Apparatus and method for providing useful audio feedback to aid in the operation of remotely controlled powered manipulators

The user of a remotely controlled, powered manipulator is provided with audible feedback signals which provide useful information relating to the force and speed of the motor. An audio tone is generated and is frequency modulated by a signal which corresponds to motor speed. The tone is amplitude modulated by a signal corresponding to motor force thus providing the user with an intuitively interpretable feedback signal having a speed-proportional pitch and force-proportional volume. In the case of an electric motor, voltage and current can be monitored to provide speed and force signals respectively. Additional modulation means are disclosed for introducing a force-proportional warble component to said tone.

BACKGROUND AND FIELD OF THE INVENTION 
The present invention generally relates to sensory feedback systems for 
improving user control of remotely operated devices. In particular, the 
invention relates to audible feedback systems utilized in connection with 
remotely operated power driven manipulators such as "robot arms" and the 
like. 
Remotely operated manipulators are finding wide use in fields ranging from 
surgery to deep sea exploration and in environments ranging from the 
subterranean to the surfaces of distant planets. The users of remotely 
operated manipulators have traditionally monitored manipulator position 
and motion through the use of closed circuit television. This enables the 
operator to see the effects of his control inputs in real time and make 
appropriate adjustments. Video feedback systems do not, however, provide 
sufficient information concerning manipulator force. Remotely operated 
manipulators are usually employed in hostile or humanly inaccessible 
environments to make repairs or conduct exploration. In the case of making 
critical repairs, the lack of force information can lead to an increase in 
damage rather than the completion of desired repair. 
Various schemes exist for providing user feedback information on force, 
speed, displacement and the like. Many of the prior art systems are 
analogous to the well known force feedback devices used in aircraft to 
give a natural "feel" to pilot controls. Other prior art systems utilize 
various transducers to sense displacement, speed, and force and use the 
information derived therefrom to operate a duplicate manipulator which is 
placed proximately to the operator for direct viewing. 
The prior art devices primarily utilize the user's visual and tactile 
senses. Most of the prior art systems which provide feedback relating to 
speed, displacement, or force, require the use of special transducers 
which add to the complexity of the remote manipulator as well as its 
expense. 
SUMMARY OF THE INVENTION 
In contrast to the aforementioned visual and tactile feedback systems, the 
present invention provides feedback in the form of audible sound. The 
instant invention supplements the visual feedback systems of the prior art 
with an audible sound input to the operator which, through various 
combinations of frequency and amplitude modulation, conveys information 
concerning the force and speed of a remotely operated manipulator. The 
essential principles of the invention can be used with all types of 
powered manipulator system including hydraulic, pneumatic, and electric. 
When used with systems which employ electric motors, the present invention 
can provide useful information relating to force and speed without 
requiring the use of special transducers. Motor voltage can be monitored 
to provide a signal corresponding to speed and motor current can be 
monitored to provide a force proportional signal. The speed signal can 
then be used to frequency modulate an audio tone and the force signal is 
used to amplitude modulate the tone. In most DC electric motor 
applications these signals, while not absolutely accurate indicators of 
speed and force, nevertheless provide sufficiently accurate information to 
effect good control of the manipulator. By sensing motor voltage and 
current at the power supply, which is typically located in close proximity 
to the user, the need for extra cabling or telemetry devices for relaying 
speed and force information from the remote manipulator itself is 
eliminated. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved feedback apparatus and method for supplying information to the 
user of remotely operated powered manipulators by generating appropriately 
modulated sound signals which can be intuitively interpreted by the 
operator as indicators of speed and force. 
A further object of the present invention is to provide a user feedback 
system which, when utilized with electric motor powered manipulators, 
eliminates the need for special force and diplacement transducers. 
A still further object is to provide a feedback system which allows a user 
to control a remotely operated manipulator with precision and speed 
heretofore unobtainable in a system of comparable simplicity. 
Another object of the present invention is to provide an audio feedback 
system which is readily adaptable for use with all types of powered 
remotely operated manipulators including electric, hydraulic and pneumatic 
types. 
The apparatus and method of the instant invention has other objects and 
features of advantage which will be set forth in and become apparent from 
the following description of the preferred embodiment and the accompanying 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, an audio feedback system is illustrated for monitoring 
and aiding in user control of a remotely operated, powered manipulator. 
For purposes of simplicity, only a single motor is illustrated along with 
its associated audio feedback circuitry. Typical applications will involve 
the use of multiple motors to effect three-dimensional movement. 
