Angular position sensing and control system, apparatus and method

• US 3,899,028 A
• Filed: 08/28/1973
• Issued: 08/12/1975
• Est. Priority Date: 03/30/1972
• Status: Expired due to Term

• Current Assignee: New SD, Inc.
• Original Assignee: Frederick J. Sigworth, Harold D. Morris
• Inventors: Morris, Harold D., Sigworth, Frederick J.
• Primary Examiner(s): Pellegrino, Stephen C.
• Application Number:
  US05/392,257
• Time in Patent Office: 714 Days
• Field of Search: 172/4.5,2 73/488,517
• US Class Current: 172/4.5
• CPC Class Codes: E02F 3/764 with the scraper blade being pivotable about a vertical axis
• CPC Class Codes: E02F 3/765 with the scraper blade being pivotable about a horizontal axis disposed perpendicular to the blade
• CPC Class Codes: E02F 3/845 using mechanical sensors to determine the blade position, e.g. inclinometers, gyroscopes, pendulums
• CPC Class Codes: G05D 3/14 using an analogue comparing device

Abstract

A structural body is monitored in angular position about at least one axis by utilizing a level sensor and a sensor responsive to angular motion of the body about the axis being monitored. An output indicative of long term angular variations of the body about the axis is provided by the level sensor and short term angular variations are indicated by the output from the sensor responsive to angular motion about the axis. The signals indicative of long term and short term angular variations are combined to provide a broad band output signal indicative of angular position which is immune to high frequency translational variations appearing at the level sensor output, but which is responsive to high frequency angular inputs. The fast response angular sensor output is compared with the slow response level sensor output and an error signal is produced which drives the angular sensor output signal into coincidence with the level sensor output signal. The combined broad band angular output indication may be utilized for angular position monitoring for controlling one part of a machine relative to a predetermined external angular position reference for driving a stable platform to assume a predetermined angle about the axis.

Claims

1. In an angular position sensor and attitude control system for controlling the tilting movement relative to a local horizontal reference of one part of a machine having a framework when the framework of the machine shifts its position relative to the reference as the machine moves relative to a surface, means carried on the machine for supplying an input reference command for control of movement of said one part, means mounted on the machine for receiving the input command, sensing means carried by said one part to sense the tilting movement of said part, said sensing means including a linear accelerometer having its sensitive axis at right angles to the axis of the tilting movement and an angular accelerometer with its sensitive axis parallel to the axis for tilting movement, means mounted on the machine for combining the outputs from the linear and angular accelerometers and comparing the combined outputs with the input command at said means for receiving the input command to supply an error signal therefrom, means mounted on the machine for receiving the error signal for and means carried on the framework driven by said mechanical force which causes tilting movement of said one part in accordance with the error signal.

2. A system as in claim 1 wherein said machine is of a type in which said one part is capable of being rotated about a vertical axis, together with means adapted to be mounted on the framework for detecting linear acceleration of the framework along the direction of movement of the machine and providing an acceleration output, a sine function means receiving said last-named acceleration output for modifying the magnitude of said last-named acceleration output in accordance with the sine of the angle between a vertical plane parallel to the direction of travel and the tilting axis and supplying the same to said sensing means so as to compensate for error induced in the sensing means carried by the one controlled part due to linear acceleration of the machine along the direction of movement of the machine when rotation of said one part aligns the sensitive axis of the linear accelerometer mounted thereon other than perpendicular to the direction of movement.

3. A system as in claim 2 wherein the sine function means is adapted to be mounted partly on said framework and partly on said one part for automatically sensing rotation of said one part relative to said framework.

4. A system as in claim 2 wherein said machine is of a type which includes surface-engaging means for shifting the position of the machine and power means including a rotating shaft for driving the surface-engaging means and wherein said means for detecting linear acceleration of the framework along the direction of travel of the framework on a surface comprises ground speed indicating means such as a tachometer adapted to be mounted on the framework and coupled to the shaft, and providing an output signal proportional to the driven speed of the surface engagine means, and means for differentiating the last-named output to provide a signal proportional to acceleration and for supplying the acceleration signal proportion to acceleration and for supplying the acceleration signal to the sensing means.

