A presentation on TORQUE, FORCE AND TACTILE SENSORS

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A presentation onTORQUE, FORCE AND TACTILE
SENSORS

• By
• ASHWIN KUMAR GOLLAMANDALA
• CHAITHANYA KUMAR VANAMA
• VIJAY KIMAR THUMMAIPALLI

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OVERVIEW

• Introduction
• Force Control
• Types of Force Control
• Strain Gages
• Torque Sensors
• Force Sensors
• Tactile Sensors
• Conclusion

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• INTRODUCTION
• The response of a mechanical system depends on
  forces and torques applied to the system.
• Examples
• include machine-tool operations such as
  grinding,
• cutting, forging, extrusion and rolling.
• The forces and torques present in dynamic systems
  are generally functions of time.
• Performance monitoring and evaluation, failure
  detection and diagnosis, testing and controlling
  of mechanical systems can depend heavily on
  accurate measurement of associated forces and
  torques.
• As Force and Torque are effort variables, the
  term force may be used to represent both these
  variables

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• FORCE CONTROL
• The important application of force sensing is in
  the area of control. Since forces are variables
  in a mechanical system, their measurement can
  lead to effective control.
• Force control is invaluable, For example
  consider precision machining of a hard work
  piece, a slight error in motion could generate
  large cutting forces, which might lead to
  unacceptable product quality or even to rapid
  degradation of the machine tool.
• In such situations, measurement and control of
  forces seem to be an effective way to improve the
  system performance.
• There are three basic types of force control
• 1) force feedback control
• 2) feed forward force control
• 3) impedance control

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• FORCE FEEDBACK CONTROL
• Consider the system shown below, which is
  connected to its environment through two ports A
  and B.

• Force variable fa and motion variable va are
  associated with port A and force fb and motion
  variable vb are associated with port B.
• fa is an input to the system and fb is an output.
• We could control the output force fb by supplying
  the predetermined fa and by subjecting B to the
  specified motion vb.
• The corresponding open loop configuration is
  shown in following figure.

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• Since it is practically impossible to achieve
  accurate system performance with this open loop
  control arrangement, the feedback control loop
  shown in the following figure has to be added.

• In feedback control ,the response force fb is
  measured and fed back into the controller that
  will modify the control input signals according
  to a suitable control law, so as to correct any
  deviations in fb from the desired value.

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FEED FORWARD FORCE CONTROL

• Suppose that fa is an actuating force that can be
  generated according to a given specification. But
  suppose that fb is an unknown input force.
• fb could be a disturbance force, such as that
  resulting from a collision , or a useful force,
  such as gripping force whose value is not known.
• As fb is unknown, it might not be possible to
  control the system response accurately. One
  solution is to measure the unknown force fb,
  using a suitable force sensor and feed it forward
  into the controller.
• If an input force to a system is computed using
  an analytical model and is supplied to the
  actuator, the associated control is
  inappropriately termed as feed forward control.

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• IMPEDENCE CONTROL
• Impedance control is particularly useful in
  mechanical manipulation against physical
  constraints, which is a case in assembly and
  machining tasks.
• A very high impedance is naturally present in the
  direction of a motion constraint, and very low
  impedance is naturally present in the direction
  of free motion.
• Problems created by using motion control in
  applications where small motion errors would
  create large forces can be avoided if stiffness
  or impedance control is used.
• The stability of the overall system can be
  guaranteed and the robustness of the system
  improved by properly bounding the values of
  impedance parameters.
• The concept of impedance control can be applied
  to situations in which the input is not a
  velocity and output is not a force.

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• STRAIN GAGES
• Force/Torque sensors are based on strain gage
  measurements. Although strain gages measure
  strain, the measurements can be directly related
  to stress and force.

• Strain gage measurements are calibrated with
  respect to a balanced bridge. When the the strain
  gages and the bridge deform, the balance is upset.

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• The bridge has a variable resistor, it can be
  changed to restore balance.the amount of this
  change measures the amount by which the
  resistance of the strain gages changed, thereby
  measuring the applied strain.this is known as
  null balance method of strain measurement.

• This method is inherently slow because of the
  time it takes to balance the bridge. Hence null
  balance method is not suitable for dynamic
  measurements.
• This approach to strain measurement can be
  speeded up by using servo balancing, in which the
  output error signal is fed back into an actuator
  that automatically adjust the variable resistance
  so as to restore the balance.

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• SEMI CONDUCTOR STRAIN GAGES
• In low strain applications the sensitivity of
  foil gage is not adequate to produce an
  acceptable strain gage signal.semi conductor
  strain gage are particularly useful in such
  situations.
• The strain element of a semi conductor strain
  gage is made of a single crystal of piezo
  resistive material such as silicon, doped with a
  trace impurity such as boron.

• The sensitivity of a SC gage is about 2 orders of
  magnitude higher that of a metallic foil gage.
  There several other advantages like, reduced
  distribution error, high sensitivity, low
  mechanical hysterisis.
• There are several disadvantages associated with
  semiconductor strain gages. They are non linear
  strain resistance relationship, higher
  temperature sensitivity and difficult to mount on
  curved surfaces.