In FIG. 1, an X-Y joystick 100 is illustrated as a typical multi-axis 
controller for a remote manipulator. In the preferred embodiment, joystick 
100 is a Measurement Systems Incorporated Model 446 or its equivalent. The 
X output 101 and the Y output 102 supply differential voltage signals 
which are amplified and otherwise processed to control reversible DC servo 
motors. The joystick 100, in the preferred embodiment, is an isometric 
force control comprising piezo-resistive semiconductor strain measuring 
devices in a four-arm bridge configuration. This configuration gives a DC 
voltage output proportional to applied force in the range of plus or minus 
10 volts DC full scale. 
In FIG. 1, the Y axis output 102 from joystick 100 is connected to a motor 
drive servo amplifier 103 which is used to drive the Y axis manipulator 
motor 104. An electrical signal proportional to motor force is developed 
across motor current sensor 105. In practice, the motor current sensor 
need be nothing more than a resistor in series with the motor power leads. 
This provides a voltage drop across the resistor which is proportional to 
motor drive current and thus can be utilized as a force proportional motor 
current signal 106. 
An electrical signal 107 proportional to motor speed is provided at the 
output of amplifier 103 by sampling motor voltage. This motor voltage 
signal 107 is proportional to motor speed and is used as the control input 
to voltage controlled oscillator (VCO) 108. Accordingly, VCO 108 provides 
an output which is frequency modulated in proportion to the motor voltage 
signal 107 thus providing at the output of 108 an oscillator signal having 
a frequency which varies in proportion to motor speed. 
Voltage controlled amplifier (VCA) 109 has as its input the frequency 
modulated output signal from VCO 108. Motor current signal 106 is 
connected to the control input of VCA 109. Accordingly, VCA 109 is an 
amplitude modulator which varies the amplitude of the VCO signal in 
proportion to the motor current signal 106. This has the effect of 
providing a signal whose volume is proportional to motor force. The output 
of VCA 109 is connected to one side of headphone 110 for audibly 
reproducing the frequency and amplitude modulated signal for user 
listening and interpretation. 
As is noted on FIG. 1, identical circuitry is used in connection with the X 
output from joystick 100 to provide an audio signal in connection with the 
operation of the electric motor for the X axis. The output from the X 
channel circuitry is supplied to the other side of headphone 110 for 
operator listening and interpretation. 
The circuit functions illustrated in FIG. 1 illustrate a very simple 
embodiment of the instant invention which is easy to implement and which 
provides useful information to the operator. It has been found that 
additional modulative processing can be employed to further increase the 
usefulness of the instant invention. In FIG. 2 a preferred embodiment is 
schematically illustrated which employs, in addition to the simple 
amplitude and frequency modulation processing illustrated in FIG. 1, 
another processing step which provides a warbling or fluttering effect to 
the audio output signal. The aural effect at the operator's ears is a 
warbling or fluttering tone whose frequency rises proportional to motor 
voltage and whose amplitude and warble rate are proportional to motor 
current. Other additional features which are illustrated in the circuitry 
of FIG. 2 include provisions for silencing the output so that with no load 
or input drive signal to the motor, there is no output to the operator's 
head set. Other features and options which increase the usefulness of the 
instant invention will be explained in the following description of the 
circuitry illustrated in FIG. 2. 
As will be explained herein, the method and apparatus of the instant 
invention can be employed on a variety of types of remotely controlled 
powered manipulators including those utilizing electric, hydraulic, or 
pneumatic power or various combinations of the foregoing. 
