3 Ağustos 2007 Cuma

robot sensors

Sensors are very critical part of any robot, whether autonomous or teleop-erated. Sensors are the robot’s contact with the outside world or its own inner workings. There was a time when ‘sensors’ for an experimenter’s robot were just a few whiskers connected to microswitches to sense walls and such. When the robot banged into a wall or obstacle, the switches were tripped and its simple logic steered it in another direc¬tion. I mentioned such a crude robot that built years ago in last month’s column. Certainly some early machines used photo tube, selenium, or cadmium sulfide cells to detect light sources and react accordingly, but sensors for the most part were some sort of bumpers or whiskers to detect tangible objects and barriers.
Those were the days before microcontrollers were available that could make sense of a suite of sophisti¬cated sensors that we have available today. A typical robot emerging from the workshops of SERVO’s readers might have upwards of 100 sensors to guide it about its daily activities. Sensors can be used to sense the robot’s environment or its own internal parameters. These might nclude active and passive IR sensors; sound and voice sensors; ultrasonic range sensors, positional encoders on arm joints, head and wheels; compasses, navigational and GPS sensors; active and passive light and laser sensors; a number of bumper switches; and sensors to detect acceleration, turning, tilt, odor detection, magnetic fields, ionizing radia¬tion, temperature, tactile, force, torque, RF, UV, video, and numerous other types. Figures 1 through 4 illustrate just a few of the industrial quality sensor types manufactured by SICK, a compa-ny who supplied many of the sensors used in DARPA’s Grand Challenge. There are many similar companies producing sensors for use on robots.
IR Sensors
I’ll start with IR sensors as they are among the most popular for experimen¬tal robots. Infrared sensors are usually divided into two basic types: the passive or PIR sensors that emit no IR radiation and the active types that emit an IR beam that is again detected by reflec¬tion. We all have used the PIR types to detect the presence of a human outside our homes and have it turn on an outside light for a specified number of minutes. Since hundreds of millions are sold each year for security purposes, the prices are quite low. The PIR sensor uses a crystal of lithium tantalate (LiTaO3), a compound that possesses unique pyroelectric properties in that it can sense the temperature range of a human being’s body. The crystal does not detect the actual temperature but a change in temperature. It does this by seeing a series of images focused upon the crysta by a row of Fresnel lenses as a person (or animal) crosses the viewing path. This cross-ng movement causes a series of images to cross the crystal’s surface, thus, a succession of temperature changes in the crystal’s charge status. Open up a typical outdoor security light fixture and you’ll find a PIR sensor like in Figure 5 — the heart of the ‘warm body’ detector.
Figure 4. Inductive, magnetic, and capacitive sensors.

To detect a non-moving warm image, an image ‘chopper’ with wide teeth like a gear can be used to interrupt the image and cause the crystal to detect a series of changes in charge status, just like the Fresnel lens array with a moving warm object. Check this technology out on Google; it’s quite interesting. Passive IR sensors can also be used for two-way control and communications from a laptop computer or a hand-held remote control. They actually become active when they send a signal back to a computer or another robot.
The active IR sensors generally use an IR LED emitting an invisible beam that is, in turn, picked up as a reflected spot on a wall or object by a photo transistor. This same technology can be used as a range finder by having a focused beam emitted from the side or front of the robot at an angle and another series of IR detectors mounted behind a lens pointing straight out. The further away the sensed object, the greater change in detected angle by the detector array. You can try this out with a laser pointer held at an angle as you close in on a wall. If the laser is pointing to the right at, say, 45 degrees, the spot will move to the left as you get closer to the wall.
Lasers are particularly adaptable to robot ranging and object detection. The very inexpensive diode lasers available as pointers and power tool line generators make great robot add-ons. I have seen very cheap (under $5) builders levels with a built-in laser with a line generat-ng lens at Harbor Freight Tools. This straight line projected onto an object and observed from above the laser can determine the shape of the object by some fairly simple edge detection software and a cheap camera. Lasers also generate a collimated beam that does not require a lens assembly to produce a small spot of light.

