ROBOTICS

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� S. MURALI KRISHNA

SENSORS FOR ROBOTSSensors bring human touch to robots. Here is how

Sensors collect useful informationas engineering parameters aboutthe internal state of robots.

These are also used to communicatewith the outside world.

The robot controller needs to knowinstantaneously or periodically where

Mr DD robot playing with ball

each link of the robot is in order toknow the robot’s configuration. Sen-sors, which are part of the robot, sendinformation about each joint or link tothe controller, which determines theconfiguration of the robot. Also, exter-nal sensory devices like vision system,touch and tactile sensors—that are partof robots—enable them to communi-

cate with the outside world.Similar to neurons in the human

body muscles that send signals to thebrain to let it determine the state of amuscle, in a robot, as the links andjoints move, sensors such as potenti-ometers, encoders and resolvers sendsignals to the microcontroller that al-low it to determine where each jointis. Also, some more sensors similar infunction to that of humans are used inrobots for vision, touch and smell.

For harsh environments likenuclear zones, a radioactive sensor isan example of a complex sensor thatthe human body does not have. Apartfrom these, a few advanced sensorsfind application in robotics and manu-facturing as engineering solutions.

Sensors for internalfeedback control

Basic sensors useful for robotics,mechatronics and automation are dis-cussed below.

Position sensors. These are usedto measure displacements (both rotaryand linear) as well as movements. Inmany cases, such as encoders, the po-sition information may also be usedto calculate velocities. Position sensorscould be either contact-type or non-contact-type. Under non-contact-typemagnetic pickup, optical encoders arethe examples.

In a simple potentiometer acting asvoltage divider, the output will be de-pendent to the resistance as:

where R1 defines the contact position.Encoder. It is a simple device that

can output a digital signal for eachsmall portion of a movement. Thereare basically two different types ofencoders, namely, incremental andabsolute encoders.

LVDT. It is actually a transformer

Vout = Vcc ×R1R2

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where the core moves along the dis-tance being measured and outputs avariable analogue voltage as a resultof this displacement. The output of anLVDT is linearly proportional to theinput position of the core. The outputvoltage is proportional to the displace-ment of the core from the centre. Thisis shown in Fig. 1.

Resolvers. These are similar toLVDTs in principle but are used tomeasure an angular motion (typically,a tank gun rotation). A resolver is alsoa transformer where the primary coil

is connected to the rotatingshaft and carries an ACthrough slip rings. Thereare two sending coilsplaced 90 degrees apartfrom each other. The coilsdevelop a voltage propor-tional to sine and cosine ofthe angle between primaryand two secondary coils as

shown in Fig. 2. Resolvers are reli-able, robust and accurate.

Velocity sensors. When a posi-tion signal is differentiated, it con-verts into velocity signal. Differen-tiation of a signal is always noisyand should be done carefully. Asimple R-C circuit with an op-amp(‘R’ as feedback and ‘C’ to coupleposition signal at inverted end) de-livers the velocity signal as output(Vout= –RC.dv/dt).

Acceleration sensor. Acceler-ometers measure accelerations.These are not used in robots butrecently acceleration measurementshave been used for high-precisioncontrol of linear actuators and for

joint feedback control of robots.Force and pressure sensors. It is a

polymer thick-film device that executesa decreasing resistance with increas-ing force applied normal to its surface.For example, with a changing force of10 to 10000 grams, its resistancechanges from about 500 kilo-ohms to1 kilo-ohm.

Torque sensors. A pair of strategi-cally placed force sensors can be usedfor torque measurement. Two forcesensors are placed on a shaft oppo-site to each other on opposite sides.

If a torque is applied to the shaft,it generates two opposing forceson the shaft’s body causing opposite-direction strains. The two force sen-sors can measure the forces, whichcan be converted into a torque asshown in Fig. 3.

Micro switches. Micro switches aresimple, robust, inexpensive and com-mon in robotics. These cut off electri-cal current through a conductor andthus can be used for safety purposes,detecting contact, sending signalsbased on displacements, etc.