The preferred embodiment schematically illustrated in FIG. 2 is designed 
for use in connection with a remotely operated manipulator employing 
electro-hydraulic actuators well suited for underwater deployment. These 
actuators utilize bidirectional DC electric motors as their primary source 
of motive power. Accordingly, the voltage and current supplied to the DC 
electric motor can be monitored to provide useful information concerning 
the speed and force of the manipulator which is driven by the motor. In 
FIG. 2, motor 1 is a permanent magnet type DC electric motor manufactured 
by the Bodine Electric Company Model No. 32D5BEPM-W2. Hydraulic actuator 4 
is shown connected to motor 1 by a dotted line indicating that the 
actuator is coupled to and driven by the motor 1. In the upper lefthand 
portion of FIG. 1 a power module 10 is shown having an input 11 for 
receiving a plus or minus 10 volt DC signal from the Y axis output 102 of 
controller 100 illustrated in FIG. 1. Power module 10 is, in the preferred 
embodiment, a Glenteck Model GA 4552-4. Output leads 12 and 13 from power 
module amplifier 14 are shown connected to motor 1 for supplying a drive 
signal of plus or minus 120 VDC at currents up to 3 amps. Resistor 15 is 
an input resistor for power amplifier 14. Resistor 16 is a current sensing 
resistor in series with power module 10 and the drive motor 1 for the 
purpose of sensing motor current and generating a DC voltage proportional 
thereto. In the preferred embodiment, resistor 16 is a 0.1 ohm precision 
power-type resistor. The nominal magnitude of motor current ranges from 0 
to plus or minus 3 amps. Accordingly, the voltage dropped across resistor 
16 due to motor current is plus or minus 0.3 VDC. Resistor 16 provides a 
means responsive to manipulator speed by sensing the analogous parameter 
of motor voltage and provides an electrical signal proportional thereto. 
IC 20 and IC 21 are precision integrated circuit (IC) operational 
amplifiers (OP amps) configured together with their associated resistors 
22 through 29 as differential amplifiers. The manipulator force or torque 
signal developed across resistor 16 is sensed and amplified by the 
differential amplifier made up of integrated circuit 20 and integrated 
circuit 21. IC 21 functions as a summing amplifier which subtracts the 
Common Mode Voltage seen at resistors 22 and 23 both of which are 20K 
0.01% tolerance resistors. Summing amplifier IC 21 thereby lowers the 
Common Mode input voltage at IC 20 to below specified maximums typical of 
monolithic linear integrated circuits. Resistors 24, 25, 26 and 27 are all 
1K in value. 
In the preferred embodiment the power module 10 is configured in a bridged 
output mode allowing bidirectional output voltage excursions up to plus or 
minus 120 VDC. Resistors 22, 23, 24 and 25 form a precision differential 
voltage divider and should therefore employ precision matched resistors 
with low temperature coefficients. The functional block created by IC 20 
and IC 21 creates a low-cost, high voltage differencing amplifier with 20 
dB gain. The output voltage gain is determined by the ratio of resistor 28 
to resistor 22. In the preferred embodiment resistor 28 is a 200K 1% 
resistor as is resistor 29. 
Resistor 30 and potentiometer 31 which are 2K and 200K respectively in 
value form an input offset control which may be omitted if the precision 
OP amp used as IC 20 has sufficiently low input offset voltage. IC 20 
should be selected for high CMRR, low input offset voltage and current, 
with low V.sub.os drift such as OP 07, LF 363, AD 521, or other OP amps. 
The output of IC 20 is direct coupled to a precision rectifier comprised of 
diodes 41, 42, 43 and 44 together with IC 45 and resistors 51, 52, 53, 54 
and 55. The aforementioned diodes are silicon small signal types (1N4148 
or equivalent) which rectify the input current. IC 45 is an OP amp 
configured an inverting unity gain amplifier. Resistors 51 through 55 are 
all 10K 1 percent types. When the input voltage to IC 45 is positive, 
diode 41 conducts, producing, at point 60, a voltage equal to 
-(+V+I.sub.in R.sub.D41), where R.sub.D41 is the forward biased diode 
resistance of diode 41 and I.sub.in is the input current. Because the 
amplifier gain is unity, I.sub.in equals I.sub.out. The voltage appearing 
at the output of IC 45 is the sum of the voltages at point 60 and the drop 
across forward biased diode 44. The output voltage from integrated circuit 
45 is accordingly equal to -(+V+I.sub.out R.sub.D41)+I.sub.out R.sub.D44. 
For negative inputs diode 42 conducts and produces voltage (-V-I.sub.in 
R.sub.D42)/2 at the noninverting input of IC 45. 
Because the potential difference across the two inputs of IC 45 is 0 volts, 
the voltage at both inputs is the same. Since diode 41 is reversed biased, 
there is no current flow through resistor 51. Therefore the voltage at the 
inverting input of IC 45 is due to the voltage divider created by 
resistors 53 and 55. This produces a voltage (-V-I.sub.out R.sub.D42) at 
point 60. Diode 44 adds an additional voltage of I.sub.out R.sub.D44 
volts. The output is again equal to -V. Diode 43 eliminates latch-ups by 
allowing the output to drive the inverting input negative when the input 
voltage is approximately equal to 0 volts. Diodes 41 through 44 should be 
matched for V.sub.f and resistors 51 through 55 should be 1% metal film 
types. IC 45 may be a low-cost general purpose OP amp, LM741 or 
equivalent. Input offset triming may be accomplished by the circuitry 
shown for IC 175. 
