and 
second positions to determine whether a higher or lower rate dampening is 
desired in response to the position of the control stick. 
In another aspect, the selective rate dampening means in accordance with 
the invention comprises a digital computer. The computer, by operating 
software in accordance with the detected position of the manual input 
control means, calculates and generates a signal of relatively high gain 
when the manual input control is at about its null position to increase 
rate dampening thereof. When the detected position is outside about the 
null position, the signal generated by the software is of relatively lower 
gain. 
In yet another aspect, the invention consists of a method of dampening 
oscillations in an active hand controller system having manual input 
control means, i.e., a control stick. The manual input control means is 
connected to a motor which causes the oscillations to occur in response to 
feedback signals controlling the motor when the manual input control means 
is at about its null position. In accordance with the method it is first 
determined whether the manual input control means is in one of a first 
position about null or a second position outside of about null. If the 
manual input control means is at about null, the position signal feedback 
to the motor is more highly dampened than if outside about null. 
Yet still other details of the invention will become apparent from a 
reading of this specification.

DETAILED DISCUSSION 
In FIG. 1 there is disclosed a block diagram illustrating the selectible 
rate dampening circuit in accordance with the invention. A control stick 
11 with a hand grip thereon has an external force 13 applied thereto by a 
user, for example, a pilot. The control stick 11 is connected to the motor 
19 which provides certain force and feel characteristics back to the 
control stick 11 to simulate a conventional mechanical system. The control 
stick 11 is connected through a gearhead 17 to the motor 19, and also has 
a force sensor 15 connected to the control stick 11 for detecting the 
magnitude of the force 13 applied to the control stick 11. A resolver 23 
which is back-driven by the gearhead 17 as an operator moves the control 
stick 11, is used to provide the motor 19 rotor position for the purpose 
of commutating the motor 19. Additionally, a command signal corresponding 
to the movement of the control stick 11 is provided by the resolver 23 to 
the system being controlled through line 25. With respect to the position 
signal generated by resolver 23 which is indicative of control stick 11 
position, as well as the signal generated by force sensor 15 indicative of 
any force applied to control stick 11, these signals are fed back into a 
control loop to be processed by controller electronics 73 to output a 
signal through line 75 to control the motor 19 torque to thereby provide 
appropriate force and feel characteristics to the control stick 11. 
A problem with these types of hand controllers is that when the control 
stick 11 is located about its null or zero position, the control stick 11 
begins to oscillate due to torque generated by the motor 19. 
Traditionally, such oscillations have been controlled by providing rate 
dampening in the control system for the motor 19. Such rate dampening 
enhances stability of the control stick but at the high cost of degraded 
transient response. Excessive rate dampening in an active controller makes 
the hand controller feel viscous in an undesirable manner. 
In accordance with the invention and the circuit illustrated in FIG. 1, 
rate dampening is provided at a very high gain only when the control stick 
11 is located within a small region about the center position. Thus, some 
stability is provided without degrading the overall feel in use of the 
control stick 11. 
The reason for providing the higher rate dampening about the center 
position is illustrated in FIG. 2 which shows that the breakout force 
about the null position relative to control stick 11 displacement is much 
greater than at other positions. This is caused by high position loop gain 
in the torque versus position curve shown in FIG. 2 for the motor 19, as 
well as because of other characteristics of operation, such as gear 
backlash at the interconnection between the control stick 11 through the 
gearhead 17 to the motor 19. 
In the embodiment of FIG. 1, the force sensor 15 detects the force exerted 
on control stick 11 and provides a signal indicative of the force applied 
through line 65. The signal 65 from the force sensor 15 is conditioned and 
scaled to create a high level (.+-.5 or .+-.10 V) bipolar analog signal 69 
that changes linearly with force applied at the control stick 11. For 
example, a strain gauge type force sensor typically has an output signal 
of .+-.5 mV which is highly susceptible to noise. Another example of a 
force sensor for use in the invention is a force sensor with a modulated 
AC output dependent on force. The conditioning and scaling electronics 67 
transforms the force sensor output 65 into a usable force signal 69. The 
force signal is provided through line 69 to controller electronics 73 in a 
manner as disclosed in copending application no. (attorney docket no. 
A34-14307(15347-153)). Likewise, the feedback loop for the position of the 
control stick 11 on the upper half of FIG. 1 is similar to that of said 
copending application with the exception that the position signal 
generated by the resolver 23 through line 27 is passed through line 29 to 
differentiating block 37 to have the derivative of said signal taken 
therein. The derivative is done, for example, by monitoring the rate at 
which the resolver-to-digital converter increments its digital output up 
or down, to result in a velocity signal which is passed through line 39 to 
position dependent scaling block 41. 
In the position dependent scaling block 41, the velocity signal 39 is 
scaled according to the position of the control stick 11 as indicated by 
the position signal 47. For example at about null, the position dependent 
scaling block 41 amplifies the rate signal 39 by a factor several times 
greater than the scale factor at other positions. This could be 
accomplished by any number of different methods. One such method would be 
by the circuit shown in FIG. 3. At about null the switch at 105 remains 
open thereby increasing the gain of the rate signal 39. At other positions 
the switch remains closed. Similarly this function can be accomplished by 
software as described with reference to FIG. 5 discussed hereinafter. 