Sensors for interactionwith the outside world

Given below is the basic operatingprinciple of some of the sensors de-ployed for interaction with externalsources or target locations. CCD sen-sors, thermal imaging sensors, radio-frequency (RF) and microwave sensorsare not covered here.

Light and infrared sensors. Theelectrical resistance of these sensorschanges when light is projected onthem. They exhibit low resistance forhigher intensity of light.

A phototransistor turns a device‘on’/‘off’ when light falls on its win-dow.

A light sensor array can be usedwith a moving light source to mea-sure displacements/small movementsin robots and other machinery.

Infrared (IR) sensors are sensitiveto infrared range. Since IR is invisibleto human eyes, it will not disturb hu-mans if used in devices that projectthe light out. If a device needs light tomeasure a large distance for naviga-tion purpose, IR can be used withoutattracting attention or disturbing any-one. IR remote controls can be used toestablish remote control between de-vices and robots.

Touch and tactile sensors. Touchsensors send a signal when physicalcontact is made. Micro switch is thesimplest touch sensor which eitherturns on or off as contact is made. Aforce sensor used as a touch sensormay not only send touch informationbut also report the magnitude of touch-ing forces. A tactile sensor is a collec-tion of touch sensors that, in addition

Fig. 1: Linear variable differential transformer (LVDT)

Fig. 2: Schematic of a resolver

Fig. 3: Arrangement of three pairs of strain gaugesalong the three major axes for force and torquemeasurements

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to determining contact, can provide in-formation about the subject.

Touch sensors are, in fact, displace-ment sensors. Therefore other typesof displacement sensors (microswitches, LVDTs, pressure sensors,magnetic sensors, etc) may be used forthis purpose.

As the tactile sensor comes in con-tact with an object, depending on theshape and size of the object, differenttouch sensors react differently in dif-ferent sequences. This information isthen used by the controller to deter-mine the size and shape of the object.Figs 4(a) and 4(b) show such sensors.

A continuous skin-like tactile sen-sor that can perform like human skinis shown in Fig. 4(c). It comprisesa matrix of sensors that are embed-

ded betweentwo polymerlayers separatedby an isolatormesh.

Proximitysensors. A prox-imity sensor de-termines that anobject is close toanother objectbefore contact ismade. This non-contact sensingcan be useful insituations rang-ing from mea-suring thespeed of a rotorto navigating arobot. Differenttypes of prox-imity sensorsincluding mag-netic, eddy cur-rent and Hall ef-

fect, optical, ultrasonic, in-ductive and capacitive areused in robots.

Sniff sensors. Similar tosmoke detectors, these aresensitive to particular gasesand send a signal whenthey detect those gases.Sniff sensors are used forsafety as well as search anddetection purposes.

Voice recognition devices. When avoice recognition system recognises aword, it sends a signal to the control-ler, which, in turn, runs the robots tothe desired location and orientation. Itis particularly useful for robots usedto aid the disabled as well as for medi-cal robots.

Voice recognition involves what issaid and taking an action based on theperceived information. The voice rec-ognition system generally works onthe frequency content of spokenwords. Any signal may be decom-posed into a series of sines and co-sines at different frequencies at differ-ent amplitudes that will reconstruct theoriginal signal if combined.

Voice synthesisers are accom-plished in two ways. One is to recre-

ate the words by combining phonemesand vowels. Here each word is recre-ated when the phonemes and vowelsare combined. Another way is torecord the words that the system mayneed to synthesise and access themfrom memory or tape as needed. Stan-dard telephone time announcements,video games and many machine voicesare prerecorded and accessed asneeded.

Range finders. Light-based (laser)range finders measure the distancefrom an object by three different meth-ods, namely, direct time delay, indi-rect amplitude modulation and trian-gulation. For shorter distances encoun-tered in navigations, triangulation isthe most accurate and gives the high-est resolution results.

Nuclear sensors. Upon detectionof a nuclear burst/contamination ofatmosphere, based on input fromgamma sensor (responds to gammaray exposure in nanoseconds) anddose rate from the GM probe (Figs 5and 6), its intensity and place of oc-currence (GPS data) can be transmit-ted as data over wireless to alert thesituation control station. Similarly, achemical sensor responding to mostcommon chemical contaminations inair and biological sensors can also behooked to the microcontroller-basedsystem to know the type of contami-nation for taking preventive measuresin time.

