Therefore, a sensor has input properties of any kind and electrical output properties. Any sensor is an energy converter. No matter what you try to measure, you always deal with energy transfer from the object of measurement to the sensor. The process of sensing is a particular case of information transfer, and any transmission of information requires transmission of energy.
Of course, one should not be confused by an obvious fact that transmission of energy can flow both ways—it may be with a positive sign as well as with a negative sign; that is, energy can flow either from an object to the sensor or from the sensor to the object. A special case is when the energy is zero, and it also carries information about existence of that particular case. For example, a thermopile infrared radiation sensor will produce a positive voltage when the object is warmer than the sensor infrared flux is flowing to the sensor or the voltage is negative when the object is cooler than the sensor infrared flux flows from the sensor to the object.
When both the sensor and the object are at the same temperature, the flux is zero and the output voltage is zero. This carries a message that the temperatures are the same. The term sensor should be distinguished from transducer. The latter is a converter of one type of energy into another, whereas the former converts any type of energy into electrical.
An example of a transducer is a loudspeaker which converts an electrical signal into a variable magnetic field and, subsequently, into acoustic waves. Transducers may be used as actuators in various systems. An actuator may be described as opposite to a sensor—it converts electrical signal into generally nonelectrical energy. For example, an electric motor is an actuator—it converts electric energy into mechanical action. Figure A sensor may incorporate several transducers. Robotics Basic Concepts 22 Transducers may be parts of complex sensors Fig.
For example, a chemical sensor may have a part which converts the energy of a chemical reaction into heat transducer and another part, a thermopile, which converts heat into an electrical signal.
The combination of the two makes a chemical sensor—a device which produces an electrical signal in response to a chemical reaction. Note that in the above example, a chemical sensor is a complex sensor; it is comprised of a transducer and another sensor heat. This suggests that many sensors incorporate at least one direct-type sensor and a number of transducers. The direct sensors are those that employ such physical effects that make a direct energy conversion into electrical signal generation or modification.
Examples of such physical effects are photo effect and See beck effect. In summary, there are two types of sensors: direct and complex. A direct sensor converts a stimulus into an electrical signal or modifies an electrical signal by using an appropriate physical effect, whereas a complex sensor in addition needs one or more transducers of energy before a direct sensor can be employed to generate an electrical output.
A sensor does not function by itself; it is always a part of a larger system that may incorporate many other detectors, signal conditioners, signal processors, memory devices, data recorders, and actuators. It may be positioned at the input of a device to perceive the outside effects and to signal the system about variations in the outside stimuli.
A sensor is always a part of some kind of a data acquisition system. Often, such a system may be a part of a larger control system that includes various feedback mechanisms.
To illustrate the place of sensors in a larger system, Fig. An object can be anything: a car, space ship, animal or human, liquid, or gas. Any material object may become a subject of some kind of a measurement. Data are collected from an object by a number of sensors. Some of them 2, 3, and 4 are positioned directly on or inside the object. Sensor 1 perceives the object without a physical contact and, therefore, is called a noncontact sensor. Examples of such a sensor is a radiation detector and a TV camera.
Robotics Basic Concepts 23 Figure Positions of sensors in a data acquisition system. Sensor 1 is noncontact, sensors 2and 3 are passive, sensor 4 is active, and sensor 5 is internal to a data acquisition system. Sensor 5 serves a different purpose. It monitors internal conditions of a data acquisition system itself. Some sensors 1 and 3 cannot be directly connected to standard electronic circuits because of inappropriate output signal formats. They require the use of interface devices signal conditioners.
Sensors 1, 2, 3, and 5 are passive. They generate electric signals without energy consumption from the electronic circuits. Sensor 4 is active.
It requires an operating signal, which is provided by an excitation circuit. This signal is modified by the sensor in accordance with the converted information. An example of an active sensor is a thermistor, which is a temperature-sensitive resistor. It may operate with a constant-current source, which is an excitation circuit.
Depending on the complexity of the system, the total number of sensors may vary from as little as one a home thermostat to many thousands a space shuttle. Depending on the classification purpose, different classification criteria may be selected.
Here, we offer several practical ways to look at the sensors. All sensors may be of two kinds: passive and active. The examples are a thermocouple, a photodiode, and a piezoelectric sensor. Most of passive sensors are direct sensors as we defined them earlier.
The active sensors require external power for their operation, which is called an excitation signal. That signal is modified by the sensor to produce the output signal. The active sensors sometimes are called parametric because their own properties change in response to an external effect and these properties can be subsequently converted into electric signals.
For example, a thermistor is a temperature-sensitive resistor. These variations presented in ohms directly relate to temperature through a known function. Another example of an active sensor is a resistive strain gauge in which electrical resistance relates to a strain.