5. A system as in claim 2 wherein said means for detecting linear acceleration of the framework along the direction of movement of the framework of the machine is comprised of a linear accelerometer adapted to be mounted on the frameworK with its sensitive axis parallel to the direction of travel of the framework.

6. A system as in claim 1 wherein said means for combining the output from the linear and angular accelerometers comprises an operational amplifier having an input and an output, a first summing junction at the input, first circuit means coupling the output from the angular accelerometer to the first summing junction, said operational amplifier including second circuit means connected between the output and the first summing junction to form a feedback loop around the operational amplifier so that the output from the operational amplifier is a double integral of the angular accelerometer output, a second summing junction at the linear accelerometer output, third circuit means connected between the output of the operational amplifier and the second summing junction, fourth circuit means coupling the output of the linear accelerometer to the second summing junction and means connecting the second summing junction to the first summing junction whereby the output from the operational amplifier is also a low-pass filtered amplification of the linear accelerometer output.

7. A system as in claim 6 wherein each of said first and second circuits comprises a resistive-capacitive network and said third and fourth circuits are resistive.

8. A system as in claim 7 wherein said first circuit consists of two serially connected resistors and a capacitor connected from between the resistors to ground, and wherein said second circuit consists of two serially connected capacitors and a resistor connected from between the capacitors to ground.

9. A system as in claim 6 wherein said means coupling the second summing junction to the first summing junction includes an error nulling amplifier in the form of a positive follower for reducing the susceptability of the system to minute bias errors produced by the angular accelerometer, and a phase-lead network to prevent oscillation from positive feedback due to the combination of negative feedback and two ninety-degree phase shifts in the double integrator.

10. A system as in claim 9 wherein said error nulling amplifier receives its input at the positive input terminal and includes a capacitive feedback from the nulling amplifier output to the negative input terminal and a resistor placing the negative input terminal above ground, whereby the output polarity follows input polarity, and amplification varies from the gain of the amplifier at zero frequency to one at a circular frequency equivalent to the quantity 1/RC.

11. A system as in claim 1 wherein the machine is a motor grader and wherein said one part is an earth moving blade.

12. A system as in claim 1 wherein the machine is a ralroad car having a framework and wherein said one part is a passenger compartment pivotally supported from the framework to permit pivotal movement about an axis parallel to the direction of movement of the car.

13. A system as in claim 6 wherein said machine is of a type in which said one part is capable of being rotated about a vertical axis together with means providing a correction signal to the means for receiving the error signal adapted to cause tilting movement of said one part to thereby maintain the tilt resulting from the input reference command as projected onto a vertical plane perpendicular to the direction of movement of the machine along the surface.

14. A system as in claim 6 where said machine is of a type in which said one part is capable of being rotated about a vertical axis and wherein said second circuit means includes means for modifying the output of the operational amplifier with a secant function, whereby the change in tilt angle as projected onto a vertical plane perpendicular to the direction of movement of the machine along the surface is compensated.

15. A system as in claim 1 wherein said means for causing tilting movement comprises first and second actuators located on opposite sides of the axis for tilting movement Of said one part, together with means for establishing an elevation reference adapted to control the operation of said second actuator, and wherein the means for receiving the error signal is adapted to control the operation of the first actuator.

16. In an angular position sensor and control system for controlling the tilting movement relative to a local horizontal reference of a blade of a road-grading apparatus of a type including a framework, utilizing an external elevation reference, having surface-engaging means carried by the framework for travelling on a surface to be graded, motive means for driving the surface-engaging means for advancing the framework over the surface, means for mounting the blade on the framework providing rotational movement of the blade about a vertical axis and tilting movement of the blade about a horizontal axis, actuating means including first and second actuators disposed between the blade and the framework on opposite sides of the horizontal axis for causing tilting movement of said blade, said control system comprising means carried by the blade for sensing the external elevation reference and providing an elevation reference signal, means for converting said elevation reference signal to a blade elevation force for urging the second actuator to an elevation determined by the external elevation reference, means mounted on the blade for sensing the angular position of the blade and providing a blade tilt signal, means for converting said blade tilt force, and means for coupling said blade tile force to said first actuator to thereby control the tilting motion of the blade to maintain a predetermined cross-grade angle regardless of lateral tilting movement of the framework as the apparatus moves over the surface, said means for sensing including linear accelerometer means carried by the blade with a sensitive axis perpendicular to the horizontal axis of tilting movement providing an output indicative of tilt of the blade about the horizontal axis relative to the local horizontal reference, and angular accelerometer means with a sensitive axis parallel to the horizontal axis of tilting movement of the blade providing an output indicative of angular acceleration of tilting movement about the horizontal axis.

