SENSOR-BASED ROBOT CONTROL
Robotics has matured as a system integration engineering field defined by M.
Bradley as “the intelligent connection of the perception to action”. Programmable
robot manipulators provide the “action” component. A variety of sensors and
sensing techniques are available to provide the “perception”.
t ROBOTIC SENSING
Since the “action” capability is physically interacting with the environment, two
types of sensors have to be used in any robotic system:
- “proprioceptors” for the measurement of the robot’s (internal) parameters;
- “exteroceptors” for the measurement of its environmental (external, from the
robot point of view) parameters.
Data from multiple sensors may be further fused into a common representational
format (world model). Finally, at the perception level, the world model is
analyzed to infer the system and environment state, and to assess the
consequences of the robotic system’s actions.
1. Proprioceptors
From a mechanical point of view a robot appears as an articulated structure
consisting of a series of links interconnected by joints. Each joint is driven by an
actuator which can change the relative position of the two links connected by that
joint. Proprioceptors are sensors measuring both kinematic and dynamic
parameters of the robot. Based on these measurements the control system
activates the actuators to exert torques so that the articulated mechanical
structure performs the desired motion.
The usual kinematics parameters are the joint positions, velocities, and
accelerations. Dynamic parameters as forces, torques and inertia are also
important to monitor for the proper control of the robotic manipulators.
The most common joint (rotary) position transducersare: potentiometers,
synchros and resolvers, encoders, RVDT (rotary variable differential transformer)
and INDUCTOSYN. The most accurate transducers are INDUCTOSYNs (+ 1 arc
second), followed by synchros and resolvers and encoders, with potentionmeters
as the least accurate.
Encoders are digital position transducers which are the most convenient for
computer interfacing. Incremental encoders are relative-position transducers
which generate a number of pulses proportional with the traveled rotation angle.
They are less expensive and offer a higher resolution than the absolute
encoders. As a disadvantage, incremental encoders have to be initialized by
moving them in a reference (“zero”) position when power is restored after an
outage.
Absolute shaft encoders are attractive for joint control applications because their
position is recovered immediately and they do not accumulate errors as
incremental encoders may do. Absolute encoders have a distinct n-bit code
(natural binary, Gray, BCD) marked on each quantization interval of a rotating
scale. The absolute position is recovered by reading the specific code written on
the quantization interval that currently faces the encoder reference marker. The
number of code tracks on the scale increases proportionally with the desired
measuring resolution, limiting the encoder’s resolution. This can be avoided by
using pseudo-random encoding which permits absolute encoders needing only
one code track.
Joint position sensors are usually mounted on the motor shaft. When mounted
directly on the joint, position sensors allow feedback to the controller with the
joint backlash and drive train compliance parameters.
Angular velocity is measured (when not calculated by differentiating joint
positions) by tachometer transducers. A tachometer generates a DC voltage
proportional to the shaft'’ rotational speed. Digital tachometers using magnetic
pickup sensors are replacing traditional, DC motor-like tachometers which are
too bulky for robotic applications.
Acceleration sensors are based on Newton’s second law. They are actually
measuring the force which produces the acceleration of a known mass. Different
types of acceleration transducers are known: stress-strain gage, piezoelectric,
capacitive, inductive. Micromechanical accelerometers have been developed. In
this case the force is measured by measuring the strain in elastic cantilever
beams formed from silicon dioxide by an integrated circuit fabrication technology.
Strain gages mounted on the manipulator’s links are sometimes used to estimate
the flexibility of the robot’s mechanical structure. Strain gages mounted on
specially profiled (square, cruciform beam or radial beam) shafts are also used to
measure the joint shaft torques.
2. Exteroceptors
Exteroceptors are sensors that measure the positional or force-type interaction of
the robot with its environment.
Exteroceptors can be classified according to their range as follows:
- contact sensors
- proximity (“near to”) sensors
- “far away” sensors
2.1 . Contact Sensors
Contact sensors are used to detect the positive contact between two mating
parts and/or to measure the interaction forces and torques which appear while
the robot manipulator conducts part mating operations. Another type of contact
sensors are the tactile sensors which measure a multitude of parameters of the
touched object surface.