To measure the resistance of a sensor, electric current must be applied to it from an external power source. Depending on the selected reference, sensors can be classified into absolute and relative. An absolute sensor detects a stimulus in reference to an absolute physical scale that is independent on the measurement conditions, whereas a relative sensor produces a signal that relates to some special case.
An example of an absolute sensor is a thermistor: a temperature-sensitive resistor. Its electrical resistance directly relates to the absolute temperature scale of Kelvin. Another very popular temperature sensor—a thermocouple— is a relative sensor. It produces an electric voltage that is function of a temperature gradient across the thermocouple wires. Thus, a thermocouple output signal cannot be related to any particular temperature without referencing to a known baseline. Another example of the absolute and relative sensors is a pressure sensor.
An absolute-pressure sensor produces signal in reference to vacuum—an absolute zero on a pressure scale. A relative-pressure sensor produces signal with respect to a selected baseline that is not zero pressure e.
Another way to look at a sensor is to consider all of its properties, such as what it measures stimulus , what its specifications are, what physical phenomenon it is sensitive to, what conversion mechanism is employed, what material it is fabricated from, and what its field of application is. Tables 1. Represent such a classification scheme, which is pretty much broad and representative. They only return a single bit of information, either 0 or 1.
A typical example is a tactile sensor on a robot, for example using a micro switch. Interfacing to a microcontroller can be achieved very easily by using a digital input either of the controller or a latch. Figure 4 shows how to use a resistor to link to a digital input. In this case, a pull-up resistor will generate a high signal unless the switch is activated.
The output signal of digital sensors can have different forms. See Figure 5 for an example of a sensor with a 6bit wide output word. There are several techniques for building an encoder. The most widely used ones are either magnetic encoders or optical encoders. Magnetic encoders use a Hall-effect sensor and a rotating disk on the motor shaft with a number of magnets mounted in a circle. Standard optical encoders use a sector disk with black and white segments see Figure 2.
The photo-diode detects reflected light during a white segment, but not during a black segment. So once again, if this disk has 16 white and 16 black segments, the sensor will receive 16 pulses during one revolution. Encoders are usually mounted directly on the motor shaft that is before the gear box , so they have the full resolution compared to the much slower rotational speed at the geared-down wheel axle.
Both encoder types described above are called incremental, because they can only count the number of segments passed from a certain starting point. They are not sufficient to locate a certain absolute position of the motor shaft. If this is required, a Gray-code disk Figure 6, right can be used in combination with a set of sensors.
This would not be the case for a standard binary encoding e. This is an essential feature of this encoder type, because it will still give a proper reading if the disk just passes between two segments. For binary encoding the result would be arbitrary when passing between and As has been mentioned above, an encoder with only a single magnetic or optical sensor element can only count the number of segments passing by.
But it cannot distinguish whether the motor shaft is moving clockwise or counterclockwise. This is especially important for applications such as robot vehicles which should be able to move forward or backward. For this reason most encoders are equipped with two sensors magnetic or optical that are positioned with a small phase shift to each other.
With this arrangement it is possible to determine the rotation direction of the motor shaft, since it is recorded which of the two sensors first receives the pulse for a new segment. If in Figure 6 Enc1 receives the signal first, then the motion is clockwise; if Enc2 receives the signal first, then the motion is counter-clockwise.
Since each of the two sensors of an encoder is just a binary digital sensor, we could interface them to a microcontroller by using two digital input lines. However, this would not be very efficient, since then the controller would have to constantly poll the sensor data lines in order to record any changes and update the sector count.
The output format also varies. Typical are either a parallel interface for example up to 8 bits of accuracy or a synchronous serial interface see Section 2. For decades, mobile robots have been equipped with various sensor types for measuring distances to the nearest obstacle around the robot for navigation purposes. Sonar sensors in the past, most robots have been equipped with sonar sensors often Polaroid sensors.
Sonar sensors use the following principle: a short acoustic signal of about 1ms at an ultrasonic frequency of 50 kHz to kHz is emitted and the time is measured from signal emission until the echo returns to the sensor. The measured time-of-flight is proportional to twice the distance of the nearest obstacle in the sensor cone.
If no signal is received within a certain time limit, then no obstacle is detected within the corresponding distance. The most significant problems of sonar sensors are reflections and interference.
When the acoustic signal is reflected, for example off a wall at a certain angle, then an obstacle seems to be further away than the actual wall that reflected the signal.
Interference occurs when several sonar sensors are operated at once among the 24 sensors of one robot, or among several independent robots. Here, it can happen that the acoustic signal from one sensor is being picked up by another sensor, resulting in incorrectly assuming a closer than actual obstacle. Coded sonar signals can be used to prevent this, for example using pseudo random codes [Jorge, Berg ].