Sensor characteristics

Resolution. It is the minimum step sizewithin the range of measurement of asensor. In a wire-wound potentiom-eter, it will be equal to resistance ofone turn of wire. In digital deviceswith ‘n’ bits, resolution is ‘full range/2n.’ An absolute encoder with 8 bitsresolution can report position up to 28

=256 levels. Therefore its resolution is360/256=1.56 degrees.

Sensitivity. It is defined as thechange in output response divided bythe change in input response. Highlysensitive sensors show larger fluctua-tions in output as a result of fluctua-tions (including noise) in input.

Linearity. It represents therelationship between input variations

Fig. 4: (a) Tactile sensors are generally a collection of simple touchsensors arranged in an array or a matrix form with a specific order to relaycontact and shape information to the controller; (b) A tactile sensor canprovide information about the object; (c) Skinlike tactile sensor

Fig. 5: Gamma detection process

(a)

(b)

(c)

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and output variations. In a sensor withlinear output, any change in input atany level within the range will pro-duce the same change in output. Whenthe sensor is not linear, its linearityrange can be recorded, and where itis non-linear it can be programmedas in a displacement sensor (by speci-fying the angle) whose output varieswith sine angle.

Range. It is the difference betweenthe smallest and the largest outputsthat a sensor can provide, or the dif-ference between the smallest and larg-est inputs with which it can operateproperly.

Response time. It is the time that asensor’s output requires to reach a cer-tain percentage of total change. It isalso defined as the time required toobserve the change in output as a re-sult of change in input; for example,ordinary mercury thermometer re-sponse time and digital thermometerresponse time (one is slow and theother is fast).

Frequency response. The frequencyresponse is the range in which thesystem’s ability to resonate (respond)to the input remains relatively high.The larger the range of frequency re-sponse, the better the ability of thesystem to respond to varying input.Similarly, it is important to considerthe frequency response of a sensorand determine whether the sensor’sresponse is fast enough under all op-erating conditions, in particular, mili-tary, underwater and aerospace ap-plications.

Reliability. It is the ratio betweenthe number of times a system oper-ates properly and the number of timesit is tried. For continuous satisfactoryoperation, it is necessary to choose re-liable sensors that last long while con-sidering the cost as well as other re-quirements.

Accuracy. It shows how close theoutput of the sensor is to the expectedvalue. For a given input, certain ex-pected output value is related to howclose the sensor’s output value is tothis value. Inaccuracy can be predicted,measured, corrected or compensatedgenerally.

Repeatability. It is a measure of

how varied the different outputs arerelative to each other. For the same in-put if the output response is differenteach time, then repeatability is poor.Also, a specific range is desirable foroperational performance as the perfor-mance of robots depends on sensors.

Repeatability is a random phenom-enon and hence there is no compensa-tion. The best way is to quality checkand choose the sensors to rule out thebasic sensor problem in the system in-tegration.

Interfacing. Direct interfacing ofthe sensor to the microcontroller/mi-croprocessor is desirable while someadd-on circuit may be necessary incertain special sensors. The type ofthe sensor output (digital or analogue)is equally important. An ADC is re-quired for analogue output sensors;for example, potentiometer output tomicrocontroller.

Size, weight and volume. Size is acritical consideration for joint displace-ment sensors. When robots are usedas dynamic machines, weight of thesensor is important. Volume or spaceis also critical to micro robots andmobile robots used for surveillance.

Cost is important especially whenquantity involved is large in the endapplication.

The perfect match

Choice of sensors plays a vital role inrobotics design as their output is anessential input to the processor con-trolling the robot or its end use. There-fore it is needless to emphasise thatthe basic understanding of the sensorcharacteristics, its matching with thecontrol system and proper utilisationby the intelligent software built intothe robot are essential for a roboticproject to succeed. �

Fig. 6: Roentgenometer cum GM probe