Force/Torque Sensors
The interaction forces and torques which appear, during mechanical assembly
operations, at the robot hand level can be measured by sensors mounted on the
joints or on the manipulator wrist. The first solution is not too attractive since it
needs a conversion of the measured joint torques to equivalent forces and
torques at the hand level. The forces and torque measured by a wrist sensor can
be converted quite directly at the hand level. Wrist sensors are sensitive, small,
compact and not too heavy, which recommends them for force controlled robotic
applications.
A wrist force/torque has a radial three or four beam mechanical structure. Two
strain gages are mounted on each deflection beam. Using a differential wiring of
the strain gages, the four -beam sensor produces eight signals proportional with
the force components normal to the gage planes. Using a 6-by-8 “resolved force
matrix”, the eight measured signals are converted to a 6-axis force/torque vector.
Tactile Sensing
Tactile sensing is defined as the continuous sensing of variable contact forces
over an area within which there is a spatial resolution. Tactile sensing is more
complex than touch sensing which usually is a simple vectorial force/torque
measurement at a single point. Tactile sensors mounted on the fingers of the
hand allow the robot to measure contact force profile and slippage, or to grope
and identify object shape.
The best known of tactile sensor technologies are: conductive elastomer, strain
gage, piezoelectronic, capacitive and optoelectronic. These technologies can be
further grouped by their operating principles in two categories: force-sensitive
and displacement-sensitive. The force-sensitive sensors (conductive elastomer,
strain gage and piezoelectric) measure the contact forces, while the
displacement-sensitive (optoelectronic and capacitive) sensors measure the
mechanical deformation of an elastic overlay.
Tactile sensing is the result of a complex exploratory perception act with two
distinct modes. First, passive sensing, which is produced by the “cutaneous”
sensory network, provides information about contact force, contact geometric
profile and temperature. Second, active sensing integrates the cutaneous
sensory information with “kinesthetic” sensory information (the limb/joint positions
and velocities).
While the tactile sensor (probe) itself provides the local cutaneous information,
the robotic manipulator provides the kinesthetic capability which moves the tactile
probe around on the explored object surface. The sequence of local cutaneous
data frames is integrated with the kinesthetic position parameters of the
manipulator resulting in a global tactile image (geometric model) of the explored
object. Various multi-sensor fusion techniques are available for this integration
process.
2.2 . Proximity Sensors
Proximity sensors detect objects which are near but without touching them.
These sensors are used for near-field (object approaching or avoidance) robotic
operations. Proximity sensors are classified according to their operating
principle; inductive, hall effect, capacitive, ultrasonic and optical.
Inductive sensors are based on the change of inductance due to the presence of
metallic objects. Hall effect sensors are based on the relation which exists
between the voltage in a semiconductor material and the magnetic field across
that material. Inductive and Hall effect sensors detect only the proximity of
ferromagnetic objects. Capacitive sensors are potentially capable of detecting
the proximity of any type of solid or liquid materials. Ultrasonic and optical
sensors are based on the modification of an emitted signal by objects that are in
their proximity.
2.3 . “Far Away” Sensing
Two types of “far away” sensors are used in robotics: range sensors and vision.
Range Sensing
Range sensors measure the distance to objects in their operation area. They are
used for robot navigation, obstacle avoidance or to recover the third dimension
for monocular vision. Range sensors are based on one of the two principles:
time-of-flight and triangulation.
Time-of-flight sensors estimate the range by measuring the time elapsed
between the transmission and return of a pulse. Laser range finders and sonar
are the best known sensors of this type.
Triangulation sensors measure range by detecting a given point on the object
surface from two different points of view at a known distance from each other.
Knowing this distance and the two view angles from the respective points to the
aimed surface point, a simple geometrical operation yields the range.