Positional Encoders
Positional encoders are probably the second most popular sensor on a robot. Most experimental robots do not have arms and do not use positional encoders to determine the positions of an arm’s different joints. They do use shaft encoders on the wheels or motor shafts to determine the number of revolutions of the wheels and thus, the distance traveled by the wheels.
These encoders can use electrica contacts, magnetic Hall-effect detectors, or the more popular optical path broken by rotating teeth or opaque and clear graphics etched on a wheel. Absolute encoders output a binary word for each ncremental position and are complex and expensive. Incremental encoders provide a pulse for each increment of shaft movement. The use of two optica channels enable the determination of the direction of rotation. Again, Google these unique devices for detailed information.
The use of potentiometric encoders is popular for the model aircraft servos (before hacking) and are used for 180° or less rotation. Multi-turn pots, partial turn pots, linear pots, and even trim pots can be used as feedback positional sensors. A 25 turn trim pot can be attached to a 25 turn leadscrew and hacked model airplane servo and be used as the feedback device for a fairly powerful linear actuator.
Compasses and Navigation Sensors
I tried to use a standard Boy Scout type of compass in one of my early robots to determine direction. It was one of those liquid filled types that dampened the moving disk that con-tained the tiny magnet and the north arrow. I pulled it apart so many times to place reflective stickers on the disk that it always leaked and the disk seemed to never point north as it was overloaded by stickers that eventually floated off the surface. It was so sluggish and my crude CdS and flashlight bulb detector array didn’t work very well. Needless to say, I soon scrapped this robot compass