17. A control system as in claim 16 wherein said means for sensing includes electronic circuitry for receiving the outputs from the linear and angular accelerometers providing a fast-response output which is the double integral of the angular accelerometer output and further providing a slow response amplified output of the linear accelerometer output, means for combining the fast and slow response outputs, means for providing an input reference command signal, and means for comparing the combined fast and slow response outputs with the input reference command signal for providing said blade tilt signal.

18. A control system as in claim 16 wherein said means for sensing includes means mounted on said framework for detecting linear acceleration of the framework along the direction of travel of the framework on said surface providing an additional acceleration output, means for modifying the additional acceleration output in accordance with the sine of the angle between a vertical plane parallel to the direction of travel and the tilting axis and for combining the modified additional acceleration output with output indicative of tilt to provide an error-connected acceleration output from the linear accelerometer means carried by the blade.

19. A control system as in claim 17 wherein said electronic circuitry providing a fast response output includes an operational amplifier and means for modifying the output of the operational amplifier in accordance with the secant of the angle between a vertical plane parallel to the direction of travel and the tilting axis.

20. In an angular position sensor and control system for controlling the tilting movement relative to a local horizontal reference of a blade of a road-grading apparatUs of a type including a framework utilizing an external elevation reference, having surface-engaging means carried by the framework for travelling on a surface to be graded, motive means for driving the surface-engaging means for advancing the framework over the surface, means for mounting the blade on the framework including a drawbar and means for providing rotational movement of the blade about a vertical axis and tilting movement of the blade about a horizontal axis, actuating means mounted between the framework and the blade including first and second actuators disposed on opposite sides of the horizontal axis for causing tilting movement of said blade, said control system comprising means carried by the blade for sensing said external elevational reference and producing an elevation reference signal, means for converting said elevation reference signal to a first force, means for applying said first force to said second actuator thereby controlling operation of said second actuator, means mounted on the drawbar for sensing the angular position of the blade and providing a blade position signal, means for converting the blade position signal to a second force, and means for applying said second force to said first actuator for controlling the operation of said first actuator to thereby control the tilting motion of the blade to maintain a predetermined cross-grade angle regardless of lateral tilting movement of the framework as the apparatus moves over the surface, said means for sensing including linear accelerometer means with a sensitive axis perpendicular to the longitudinal tilt axis of the drawbar providing an output indicative of tilt of the blade about the horizontal axis relative to the local horizontal reference and angular accelerometer means with a sensitive axis parallel to the longitudinal tilt axis of the drawbar providing an output indicative of angular acceleration of tilting movement about the horizontal axis and means for combining said outputs indicative of tilt and angular acceleration for providing said blade position signal.

21. In a motor grading apparatus for use with an external elevation reference, a framework, surface engaging means carried by the framework and adapted to travel on a surface to be graded, motive means for driving the surface engaging means for advancing the framework over the surface, a surface-grading blade, means mounting the blade on the framework and permitting rotational movement of the blade about a vertical axis and tilting movement of the blade about a horizontal axis, means including first and second actuators disposed on opposite sides of the horizontal axis for causing tilting movement of said blade, means carried by the blade and adapted to sense said external elevation reference for controlling operation of said second actuator, means mounted on the blade for sensing the tilting movement of the blade as the apparatus moves over the surface, said last named means including a linear accelerometer carried by the blade with its sensitive axis perpendicular to the horizontal axis of tilting movement of the blade, an angular accelerometer carried by the blade with its sensitive axis parallel to the horizontal axis of tilting movement of the blade, and for combining the outputs of the linear and angular accelerometers providing a fast response double integrated output from the angular accelermoeter and a slow-response amplified output from the linear accelerometer, means mounted on the framework adapted to be controlled by the operator of the apparatus for supplying an input reference signal, means carried by the apparatus for receiving the combined outputs of the linear and angular accelerometers and comparing the combined outputs with the input reference signal, means for supplying an error signal resulting from the comparison, means mounted on the apparatus for receiving and converting the error signal to a mechanical force, and means for applying said mechanical force to said first actuator whereby a predetermined crosS-grade angle is maintained on the surface grading blade regardless of lateral tilting movement of the framework during operation of the apparatus.