The output from IC 45 is negative in polarity and equal in magnitude to the 
output of the differencing amplifier formed by IC 20. Due to the +20 dB 
gain achieved in IC 20, the output from IC 45 is -10 .vertline.V.sub.R16 
.vertline.. IC 46 is configured as a direct coupled inverting amplifier 
with a gain of 10 dB. Potentiometer 62 and associated resistors 63 and 64 
form an adjustable offsetting input to IC 46 and reference the output 
voltage to V+/2 which is the zero reference for the AM input of VCO IC70. 
Potentiometer 62 is in the preferred embodiment a 200K pot. Resistors 63 
and 64 are 6.8K in value. 
IC 70 and IC 80 are both EXAR 2206 monolithic IC Function Generators 
capable of producing sine, triangle, or square wave outputs. IC 70 and IC 
80 are shown in outlined form so that the pin arrangement seen in FIG. 2 
corresponds to the pin arrangement on the actual chip itself. Pins 1 and 
16 are labeled on both IC 70 and IC 80 to show that pin numbering starts 
at 1 and proceeds sequentially counterclockwise to pin number 16. 
References to pin numbers on IC 70 and IC 80 are references made to the 
actual chip pin numbers. The output of IC 70 at pin 2 is a low-frequency 
sine wave whose voltage and frequency vary with the magnitude of the 
output voltage at IC 46. Resistor 65 is a feedback resistor for IC 46 and 
has a value of 33K. Due to the gain developed in IC 46, the output voltage 
at pin 2 of IC 70 drives the output stage to saturation before the motor 
current reaches its full range of plus or minus 3 amps, however the output 
frequency continues to rise. Potentiometer 73 shown connected to pin 3 of 
IC 70 sets the gain of the output at pin 2 of IC 70. Potentiometer 73 is a 
200K pot and resistor 71 is 5.1K. Resistor 78 is also 5.1K and is 
paralleled by capacitor 77 which is a 10 microfarad 10 volt capacitor. 
Capacitor 79 is a one microfarad capacitor. Potentiometer 74 is a 25K pot 
and potentiometer 75 is 200K with a 1K resistor 91 placed in series with 
ground. The ranges and values of potentiometers 74 and 75 and capacitor 92 
may be adjusted to achieve appropriate scaling of the desired output 
signal. Potentiometer 93 is a 500 ohm pot which allows adjustment of the 
sine wave shape to minimize distortion. 
The low-frequency sine wave output of IC 70 is AC coupled to IC 90 through 
capacitor 95 and resistor 96 which have values of 10 microfarads and 10K 
ohms respectively. IC 90 is a unity gain summing inverting amplifier. This 
amplifier sums the signals developed across resistor 96 and 10K ohm 
resistor 97. IC 90 is an absolute value circuit similar to the one 
discussed in connection with IC 45 except that the output of IC 90 is 
positive polarity with magnitude from 0 to 10 VDC proportional to the 
control voltage present at the input 11 to power module 10. Diodes 115 and 
116 and their associated 1K ohm resistors 121 and 122 form an input 
protection circuit to prevent voltage spikes or inadvertent high voltages 
from damaging the inputs to IC 175. Resistors 123 and 124 are 10K 1% 
tolerance resistors and together with diodes 117 and 118 perform the same 
function as do the related components in the input circuit to IC 45. 
Similarly, feedback resistor 125 is a 10K 1% tolerance resistor. Diodes 
126 and 127 are similar in type and circuit function to the previously 
described diodes 43 and 44 which are used on the output of IC 45. 
The summed inverted output of IC 90 provides the frequency sweep control 
voltage at pin 7 of IC 80. The magnitude of the output modulated signal 
from IC 80 is determined by the DC voltage at pin 1 of IC 80. This voltage 
has been established by the output of IC 46 and is proportional to motor 
current and hence proportional to manipulator force. Accordingly, the 
output signal at pin 2 of IC 80 is a sine wave swept across a mid range 
frequency band, i.e., (400 hertz to 4 kilohertz) when driven by a 0 to 10 
volt motor voltage control signal from joystick 100. This sine wave is 
modulated by the low-frequency AC output generated in IC 70. The amplitude 
of the swept modulated output is determined by a DC control signal 
proportional to the motor current signal developed across resistor 16. The 
output at pin 2 of IC 80 is AC coupled to a low power audio amplifier with 
a variable gain capable of powering standard stereo headphones. 