(but what a clever idea! - Ed.)
Much better electronic compasses are available today for the experimenter. Magnetometer and fluxgate compasses began to be available to the experi¬menter in the ‘80s and first saw uses in consumer automobile compasses. These devices made autonomous navigation possible for the first computer-controlled machines. However, it was the nexpensive compass modules made by Devantech, Parallax, and others that made interfacing compasses to micro-controllers so easy. As Parallax states: “This compass module made exclusively by Parallax is a dual-axis magnetic field sensor built around the Hitachi HM55B IC. Parallax has made this compass IC accessible by providing Hitachi’s surface mount sensor chip with a 3V onboard voltage regulator and resistor protection, all in a 0.3” wide six pin DIP module. The Hitachi HM55B compass module is compatible with the BASIC Stamp’s 5V supply and signal levels. Acquiring measurements from the module is made easy with a synchronous serial interface, and even easier with the BASIC Stamp 2 commands SHIFTIN and SHIFTOUT” (See Figure 6.) I’ve used this one as it is quite small and easily placed in small robots.
Devantech uses a magnetic field sen¬sor as the core of their device and states that “This compass module has been specifically designed for use in robots as an aid to navigation. The aim was to produce a unique number to represent the direction the robot is facing. The compass uses the Philips KMZ51 magnet¬ic field sensor, which is sensitive enough to detect the Earth’s magnetic field. The output from two of them mounted at right angles to each other is used to compute the direction of the horizonta component of the Earth’s magnetic field.” (See Figure 7.) Both of these devices are available from many of the advertisers in SERVO and Nuts & Volts.
Ultrasonic Range Sensors
Back in the ‘80s, Polaroid devel-oped an automatic focusing camera that used an ultrasonic range sensor (transducer) to focus the camera’s lens on an object, usually a person. I bought one, not to take the fairly expensive instant pictures, but to hack it. After getting into it a bit, I decided to not completely ruin it so I ended up using it as a camera. It piqued the interest of other experimenters also who soon saw many uses for the technology and Polaroid was quick to serve this unique market. They began marketing an experimenter’s kit that contained two of the electrostatic transducers, an electronics board that was developed for the camera with a separate LED distance readout, and several flat 6V battery packs. I was fortunate to be given two of the kits by Polaroid to experiment with. (Actually I was accidentally given four, but that’s another story!)
The transducers and drivers required a high current pulse, too high for AA cells, so Polaroid also developed a higher current flat battery pack that was behind each film pack. These range finders found their way into almost every robot that was marketed in the ‘80s. The October ‘00 issue of the Seattle Robotics Society’s Encoder featured an article by Dennis Clark entitled “SONAR as I Have Done it” on the hacking of a Polaroid camera for the sonar module and transducer.
Figure 8 shows the uniquely shaped circuit board in the camera and the transducer on the table. A bit of a warning if you ever start playing with one of these — there’s a pretty high voltage, like 300 VDC within the module that will certainly wake you up if you touch the wrong areas. I have been awakened a few times in the past. Newer units by other manufactur-ers use piezoelectric transducers that are a bit smaller.
Sensors for Toys
I have taken Design News Magazine for many years, an excellent source of mechatronics ideas for engineers, and the April 30th edition had a supplement highlighting sensors for toy design. We sometimes dismiss toys as low-cost, low-quality products just for kids to play with and eventually compass module.
tear up, but toy manufacturers have come up with some of the most unique and complex things for just a few dollars. Such is the case with the Zig Zag Zog UFO Saucer featured on the front cover of the “Sensors ... in Toy Design” sup¬plement of Design News (see Figure 9). Designed for toddlers, I know that this $30 toy will be purchased, disassembled, and hacked by any number of adult robot experimenters before spring is over. The objective of the toy is for the little alien to try to avoid being captured by a toddler utilizing three IR sensors to detect obstacles in its path and two more IR sensors in its head to detect the kid trying to bop it there. If it is caught, the toy operates at successively higher speeds to avoid capture.
NEC’s PaPeRo Personal Robot
The large Japanese company, NEC, recently brought a personal robot to the market — the R-100 called PaPeRo through its Personal Robot Research Center. There is no mistaking that this machine is of Japanese origin as it’s a bit too cute for most adults but the technology is amazing, as is the case with most Japanese robots. Just as with Sony’s Aibo — rest its soul — this robot has all types of sensors strewn about its body to assist in its interaction with humans. Voice and video image (people) recognition are among its most noteworthy. Various Other Sensors for Robots
There are hundreds — if not thousands — of types of sensors made today to sense virtually any type of phenomenon you can think of. Besides some of the ones that I delved into, one type that I’ve used on several occasions is the Figaro gas sensor. Back in the early 70s when I first heard of it, the Figaro gas sensor was used to detect — you guessed it — gas vapors. The company has evolved it line of sensors to detect virtually all types of anything that can be sensed as a vapor. The sensing surface is made of sintered metal oxides of various ratios and metals according to the type of gas to be sensed.
The surface’s conductivity changes according to concentration of the gas when reducing gases are absorbed on the sensor’s surface. These sensors can detect alcohol in one’s breath, propane, HVAC air, gasoline vapor, natural gas, and even cooking smells Figure 11 shows a few of the gas sensors available today.
Temperature is easily determined by a simple thermistor, a resistor that changes resistance according to

temperature. Atmospheric pressure and other weather-related phenome¬na can be determined from home weather station components. Smoke detectors are very cheap and great add-ons for a home security robot. Accelerometers and gyro systems are available from SERVO advertisers and model helicopter suppliers. Cheap GPS receivers abound and are great for longer-range outdoor robots. Radiation detectors can be used for high security robots. There is no limit of what type of sensor you can place on a robot.

If you want to know more about the hundreds of types of sensors available on the market today, visit the Internet and Google through millions of sites. A book I highly recommend is H.R, ‘Bart’ Everett’s Sensors for Mobile Robots -Theory and Applications. This 1995 book still has a great amount of information that is applicable for today’s experimental machines and is written by one of the world’s most respected robot designers and builders. Bart has built a series of robots that are second to none, especially when it comes to sensor technology
Gönderen zaman:

Hiç yorum yok:

Yorum Gönder

Kaydol: Kayıt Yorumları (Atom)
Cascode stage
or “collector follower”
Jean-Paul Brodier
All microprocessors from the 8051 family have inputs and outputs that are ‘quasi-bidirectional’. This means that when power is first applied, the ports behave as inputs with a logic high level and a weak pull-up.,