22. Apparatus as in claim 21 together with means mounted on the framework for measuring the acceleration of the framework in the direction of travel of the framework providing an additional acceleration output, means for modifying the additional acceleration output with the sine of the angle between a vertical plane parallel to the direction of travel and the tilting axis, means for combining the modified additional acceleration signal and the first named acceleration signal, and means for supplying the last named combination to the means for combining the linear and angular accelerometer outputs.

23. Apparatus as in claim 22 wherein said means for combining the linear and angular accelerometer outputs includes an operational amplifier and means for modifying the output of the operational amplifier with the secant of the angle between a vertical plane parallel to the direction of travel and the horizontal tilting axis, whereby the tilt angle as projected onto a vertical plane perpendicular to the direction of travel of the framework is maintained at the magnitude selected by control of the input reference signal.

24. Apparatus as in claim 22 wherein said means for supplying the combined linear acceleration signals to the means for combining the linear and angular accelerometer outputs is comprised of an error nulling amplifier in the form of a positive follower for reducing the susceptibility of the system to minute bias errors produced by the angular accelerometer, and a phase-lead network to prevent oscillation from positive feedback due to the combination of negative feedback and two ninety degree phase shifts in the double integration.

25. Apparatus as in claim 23 wherein said means for supplying sine and secant information is in the form of potentiometers mounted on said framework and having input shafts which are coupled to the blade so that they measure angular rotation of the blade about a nominally vertical axis.

26. Apparatus as in claim 22 together with means for attenuating the output of the combined first named and additional acceleration signals by approximately a 30 to 1 ratio at approximately 2.0 Hz.

27. Apparatus as in claim 21 wherein said means carried by the blade adapted to sense the external elevation reference includes means coupled to the blade and providing an output signal which represents the actual elevation position of the blade as determined by the second actuator, means mounted on the framework available to the operator for selecting an elevation reference command signal, and means for comparing the first named signal with the reference command signal to provide an error signal for causing operation of said second actuator.

28. In a method for controlling the tilting movement relative to a local horizontal reference of one part of a machine having a framework where the framework of the machine shifts its position relative to the reference as the machine moves relative to a surface, comprising the steps of providing a reference command signal for controlling the movement of said one part, sensing the resultant linear acceleration including the acceleration of gravity of said one part along an axis at right angles to the axis of tilting movement of said one part, sensing the angular acceleration of said one part about an axis parallel to the axis for tilting movement, obtaining fast response outputs from the angular accelerometer and slow response outputs from the linear accelerometer, combining the two last named outputs, comparing the combined output with the input command signal to provide an error signal, converting the error signal into a mechanical force and coupling the mechanical force to the one part for controlling the movement of said one part.

29. A method as in claim 28 together with the steps of measuring the linear acceleration of the fRamework in the direction of movement of the machine modifying the last named linear acceleration measurement by the sine of the angle between a vertical plane parallel to the direction of travel and the tilting axis and summing the modified linear acceleration measurement with the sensed acceleration at right angles to the axis of tilting movement thereby compensating for error from acceleration in the direction of machine movement.

30. A method as in claim 28 together with the step of modifying said combined fast and slow response outputs by the secant of the angle between a vertical plane parallel to the direction of movement of the machine and the tilting axis whereby a constant angle of tilt is maintained as projected onto a vertical plane perpendicular to the direction of movement.

31. A method as in claim 28 together with the step of providing attenuation of the linear acceleration signal in the frequency range of resonance of the machine to provide control system stability.