Laser sensors today, in many mobile robot systems, sonar sensors have been replaced by either infrared sensors or laser sensors. The current standard for mobile robots is laser sensors for example Sick Auto Indent [Sick ] that return an almost perfect local 2D map from the viewpoint of the robot, or even a complete 3D distance map.
Unfortunately, these sensors are still too large and heavy and too expensive for small mobile robot systems. This is why we concentrate on infrared distance sensors. Robotics Basic Concepts 32 Figure infrared sensor Infrared IR distance sensors do not follow the same principle as sonar sensors, since the time-of-flight for a photon would be much too short to measure with a simple and cheap sensor arrangement.
Instead, these systems typically use a pulsed infrared LED at about 40 kHz together with a detection array see Figure 9. The angle under which the reflected beam is received changes according to the distance to the object and therefore can be used as a measure of the distance.
The wavelength used is typically nm. Although this is invisible to the human eye, it can be transformed to visible light either by IR detector cards or by recording the light beam with an IR-sensitive camera. Figure 2. The digital sensor has a digital serial interface.
It transmits an 8bit measurement value bit-wise over a single line, triggered by a clock signal from the CPU as shown, the relationship between digital sensor read-out raw data and actual distance information can be seen.
From this diagram it is clear that the sensor does not return a value linear or proportional to the actual distance, so some post-processing of the raw sensor value is necessary. The simplest way of solving this problem is to use a lookup table which can be calibrated.
For each individual sensor. Since only 8 bits of data are returned, the lookup table will have the reasonable size of entries. With this concept, calibration is only required once per sensor and is completely transparent to the application program. Robotics Basic Concepts 33 1. Most of them cannot handle jitter very well, which frequently occurs in driving or especially walking robots.
As a consequence, some software means have to be taken for signal filtering. A promising approach is to combine two different sensor types like a gyroscope and an inclinometer and perform sensor fusion in software. A number of different accelerometer models are available from Analog Devices, measuring a single or two axes at once. The acceleration sensors we tested were quite sensitive to positional noise for example servo jitter in walking robots.
For this reason we used additional low-pass filters for the analog sensor output or digital filtering for the digital sensor output. Most motors convert the power to a magnetic field using coils. The power fed in to the motor coils can come from the AC power mains, DC power supplies, or from controllers that control the coils for specific purposes. Motors are divided into classes based on the type of power they use. They are present in most motorized appliances that use AC power.
Motors differ in their construction, speed control, cooling methods, control systems, size, and weight. Construction AC motors have the coils built in to the outside casing the stator and magnets that spin in the middle on the rotor.
Speed The number of windings and the frequency of the power fed to the coils fix the speed of the motor. The speed of AC motors is basically constant. As such, they may not be the best for robots.
If just three windings form a single rotating field one pole , the motor spins at 60 Hz or 3, revolutions per minute RPM. As three more winding coils are added, the number of poles goes to 2 and the RPMs go down to The following equation is used to determine the RPM, where p is the number of three winding coils poles , f is the frequency of the power, and so is the speed of the motor in 4 RPM: cooling the windings are on the outside case, where they can be cooled easier.
It is possible to build an electronic controller to trim the speed and power consumption of an AC motor, but it is best used in situations where only gross mechanical power is needed, especially for constant speed applications. AC motors only have fewer styles because their architecture attempts to take advantage of the existing movement waveform of the AC power. Like most motors, DC motors generate movement by creating magnetic fields within the motor that attract one another. By and large, DC motors have permanent magnets in the stator and the rotor has the coils the reverse of AC motors.
But since DC power has no movement waveform of its own, the motor electronics must create a change in the DC waveform as the motor rotates. This can happen in several ways. By altering the polarity of the DC voltage on the coil as it rotates, we can continually make its field attract the next magnet in the stator.
As the rotor rotates, a set of position-dependent switches in the rotor switch the field on the rotor coils. The switches are implemented with a stationary, partitioned slip ring on the rotor bearing for incoming power and brushes that drag around the ring to power the coils. After the rotor rotates enough, the brushes move to the other part of the slip ring and reverse the polarity on the coils. Further, since the voltages change abruptly, the power supply noise can be severe.
Brushes can wear out and get clogged with dirt. Speed Higher-voltage motors are generally more powerful. Furthermore, since the speed is controlled by linearly varying the power to the coils, the dissipation in the power supply can become a problem. By and large, most DC motor controllers use a chopping waveform to control the average DC voltage as opposed to a linear regulator.
By turning the DC coil voltage off and on to full voltage very rapidly, the average DC voltage on the coil can be controlled by means of a duty cycle. Such motor drives are more efficient. Further, since the coils are on the rotor, they have a considerable gyroscopic effect.