Glitch
a relay or some other load such as When driving an optocou-pler or LED, there is a problem
at power on: the NPN transistor in the common emitter connec­tion (Figure 1) causes an unde­sirable excitation of the load from the moment power is applied until the microprocessor has had the chance to turn the output low. In addition, logic high outputs are seldom able to deliver enough current to drive the transistor into saturation because they have been designed to be active low.
Figure 1. An NPN transistor drives a load.
To solve both of these problems in one hit, we have to make the active level logic low. This can be done in three different ways: use an emitter follower as a buffer stage (Figure 2a), an inverter in a common emit­ter circuit (Figure 2b) or an inverter/open collector circuit (Figure 2c). The disadvantage of solution 2a is the fact that the voltage to the load is reduced. In the case of a relay with a 5-V coil there is the risk that the resulting voltage is too low. The disadvantage of examples 2b and 2c is that they require more parts.
Collector follower
That leaves the open collector buffer in the form of an IC type 7404. This solution, however, also has a few disadvantages. You do not always need all of the 6 buffers in one IC. Also, the SMD version can only handle 12 V. This is too low and dan­gerous if we happen to supply the load from an unregulated voltage.
The solution presented here com­bines in one transistor the advan­tages of the emitter follower (inactive when power is first applied) and open collector (higher power supply voltage, lower current). This circuit has been known since the valve era by the name cascode (drive via the cathode). The goal was to reduce the Miller-effect of the internal (parasitic) capacitances. Not having the option of reduc­ing the capacitance between the internal electrodes, a lower volt­age was used instead. The cas-code circuit is often used in pow­erful transmitters (tens of kW) to minimise the Miller-effect. This circuit was also used to limit tran­sistor conduction and to keep the dissipation within bounds, which increased the life of bipolar tran­sistors. This was in the IGBT and VMOS era.
The transistor conducts only when the output from the micro­processor is low (refer Fig­ure 3). The base current is lim­ited by resistor R. This current is determined by the current flow­ing through the load. When the power is switched on, both the base and emitter see the same potential, VCC, so the transistor remains blocked. One thing we have to keep in mind: we may not exceed the current rating of the microprocessor output because it has to cope with all the current flowing in the emitter of the transistor.
In the case of the quite common 80C51, this maximum current is typically 3.2 mA (two LS TTL loads). This is sufficient to drive an LED without overloading the 5-V regulator, or for driving a PNP power stage at the high side (Figure 3b). The parallel Philips PCF88574 I2C interfaces can handle 25 mA. For the Atmel AT89Cx051 as well as for the Philips P89LPC9xx the limit is 20 mA. For the latter type the cascode circuit or ‘collector fol­lower’ is even more interesting when the outputs are configured as open-drain because the nom­inal voltage is only 3.6 V. In all cases we have to make sure that the maximum dissipation of the
Figure 3. Cascode driver stage with discrete transistor.
package is not exceeded. 24 V is sufficient to energise its are determined by the power
Should this be the case, then the half Watt relay coil, which in PNP (or VMOS) transistor.
number of open collectors turn can drive a load of 16 A at The cascode transistor can be a required will probably justify 230V. ‘digital’ type with integrated resorting to a 7404. For loads driven from the positive base and emitter resistors.A current of around 20 mA at side, the voltage and current lim-


