Sensors Notes 47r1p

SENSORS FOR ROBOTIC SYSTEMS

1.0 Transducers & Sensors A transducer is a device that converts one type of physical variable (e.g, force, pressure, temperature, velocity, flow rate, etc) into another form. A common conversion is to electrical voltage, and the reason for making the conversion is that the converted signal is more convenient to use and evaluate. A Sensor is a transducer that is used to make a measurement of a physical variable of interest. Any sensor or transducer requires calibration in order to be useful as a measuring device.

Transducers and sensors can be classified into two basic types: - Analog transducers - Digital transducers - Analog transducers provide a continuous analog signal such as electrical voltage or current. This signal can then be interpreted as the value of the physical variable that is being measured. - Digital transducers produce a digital output signal either in the from of a set of parallel status bits or a series of pulses that can be counted. In either form, the digital signal represents the value of the measured variable.

1.1 Main types of sensors The function of robot sensors may be divided into two principal categories: - Internal & External State Sensors.

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- Internal State Sensors deal with the detection of variable such as the position and the rate of movement (velocity) of robot arm ts, which are used for control control. - External State sensors on the other hand deal with the detection of variables such as proximity, range and touch. External sensors are used for robot guidance as well as for object identification and handling (Vision and tactile sensors). External sensors can be further classified as & Non types. ing sensors respond to a physical such as touch, slip, force and torque. Non ing sensors rely on the response of a detector to variations in optical, accoustic or electromagnetic radiation.

1.2 Sensor Characteristics These can be divided into Static and Dynamic characteristics. - Static:

Linearity Hysteresis Resolution

- Dynamic:

Frequency Response Step Response

1.3 Internal Sensors : - Position - Velocity 1.3.1 Position Sensors : - Angular:

- Linear:

- Tachometers - Incremental Encoders - Absolute (Shaft) Encoders - Analog Resistive - Digital (encoded) types. 2

Absolute encoders are used in situations when : - the control system is subjected to frequent power shut down and the current positional information is to be retained. - frequent resetting of the robot arm positions is to be avoided such as high flexibility and reliability position sensing systems. Absolute encoders generally are limited to one single revolution. Analog Resistive sensors: linear insulator bar having a resistance wire wound on it. 1.3.2 Velocity Sensors: - D.C Tachogenerators - Optical Incremental Encoders

1.4 External Sensors External Sensors



Tactile

Non-

Force/Torque

Visual

Proximity/Range

1.4.1 Proximity Sensors Proximity Sensors

Pneumatic

Inductive

Capacitive

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Optical

Ultrasonic

(i) Pneumatic - known as Nozzle Flapper: Air is supplied and by moving a flapper in front of the nozzle the output pressure is varied. (ii) Inductive - based on the principle of change of inductance due to the presence of a ferromagnetic object. (iii) Hall effect - based on the principle of Lorenz force. (iv) Capacitive - can detect all objects made of electrically conductive materials. (v) Optical - narrow beam is produced by an L.E.D, and a lens. Reflected light on a photodiode depends on the distance of the object reflecting the light. (vi) Ultrasonic - sensor consists of a piezo crystal and microphones.

1.4.2 Tactile Sensors: These are a generalised analog Touch Sensors since they measure forces at several points (in an array) at the ing area between the robot’s gripper and the workpiece. Tactile Sensors

Resistive

Optical

Magnetic

Capacitive

General requirements for tactile sensors are as follows: - Should provide an array of sensing elements at a density of 10*10 per square inch constructed on a compliant surface. - Each element should have a response time of 1 to 10 msec (step). - The elements must be stable, repeatable and have low hysteresis. - the threshold sensitivity for the elements should be about 1 grammes, with an upper limit of 1000 grammes. - the sensing material must be robust, capable of standing upto harsh industrial environments.

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