32. In a method for controlling the position of a moldboard about a tilt axis extending in a direction perpendicular to the long dimension of the moldboard being carried by a framework of a motor grader being adapted to travel on a surface, the position being referenced to a local horizontal plane, where said plane is defined as a plane perpendicular to the local long-term acceleration vector which includes the acceleration of gravity as a component, the method comprising the steps of providing an input command signal for controlling the position of the moldboard, sensing the motion of the moldboard by detecting angular acceleration about the tilt axis and linear acceleration along an axis extending at right angles to the tilt axis, compensating the linear acceleration output for error induced by framework acceleration along the direction of travel of the framework, combining the compensated linear acceleration output signal and the angular accelerometer output signal, filtering the linear acceleration output to provide a slow response output proportional to tilt angle, attenuating the slow response output in the frequency range of machine resonance, obtaining a fast response output proportional to tilt angle from the angular acceleration output, combining the slow and fast response outputs, obtaining a correction signal proportional to the secant of the angle between the tilt axis and a vertical plane parallel to the direction of travel of the framework, summing the correction signal with the combined slow and fast response outputs to insure maintenance of the commanded cross-grade angle as the tilt axis departs from a parallel to said vertical plane, comparing the summed signal with the input command signal to provide an error signal, converting the error signal into a mechanical force, and coupling the mechanical force to the moldboard for positioning the moldboard.

33. A method as in claim 32 wherein the step for providing compensation for linear accelerometer output error induced by framework acceleration along the direction of travel of the framework is comprised of sensing linear acceleration along the direction of travel of the framework, providing an additional linear acceleration output therefrom, sensing the sine of the angle between the vertical plane parallel to the direction of travel of the framework and the tilt axis, multiplying the sensed sine of the angle by the additional linear acceleration output thereby providing an output signal proportional to the sine of the angle, and combining the last named output signal with the first named linear accelerometer output.

34. In an angular position sensing system for detecting the angular position of a structural body referenced to earth'"'"''"'"'s coordinates about at least one axis, a level sensor mounted to detect the angular position about said one axis of a plane referenced to local horizontal and producing an output related thereto, means for detecting angular motion about said one axis substantially solely sensitive to inertial angular motion and producing an output related thereto, means for combining the outputs from said level sensor and said means for detecting angular motion, said means for combining including means for accepting solely long term level sensor output and short term angular motion output and providing a continuous broad band output containing said long and short term outputs related to angular position, together with a platform mounted on the structural body, means for allowing rotation of said platform relative to the structural body about at least one axis, a device of the type which is to be stablized about said one axis mounted on the platform, means for providing a reference signal for said one axis indicative of a predetermined angular position of the structural body about said one axis, means for comparing the combined outputs with said reference signal for generating an error signal, and means for receiving said error signal, wherein said means for receiving said error signal includes means for applying torque about said one axis to urge said platform to assume said predetermined angular position.

35. An angular position sensing and stabilization system for detecting changes in angular position of a structural body relative to earth'"'"''"'"'s coordinates, comprising a platform mounted on the structural body, means for allowing rotation of said platform relative to the structural body about at least one axis, a device of the type which is to be stabilized mounted on the platform, means for providing a reference signal for said one axis of rotation indicative of a predetermined angular position of said platform, means for sensing the rotational movement of said platform about said one axis of rotation and providing output responsive thereto, said last named means including a level sensor mounted on said platform producing an output indicative of deviation from local horizontal, means for detecting angular motion about said one axis substantially solely sensitive to inertial angular motion and providing an output related thereto, means for combining the outputs from said level sensor and said means for detecting angular motion, said means for combining including means for accepting solely long term level sensor output and short term angular motion output thereby providing a continuous broad band output related to angular position, means for comparing the combined outputs with said reference signal for said one axis of rotation for generating an error signal, means for receiving and converting said error signal to a torque, and means coupling said torque to said platform for causing rotation of said platform about said one axis of rotation to assume said predetermined angular position.