pot as interrupt generator

In battery-powered, microcon­troller driven circuits, as well as with microcontrollers operating in cars, it is desirable to switch the micro into power-down mode once a task has been completed. An interrupt request is then required to wake up the micro. This circuit allows an interrupt to be generated in a simple way using a common potentiometer. In the example circuit, the pot may also copy its spindle position to the ADC. This enables the pot to be used for continuously variable settings (like volume) as well for getting the micro out of its power-down mode.
IC1A is configured as a differen­tiator with R3 preventing oscillation by keeping the gain down to 10 times. Because the opamp oper­ates off a single-rail supply voltage, an 18k/10k potential divider (R1/R2) is able to create a virtual ground level at +1.75 V. This can be done because the LM358 can handle input levels of up to 3.5 V when supplied at 5.0 volts. IC1A supplies a brief High pulse at a falling input voltage, and a similar Low pulse when the input voltage rises. In order to get a High pulse when the potentiome­ter spindle is turned cw or ccw, IC1B is set up as an inverter. Next, each opamp output drives the base of a BC547 transistor. The 5 V-to-0 V transitions at both collector outputs are shaped and combined into a usable interrupt pulse by three NOR gates IC2A, IC2B and IC2C.If the potentiometer spindle is turned very slowly, it is possible that the circuit does not respond
That is why an LED has been added that lights briefly when a pulse is generated. Finally, a tip: a 100-pF capacitor may be connected in parallel with R5 for additional suppres­sion of self-oscillation.

elektor time standart


Elektor Time standard (1988)
Jan Buiting

The Elektor Time Standard and associated Slave Unit were spin-offs of another hugely successful project, the DCF77 Receiver / Locked Frequency Standard. The receiver was published in the January 1988 issue, the Time Standard and Slave display in the next two issues. All units were housed in then very fash­ionable and (expensive!) Ver-obox two-part ABS enclosures which had also been used for a number of Elektor test instrument designs published between 1984 and 1987. The Time Standard box was designed to process seconds pulses received from the VLF (77.5 kHz) DCF77 time standard transmitter in Mainflingen, Ger­many, and display time (with atomic accuracy) and date on an LC display. The circuit was based on then extremely popular 8052AH-BASIC microcontroller from Intel, a device, we can safely claim, that made it to fame & glory thanks to Elektor Electron-
ics. The 40-way DIL chip con­tained a BASIC interpreter capa­ble of executing ‘tokenised’ code from an external EPROM. This, we were told by our resident designer Peter Theunissen, made writing the DCF77 time signal decoding routines ‘a doddle’ using his specially adapted BASIC computer and interpreter. For example, when concerns were raised (by myself) that not all of Europe was in the time zone served by DCF77 (i.e., CET or GMT+1h), a menu option was quickly added to allow users to select between UTC and GMT+1h. As a relative novelty, a ready-made self-adhesive front panel foil with built-in membrane keys was designed into the proj­ect. This expensive item had been produced specially for Elektor. However, when the article went into print (using a rather glum page layout and black & white print), there were yet other con­cerns regarding the range of the DCF77 transmitter. This is offi­cially claimed as “approximately 1,000 km by groundwave propa-

gation”. A quick use of a com­pass and a map of Europe sug­gested that the signal would only cover the south-eastern part of the UK, possibly including Greater London. For a couple of months we waited with baited breath for readers’ responses, only to receive two enthusiastic reception reports, one from the East coast of Ireland and another from Riyadh, Saudi Arabia! The latter report came from a reader work­ing at a chemical laboratory. I remember he wrote that DCF77 could be received for a few min­utes a day only, synchronising the clock, usually around nightfall despite heavy ‘static’. A huge wire antenna was used (nothing like the 1-inch ferrite rods we used in our lab, which is less than 100 km away from Mainflingen). Although the BASIC program list-
ing for the Time Standard was freely distributed to interested readers (on paper, in an enve­lope, by snail mail!), only very advanced readers were able to compile the program into tokenised code and burn it into an EPROM. Most other readers had to rely on a ready-pro­grammed 27C64 supplied through our Readers Services. Apart from displaying time and date at atomic accuracy, the Time Standard was also capable of outputting time/date information

in the form of ASCII character strings for other (intelligent) equip­ment to use, for example, a timer or switching clock. Although sales figures of the PCB and EPROM were in the hundreds, I never heard from anyone actually hav­ing enjoyed the wonders of the ASCII output so extensively described in the article. The Slave unit published in March 1988 was connected to the Time Standard via screened (micro­phone) cable, the idea being that one or more Slave units could be installed on walls in rooms at some distance from the main clock unit. Central timekeeping deluxe for offices, labs, schools and workshops, but at what an expense and design effort! Not too many PCBs were sold for this extension of the Time Standard.