The aural effect heard by the user through the headphone is a warbling or 
fluttering tone whose frequency rises proportional to motor voltage and 
whose amplitude and warble rate are proportional to motor current. With no 
load or input drive signal to the motor, the output to the user's headset 
is 0 volts. The amplitude control signal at pin 1 of IC 80 is weighted to 
produce a steeply rising output voltage over the lower range of motor 
current values (e.g. less than plus or minus 1 amp). As motor currents 
rise above this range, the effect becomes less noticeable, given the 
operator greater sensitivity to motor load and force changes in the low 
current range. This characteristic may be adjusted then modified by 
selecting resistor values and may be changed to give greater linearity or 
greater sensitivity as may be appropriate for controlling the manipulator 
in different situations. 
IC 130 is configured as a comparator with an output clamp whose input is 
driven by the negative going motor current signal. The switching point is 
set at approximately minus 3.0 volts by the ratio of the 10K potentiometer 
131 and the 33K resistor 132. Input resistor 133 is a 4.7K current 
limiting resistor placed in series between the output of IC 45 and the 
inverting input of IC 130. Feedback resistor 134 is a 3.3 megohm unit 
connected between the output and the noninverting input of IC 130. When 
the input to IC 130 taken between potentiometer 131 and resistor 132 drops 
below negative 3.0 volts, the output switches from V- to V+. Due to the 
diode clamp and voltage divider circuit formed by 68k resistor 135, 33k 
resistor 136, and diode 137. The comparator output switches from 
approximately -0.7 volts to +5.0 volts. This provides a logic level 
control signal to the FSK input at pin 9 of IC 80. When the comparator IC 
130 switches, pin 9 on IC 80 goes high which switches the output of IC 80 
to a steady state sine wave. This gives the operator a positive indication 
when the motor current magnitude reaches a preset limit as defined by the 
ratio of the resistances 131 to 132. Potentiometer 131 may be adjusted to 
change this threshold as is required in different applications. 
The XR 2206 is a commonly used monolithic function generator and hence the 
methods for selecting the values of the associated timing resistors and 
capacitors are well documented in the manufacturers literature. Circuit 
component values given herein have been found satisfactory when used in 
connection with the embodiment illustrated in FIG. 2. The values of the 
remaining components associated with the IC OP amps will be given for 
purposes of completeness. Resistors 141 and 142 are both 10K and perform 
identical functions to resistors 54 and 55 associated with IC 45. 
Potentiometer 143 is a 200K pot in series with a 2K resistor 144. Resistor 
145 is connected between the noninverting input of IC 90 in ground and has 
a value of 3.3K. IC 90 is configured as a 0 dB summing amp. Feedback 
resistor 146 has a value of 10K. IC 150 is used to amplify the output from 
IC 80 to a level suitable for driving headphones. Input resistor 151 is 
1K. Capacitor 152 is a 20 microfarad device. Resistor 153 has a value of 
100K. Feedback resistor 154, connected between the input and output of IC 
150, is a 10K resistor. Potentiometer 155 is a 20K pot in series with a 10 
microfarad capacitor 16. Capacitors 157 and 158 are both 10 microfarad 
units. Capacitor 180 is a 1 microfared unit. Resistors 159 and 160 are 
both 5.1K. Potentiometer 161 is a 200K pot. Timing capacitor 162 connected 
between pins 5 and 6 of IC 80 is a 1 microfarad device. Timing capacitor 
92 connected between pins 5 and 6 of IC 70 is a 10 microfarad device. 
Capacitor 165 is a one microfarad device. Potentiometer 166 has a value of 
25K ohms. Potentiometer 167 is a 200K pot in series with resistor 168 
which has a value of 5K ohms. Resistor 169 is a 4.7K unit. Potentiometer 
170 is connected between pins 13 and 14 of IC 80 and is a 500 ohm unit 
used to adjust for minimum sine wave distortion. Headphone 171 has one 
side driven by the output of audio amplifier 150. In typical multi-motored 
manipulators, the other side of headphone 171 would be driven by circuitry 
similar to that shown in FIG. 2 which would be used to process signals 
associated with another axis such as the X axis. 