The resolver 23 produces a modulated ac output signal 31 which must be 
demodulated in signal conditioning block 43 in order to produce a usable 
control signal in a control system. Typically, a resolver-to-digital 
converter is used as signal conditioning block 43 to transform the 
resolver output to a digital or binary representation of position. This 
digital representation of position is then converted to a high level 
analog signal 45 by a digital-to-analog converter chip also in signal 
conditioning block 43. The resulting position signal 45 is passed through 
line 47 to be operated on along with the velocity signal in position 
dependent scaling block 41. The operation in position dependent scaling 
block 41 will be discussed in greater detail herein with the reference to 
FIG. 3. The resultant signal from block 41 is then passed through line 61 
to summing device 63. This signal from block 41 serves to have the 
controller electronics 73 provide either a high or a low gain dampening 
signal through line 75 to motor 19 depending upon whether the control 
stick 11 is at or about its null position, or outside of its null 
position. 
As also illustrated in FIG. 1, the position signal is also passed through 
line 49 into scaling block 51 and after scaling in a conventional manner 
is passed through line 59 into summing device 63. The position signal is 
also passed through line 53 to account for breakout force characteristics 
of the control stick 11 by being processed in breakout signal generation 
block 55 and passed through line 57 in summing device 63 from where the 
combined signals are passed through line 71 into controller electronics 73 
to provide the appropriate control signal to motor 19. 
As illustrated in FIG. 3, the amount of dampening provided to the control 
stick 11 is dependent on the magnitude of the signal resulting from 
position dependent scaling block 41 at line 61. The dampening is 
controlled by the circuit of the position dependent scaling block 41 by 
means of the gain provided through operational amplifier 113, hereinafter 
op-amp 113. As can be seen from FIG. 3, the op-amp 113 is arranged in an 
inverting amplifier circuit configuration with the velocity signal through 
line 39 being passed through resistor 111 into the negative input of 
op-amp 113. Across the input and output of the op-amp 113 are located a 
pair of resistors 107 and 109 arranged in parallel with a switch 105 being 
controlled by control function block 101. The control function block 101 
serves to issue a signal through line 103 to switch 105 to either open or 
close the switch 105. When the switch 105 is closed the position signal is 
passed across resistor 107 along with the velocity signal incoming through 
line 39. Whether the switch 105 is opened or closed is dependent upon the 
position signal coming through line 47 from resolver 23. As can be 
appreciated, in accordance with the invention, the gain for the position 
dampening signal from position dependent scaling block 41 is controlled in 
accordance with the actual position of the control stick 11 detected. 
FIG. 4 illustrates an alternative implementation of the invention where a 
digital computer is used to close the control through a specific software 
implementation described with reference to FIG. 5. In FIG. 4 like elements 
are numbered the same as for FIG. 1 and function the same unless otherwise 
noted. In the control system of FIG. 4, the high level torque signal at 
line 69 and the rate signal at line 39 are converted to digital signals by 
analog to digital converters 76 and 77. A digital computer 201, shown in 
dashed lines replaces the elements shown enclosed thereby to calculate, 
through the software described with reference to FIG. 5, and based on the 
input digital rate, position and torque signals, a desired and calculated 
motor torque signal which through line 78 is passed to digital to analog 
converter 79 to be acted on by controller electronics 73 in a manner 
similar to FIG. 1. In the case of FIG. 5, the signal conditioning block 43 
is now shown as resolver to digital converter 43' which was previously 
discussed with reference to FIG. 1 as an example of the type of device 
that could be used as the signal conditioning block 43. 
The system of FIG. 4 operates as more particularly described with reference 
to the flowchart of FIG. 5 wherein the software version of the invention 
is implemented in an endless loop which provides rate dampening as a 
function of position. Components unrelated to rate dampening which are 
needed to form the complete control loop are shown for reference purposes 
in the block in dashed lines, but are not necessary to illustrate the 
concept of the invention. Such component steps in the program are 
conventional and well known to those of ordinary skill in the art. 
Turning now to the software implementation of the invention, at the top of 
the control loop current real-time measurements of position and velocity 
are input. Specifically, position and velocity in this implementation may 
be vector quantities, for example, for a conventional six axis active hand 
controller, the position and velocity will be six component vectors. 
Subsequent to such input step, a list of pre-defined non-overlapping 
position bands or intervals are examined to determine in which band the 
current position is located. For multi-axis systems where the position is 
represented as a vector, a band is determined independently for each axis. 
The "velocity--command--component" is the component of the output feedback 
command which is responsible for implementing rate dampening. The amount 
of rate dampening is computed for each axis by multiplying the 
"rate-factor" for the current band with the velocity detected for movement 
of the control stick 11. Typically, "rate-factors" are negative numbers 
used to achieve the dampening, i.e., a force in opposition to the current 
velocity. The "rate-factors" are tabulated by the band for each axis 
independently. The feedback command is then computed by summing the 
"velocity--command--component" with any other command components which may 
be needed for reasons other than rate dampening. The summed components 
result in a vector sum for a multi-axis active hand controller system. 
Finally, the feedback command is output to the system being controlled and 
the loop is closed by repeating the above sequence for the next iteration. 
It is noted that the concept of the invention as illustrated in hardware in 
FIG. 1 and software in FIG. 5 can be expanded to be employed with multiple 
position bands with multiple degrees of rate dampening for hand 
controllers having multiple axes. One such application where multiple 
bands are needed include a system with "soft stops" programmed into the 
force displacement curve as illustrated in FIG. 2. Soft stop is a 
conventional and well known concept to those of ordinary skill in the art 
and need not be disclosed in greater detail herein. With soft stops 
increased dampening will be needed at the soft stop non-linearity on the 
force versus displacement curve. Thus, additional position bands would be 
needed around the two positions to keep the systems stable as the 
controller passes through the soft stop non-linear portion of the curve. 
Having generally described the invention, the same will become better 
understood as defined in a non-limiting manner from the appended claims.