A lot of spinning mass exists on the rotor. As the rotor rotates, electrical controls switch the field on the stator coils. Further, far less mass takes place on the rotor. Higher-voltage motors are generally more powerful. Since no brushes are used, the controller must also sense the motor position. This makes the controller much more expensive. Further, make sure the motor does not have delicate sensing wires to sense position. Try to get the kind where the controller senses the motor position automatically.
It makes the controller more expensive, but the motor will be more mechanically reliable. The rotor has permanent magnets, and the coils are on the case stator. By altering the polarity of the DC voltage on the stator coils as the rotor rotates, we can continually make its field attract the next magnet in the rotor. For this reason, they tend to have less rotational mass. DC motors can perform the same feat but must have carefully designed servo systems to sense and hold their position.
Steppers hold the position that is defined by the motor geometry. If they go too fast, they may lose their position by slipping over one too many poles. They have to move deliberately. They are also not well geared for changing loads; they can lose track of their position if the load varies in a sudden manner.
If they remain stationary for some time, the current in the coil can be reduced. A good controller will do that automatically.
They are generally computerized since the computer must keep track of the position and momentum of the motor. More complex controllers have more than just on-off control of the coil voltage and current. They are not particularly good with large or varying loads, but they function reliably in most applications. Watch alarms and phone ringers are the most common applications of such materials. They are used for small motions like creeping and fine adjustments. If the robot must have very fine, accurate positioning, piezo-electrics can provide the movements.
They can move large loads, albeit slowly. No simpler motor exists. Unfortunately, these tend to be very fragile. These are stepper motors discussed in this section and servos, introduced in the following section. Stepper motors differ from standard DC motors in such a way that they have two independent coils which can be independently controlled. As a result, stepper motors can be moved by impulses to proceed exactly a single step forward for backward, instead of a smooth continuous motion in a standard DC motor.
A typical number of steps per revolution is , resulting in a step size of 1. Some stepper motors allow half steps, resulting in an even finer step size. Figure stepper motor schematic Figure 10 demonstrates the stepper motor schematics.
Executing the sequence in reverse order will move the rotor one step back. Note that the switching sequence pattern resembles a gray code. For details on stepper motors and interfacing. Stepper motors seem to be a simple choice for building mobile robots, considering the effort required for velocity control and position control of standard DC motors.
However, stepper motors are very rarely used for driving mobile robots, since they lack any feedback on load and actual speed for example a missed step execution. Robotics Basic Concepts 39 1. Such a motor must be able to handle fast changes in position, speed, and acceleration, and must be rated for high intermittent torque. A servo, on the contrary, is a DC motor with encapsulated electronics for PW control and is mainly used for hobbyist purposes, as in model airplanes, cars, or ships.
For example, a width of 0. Exact values of pulse duration and angle depend on the servo brand and model. Like stepper motors, servos seem to be a good and simple solution for robotics tasks. However, servos have the same drawback as stepper motors: they do not provide any feedback to the outside.
Base machine of independent type assembly cell. Base machine of line type assembly cell. When placed between 2 turn tables, handling of both tables is possible.
Screw tightening work using robot vision iVY system Process-to-process transfer using inverse specifications Process-to-process transfer of heavy workpieces Finished product inspection, touch-panel type evaluation machine Conveying masks for wafers Tall work pieces conveying and stacking machine Assembly cell independent cell Assembly cell line cell Assembly cell Handling unit for special purpose tester.
Various conditions can be handled by adding the iVY system's position detection function. For example, the robot can be easily incorporated in cases such as when the screw hole positions are inconsistent, the workpiece position on the conveyor is not consistent, or when multiple types of workpieces are supplied.
The iVY system can be calibrated with simple operations. The teaching process can be reduced, so the system startup time can be shortened and labor costs can be reduced, etc. Process-to-process transfer using inverse specifications Transfer workpieces between processes using the inverse specifications.
The inverse specifications allow the workpiece to be held from below, so the dropping of foreign matter onto the workpiece being transferred can be prevented. The robot mechanism performance is equivalent to the standard specifications. Yamaha proposes various ideas when designing your system. Always select the inverse dedicated specification YK-XS-U when considering a system as explained in this example.
Process-to-process transfer of heavy workpieces Process-to-process transfer of heavy workpieces. The timing belt-less drive using the built-in structure realizes a high tolerable inertia for the R axis. A large hand can be used with this high tolerable inertia for the R axis. The transferrable quantity per session increases, and attains a higher efficiency. With a low inertia, the R axis can be moved with a high acceleration, and the cycle time can be shortened. Harmonic gears are adopted for the XYR axis reduction gears.
Finished product inspection, touch-panel type evaluation machine Finished product function test.
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