Alternatively, instead of sampling motor current, manipulator force can be 
determined by placing a differential hydraulic pressure sensor in an 
appropriate part of hydraulic actuator 4. Differential pressure transducer 
180 is shown mounted on actuator 4 to sense the difference in pressure 
between the outside ambient pressure and the internal hydraulic pressure 
within actuator 4 and is of a type which provides a proportional DC output 
signal. Switch 181 can be used to select between hydraulically or 
electrically derived manipulator force signals. In practice, it will 
usually be cheaper to use motor current sensing, but certain applications 
may make the use of hydraulic pressure transducers attractive. Although 
not shown, manipulator speed signals could be derived by appropriate 
measurement of hydraulic flow rather than measurement of motor voltage as 
shown herein. The advantages of measuring motor voltage and current 
parameters include the elimination of the need for additional cabling from 
remotely mounted sensor transducers. Motor voltage and current can easily 
be sensed, for example, on the deck of a ship which is suppling voltage 
and current to an undersea manipulator. It is not necessary to run extra 
cabling up from the remotely situated electric motor powered manipulator 
for the purpose of relaying manipulator speed and force data. While motor 
voltage and current are not absolutely accurate indicators of manipulator 
force and speed, they provide sufficiently close approximations to greatly 
aid in user control. 
Another alternative means of deriving manipulator force signals include the 
use of external transducers such as strain gauges, load cells, and other 
force transducers well known in the art. Similarly, speed signals can be 
derived from the more traditional methods and means including tachometers 
and displacement transducers. 
In FIG. 1 it is seen that the means responsive to manipulators speed is 
simply a direct connection to the motor voltage signal 107 which appears 
at the output of motor drive amplifier 103. An alternative and equally 
satisfactory configuration is seen in FIG. 2 where the means responsive to 
manipulator speed is a connection 11 to the Y axis output 102 of joystick 
100. A signal which appears at 11 is, in the preferred embodiment, a DC 
voltage which is amplified and used to power the Y axis electric motor. 
The output 102 from joystick 100 can be viewed as a user adjustable speed 
control signal. Either this user adjustable speed control signal or motor 
drive voltage itself may be monitored and used to provide an electrical 
signal which is proportional to manipulator speed. 
The circuitry shown in FIG. 2 has an additional feature not shown in the 
basic embodiment illustrated in FIG. 1. In FIG. 2, IC 70 and IC 80 are 
configured together to provide, in addition to the before mentioned 
amplitude and frequency modulation features shown in FIG. 1, an additional 
modulation means for repetitively varying the frequency of the output from 
IC 80 at a repetition rate proportional to manipulator force. It would be 
possible to instead provide a repetitive variation of a user detectable 
output characteristic other than frequency. Instead of warbling the output 
frequency a variable rate gating could be used to pulse the output or, 
alternatively, other user detectable characteristics such as harmonic 
content could be varied through the use of filters or other well known 
methods. In FIG. 2, the output from IC 80 is sweep frequency modulated up 
and down to produce a warble effect having a repetition rate proportional 
to the force signal derived from the motor current sensed by resistor 16. 
The summed inverted output of IC 90 provides the frequency sweep control 
voltage at pin 7 of IC 80. This frequency sweep control voltage can be 
varied for adjusting the bandwidth over which the output frequency of IC 
80 is swept. Referring to FIG. 2, the output from headphone amplifier 150 
is heard by the user as a warbling tone having a center frequency 
proportional to manipulator speed. The term center frequency is used to 
refer to the pitch of the speed proportional tone. The warble component 
varies this pitch up and down at a warble rate proportional to manipulator 
force. The warbling tone has an amplitude or volume which is proportional 
to manipulator force. In FIG. 2, IC 80 is coupled to manipulator force 
signals in such a way that with no load or input drive signal to the 
motor, the output from IC 80 is zero volts. In this way means are provided 
for silencing audible outputs from the generator formed by IC 70 and 80 
when signals indicating zero manipulator speed and zero manipulator force 
are detected. 
When the instant invention is used in connection with manipulators 
utilizing hydraulic or pneumatic power the manipulators speed and force 
can be sensed through appropriate installations of pressure and flow 
transducers. In FIG. 2, a differential pressure transducer 180 is shown 
connected for sensing the difference between ambient outside pressure and 
pressure within the hydraulic portion of the manipulator. The differential 
pressure proportional DC output of 180 can be used in place of the motor 
current proportional voltage derived across resistor 16. As previously 
mentioned, manipulator speed could alternatively be sensed by a flow 
sensor installed at an appropriate point in a pneumatic or hydraulic 
system where flow rate is proportional to manipulator speed.