36. An angular position sensing system as in claim 35 wherein said level sensor comprises a linear accelerometer having a sensitive axis oriented perpendicular to said one axis of rotation for providing a steady state output signal responsive to rotational movement, wherein said means for detecting inertial angular motion comprises an angular accelerometer having a sensitive axis oriented parallel to said one axis of rotation for providing a transisent output signal responsive to rotational movement, wherein said means for combining includes a double integrator for receiving said transient output from said angular accelerometer, wherein said long term level sensor outputs are below a corner frequency and said short term angular motion outputs are above said corner frequency, and a phase lead network for receiving the output from said means for combining.

37. An angular position sensing system as in claim 35 wherein said level sensor comprises a linear accelerometer having a sensitive axis oriented perpendicular to said one axis of rotation for providing a steady state output signal responsive to rotational movement of said platform, wherein said means for detecting inertial angular motion comprises an angular velocity transducer having a sensitive axis oriented parallel to said one axis of rotation for providing a transient output signal responsive to rotational movement of said platform, wherein said means for combining include an integrator for receiving said transient output from said angular velocity transcucer, and wherein said long term level sensor outputs are below a corner frequency and said short term angular motion outputs are above said corner frequency.

38. An angular position sensing system as in claim 37 wherein said means for receiving and converting said error signal to a torque is a torque motor producing a torque vector substantially colinear with said one axis, and wherein said angular velocity transducer is a non-gyroscopic device.

39. An angular position sensing system as in claim 36 adapted to receive an additional reference signal indicative of structural body azimuth, wherein said reference signal is indicative of local horizontal and wherein the structural body has a yaw axis together with means for allowing rotation of said platform about said yaw axis, means for providing an output indicative of platform azimuth, an additional angular accelerometer mounted on said platform having a sensitive axis aligned with the local vertical providing an output related to platform yaw axis angular acceleration, an additional double integrator for receiving said additional angular accelerometer output, producing an output related to yaw angle, an additional phase lead network for receiving said additional double integrator output, additional means for combining said additional phase corrected double integrator output and said output indicative of platform azimuth, said last named additional means rejecting said output indicative of platform azimuth above a specified frequency and rejecting said double integrator output below said specified frequency, means for comparing said additional combination with the additional reference signal for providing an azimuth error signal, and means for receiving said azimuth error signal for controlling said platform azimuth position.

40. An angular position sensing system as in claim 35 adapted to receive an additional reference signal indicative of structural body azimuth wherein said reference signal is indicative of local horizontal and wherein the structural body has a yaw axis together with means for allowing rotation of said platform about said yaw axis, means for providing an output indicative of platform azimuth, an angular velocity transducer having a sensitive axis oriented parallel to the local vertical mounted on said platform providing an output signal related to platform azimuth angular velocity, an integrator for receiving said angular velocity output providing an output related to yaw angle, additional means for combining said integrator output and said output indicative of platform azimuth, said last named additional means rejecting said output indicative of platform azimuth above a specified frequency and rejecting said integrator output below said specified frequency, means for comparing said additional combination with the additional reference signal for providing an azimuth error signal, and means for receiving said azimuth error signal for controlling said platform azimuth position.

41. An angular position sensing system as in claim 35 wherein said means for sensing rotational movement provides an additional signal related to angular velocity of said platform about said one axis of rotation, and wherein said means for receiving said error signal is a torque motor, said torque motor being connected to receive said additional signal, whereby said platform is damped as it is driven continuously towards zero error signal thereby preventing inter axis resonance.

42. An angular position sensing system as in claim 41 wherein said platform is controlled about two mutually perpendicular horizontal axes, and wherein said device to be stabilized is an additional linear accelerometer mounted on the platform having a sensitive axis aligned with the local vertical, said Additional accelerometer providing an output indicative of vertical acceleration.

43. A stable platform as in claim 36 wherein said means for combining outputs comprises means for comparing the double integral of said transient output signal with said steady state output signal for providing an additional error signal, feedback means for connecting said additional error signal to the input of said double integrator for driving said double integrator output signal into coincidence with said steady state output signal.