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• TORQUE SENSORS
• Torque is sensed by detecting either an effect or
  the cause of torque.
• Torque and force sensing is useful in many
  applications like control of fine motions,
  process testing, measurement of power transmitted
  through a rotating device etc.
• Types of torque sensors
• Strain gage torque sensor
• Deflection torque sensor
• Direct deflection torque sensor
• Variable reluctance torque sensor
• Reaction torque sensor
• Motor current torque sensor

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• STRAIN GAGE TORQUE SENSOR
• This is a straight forward method of torque
  sensing in which a torsion member is directly
  connected between the drive unit and the load in
  series.
• If a circular shaft is used as a torsion member,
  the torque strain relationship is relatively
  simple.
• The torque sensing using a circular shaft torsion
  member is as shown below
• The torsion member and strain gage combination is
  used to sense the torque.

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• DEFLECTION TORQUE SENSOR
• In a deflection torque sensor the actual
  deflection is measured and used to determine
  torque instead of measuring strain in the sensor
  element.
• There are two types of displacement torque
  sensors
• Direct deflection torque sensor
• Variable reluctance torque sensor
• In direct deflection torque sensor the angle of
  twist is directly measured using an angular
  displacement sensor, which can be used to
  determine torque.

Direct deflection Torque sensor
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• In variable reluctance torque sensor the change
  in magnetic induction associated with sensor
  deformation is measured. Here a Ferro magnetic
  tube is used as torque sensing element.

Variable reluctance torque sensor
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• REACTION TORQUE SENSOR
• The major drawback of strain gage torque sensor
  is the use of sensing element connected between
  drive member and driven member, which is overcome
  in reaction torque sensor.

• 
  schematic diagram of a reaction torque sensor
• This method can be used to measure torque in a
  rotating machine, in which the transmitted power
  in rotating machinery through torque and shaft
  speed are measured.

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Relationship between reaction torque and load
torque
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• MOTOR CURRENT TORQUE SENSOR
• Here the concept of electromagnetic interaction
  is used to measure the torque.
• Torque in an electric motor is generated as a
  result of the electromagnetic interaction between
  the armature winding of the motor and the field
  winding.
• For a D.C motor, the armature winding is located
  on the rotor and the field winding is on the
  stator.
• Here the field current and the armature current
  are used to measure the torque but the motor
  current provides only an estimate for motor
  torque.
• The actual torque that is transmitted through
  the motor shaft is different from the motor
  torque generator.

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• FORCE SENSORS
• These are used to measure impact forces. Direct
  measurement of forces is useful in non-linear
  feedback control of mechanical systems.
• These are based on measuring a deflection caused
  by the force, both impulsive forces and slowly
  varying forces can be monitored using these
  sensors.
• Several sensors having thin film and foil sensors
  that employ the strain gage principle for
  measuring forces and pressures are shown below.

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• The stability of the control system can depend on
  the location of force sensors used.
• The non-linear control feedback using force
  acceleration and displacement measurements is
  shown below.

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• TACTILE SENSING
• Tactile sensing is usually interpreted as touch
  sensing, but tactile sensing is different from a
  simple touch where very few discrete force
  measurements are made.
• In tactile sensing a force distribution is
  measured using a closely spaced array of force
  sensors.
• Tactile sensing is particularly important in two
  types of operations
• Grasping and
• object identification.
• In grasping object has to be held in a stable
  manner without being allowed to slip and without
  being damaged.
• Object identification includes recognizing or
  determining the shape, location and orientation
  of object.
• The above tasks would require two types of
  sensing.
• Continuous variable sensing of contact forces.
• Sensing of the surface deformation profile.

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• These two types of data are generally related
  through the constitutive relations of the tactile
  sensors, touch surface or of the object that is
  being grasped.
• TACTILE SENSOR REQUIREMENTS
• Applications which are very general and numerous,
  include automated inspection of surface profiles
  and joints for defects, parts identification and
  gaging in manufacturing applications and fine
  manipulation tasks.
• The tactile sensor should have a compliant
  covering with skin like properties along with
  enough degrees of freedom for flexibility and
  dexterity, adequate robustness and some local
  intelligence for identification and learning
  purposes.
• Typical specifications for and industrial tactile
  sensor are
• Spacial resolution of about 2mm.
• Forces resolution of about 2gm.
• Force capacity of about 1kg.

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• 4. Response time of 5ms or less.
• 5. Low hysteresis.
• 6. Durability under harsh conditions.
• 7. Insensitivity to change in environmental
  conditions.
• 8. Capability to detect and even predict slip.
• OPERATION OF TACTILE SENSORS
• A schematic representation of a optical tactile
  sensor is shown below.
• The light source, the beam splitter and solid
  state camera form an integral unit.
• The splitter plate reflects part of the light
  from the light source onto a bundle of optical
  fiber.
• The image processor computes the deflection
  profile and the associated tactile force
  distribution.

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CONCLUSION
In this presentation we have described you the
concept of force control and its types, Strain
gages, torque sensors, force sensors and
tactile sensors.
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Thank You