44. In an attitude control system for controlling the tilting movement relative to a local horizontal reference of one part of an apparatus having a framework when the framework of the apparatus shifts its position relative to the reference, means carried on the apparatus for supplying an input reference command for control of movement of said one part, means mounted on the apparatus for receiving the input command, sensing means carried by said one part to sense the tilting movement of said part, said sensing means including a linear accelerometer having its sensitive axis at right angles to the axis of the tilting movement and an angular accelerometer with its sensitive axis parallel to the axis for tilting movement, means mounted on the apparatus for combining the outputs from the linear and angular accelerometers and comparing the combined outputs with the input command to supply an error signal therefrom, means mounted on the apparatus for receiving the error signal for converting the error signal to a mechanical force, and means coupling the mechanical force to said one part to cause tilting movement of said one part in accordance with the error signal.

45. An attitude control system as in claim 44 wherein said apparatus is of a type in which said one part is capable of being rotated about a vertical axis, together with means adapted to be mounted on the framework for detecting linear acceleration of the framework along the direction of shift of position of the apparatus and providing an electrical output related thereto, a sine function means receiving said last-named output for modifying the magnitude of said electrical output in accordance with the sine of the angle between a vertical plane parallel to the direction of shift of position and the axis of tilting movement and supplying said modified electrical output to said sensing means so as to compensate for error induced therein due to linear acceleration of the apparatus along the direction of shift of position of the apparatus when rotation of said one part aligns the sensitive axis of the linear accelerometer mounted thereon with a component of the direction of shift.

46. In an angular position sensing system for detecting the angular position of a body referenced to earth'"'"''"'"'s coordinates about at least one axis, first sensing means mounted on the body to detect the long term angular position of the body about said one axis referenced to local horizontal and producing a first output related thereto, second sensing means mounted on the body for detecting the short term angular position of the body about said one axis and producing a second output related thereto, means for combining the first and second outputs to produce a combined angular position output, means for converting the combined output to a mechanical force, and means coupling said mechanical force to said body to cause movement of said body in accordance with the combined output.

47. An angular position sensing system as in claim 46 wherein said first sensing means comprises a linear accelerometer having a sensitive axis oriented perpendicular to said one axis, wherein said second sensing means includes an angular accelerometer having a sensitive axis oriented parallel to said one axis of rotation, and wherein said means for combining includes a double integrator for receiving said second output and producing output indicative of angular position.

48. An angular position sensing system as in claim 46 wherein said firsT sensing means comprises a linear accelerometer having a sensitive axis oriented perpendicular to said one axis of rotation and said second sensing means comprises an angular velocity transducer having a sensitive axis oriented parallel to said one axis of rotation, said means for combining the first and second outputs including an integrator for receiving said second output, said first outputs being below a corner frequency and said second outputs being above said corner frequency.

49. In apparatus for controlling the attitude relative to a local horizontal reference of a part of an apparatus having a framework supporting said part, while said framework is moveable relative to the reference, a linear accelerometer adapted to be mounted on said part with its sensitive axis at right angles to the axis of tilting movement for producing a first signal representing a slow response angular displacement of the part, means adapted to be mounted on said part and sensitive to rotational motion of the part for producing a second signal representing a fast response angular displacement of the part, means for combining the slow response and the fast response signals representing angular displacement, means for receiving said combined signal and producing a mechanical force, and means coupling said mechanical force to said part, whereby said part is servoed to said controlled attitude over a broad band of frequencies of angular displacement of said framework.

50. An angular position sensing apparatus for sensing the attitude of one part of a machine relative to a local reference as the machine changes attitude relative to the local reference, comprising a linear sensor for detecting angular deviation of the one part relative to the local reference and providing a first electrical output indicative thereof, an angular sensor for detecting angular motion of the one part relative to the local reference and providing a second electrical output indicative thereof, means for combining said first and second electrical outputs including means for suppressing said first electrical outputs above a predetermined intermediate frequency and means for suppressing said second electrical output below said predetermined intermediate frequency, whereby said combined output is substantially free of angular position error due to linear motion of said one part at frequencies above said intermediate frequency, means for receiving said combined output and producing a mechanical force, and means coupling said mechanical force to said one part, whereby said one part is controlled to assume a predetermined attitude over a broad band of frequencies of angular displacement of said machine.