Vision
Robot vision is a complex sensing process. It involves extracting, characterizing
and interpreting information from images in order to identify or describe objects in
environment.
A vision sensor (camera) converts the visual information to electrical signals
which are then sampled and quantized by a special computer interface
electronics yielding a digital image. Solid state CCD image sensors have many
advantages over conventional tube-type sensors as: small size, light weight,
more robust, better electrical parameters, which recommends them for robotic
applications. Currently, there is a multitude of commercial computer interface
boards (“frame buffers”) providing 512-by-512 digital images with 8 bit/pixel at
standard TV video-rate (single frame time of 1/30 sec). Virtually all existent
vision sensors are designed for television which is not necessarily best suited for
robotic applications. Because of the reduced resolution, parallax errors, and
robot hand obstructing the field of view, the common wisdom approach of placing
camera above the working area is of questionable value for many robotic
applications. Mounting the vision sensor in the robot hand may be a better
solution which eliminates these problems.
Illumination is a very important component of the image acquisition. Controlled
illumination offers expedient solutions to many robotic vision problems.
Backlighting enhances the contract to a level which simplifies further image
processing. In structured lighting, special light stripes, grids or other patterns are
projected on the scene. The shape of the projected patterns on different objects
offers valuable cues from which to recover 3-D object parameters from a 2-D
image. Strobe lighting with high-intensity short pulses may be used to reduce the
negative effect of ambient light or eliminate the effect of object motion.
The digital image produced by a vision sensor is a mere numerical array which
has to be further processed till an explicit and meaningful description of the
visualized objects finally results. Digital image processing comprises more steps:
preprocessing, segmentation, description, recognition and interpretation.
Preprocessing techniques usually deal with noise reduction and detail
enhancement. Segmentation algorithms, like edge detection or region growing,
are used to extract the objects from the scene. These objects are then described
by measuring some (preferably invariant) features of interest. Recognition is an
operation which classifies the objects in the feature space. Interpretation is the
operation that assigns a meaning to the ensemble of recognized objects.
t ROBOT CONTROL
Computer-based robot controllers perform the following tasks :
· maintain a model of relationships between the references to the actuators and
their consequential movements using measurements made by the internal
sensors;
· maintain a model of the environment using the exteroceptor sensor data;
· plan the seque nce of steps required to execute a task;
· control the sequence of robot actions in response to perform the task;
· adapt robot’s actions in response to changes in the external environment;
Robot controller can have a multi-level hierarchical architrcture:
1. Artificial intelligence level, where the program will accept a command such
as, ‘Pick up the bearing ‘ and decompose it into a sequence of lower level
commands based on a strategic model of the task.
2. Control mode level where the motions of the system are modelled, including
the dynamic interactions between the different mechanisms, trajectories
planned, and grasp points selected. From this model a control strategy is
formulated, and control commands issued to the next lower level.
3. Servo system level where actuators control the mechanism parameters
using feedback of internal sensory data, and paths are modified on the basis
of external sensory data. Also failure detection and correction mechanisms
are implemented at this level.
There also are different levels of abtraction for the robot programming
languages:
1. Guiding systems, in which the user leads the robot through the motions to
be performed.
2. Robot-level programming in which the user writes a computer program to
specify motion and sensing.
3. Task-level programming in which thed user specifies operations by their
actions on the objects the robots is to manipulate
Protector Enhancements
- Improved response to crashes from any and all directions
- Sensitivity adjustable by customer
- Automatic reset at lower pressure setting
- Field-rebuildable design (with prior customer training and special tools)
- Field-replaceable connector block assembly
- IP 65 environmental protection rating - extra cost option
- Longer life
Robotic Collision Sensor Benefits:
Robot Sensors are essential components in creating autonomous robots as they are the only means for a robot to detect information about itself and its environment. As little as one sensor is needed by a robot, though increasing the number and variety of sensors tends to increase the robot’s ability to get a more thorough understanding of the world around it. There are a wide variety of sensors available which are capable of measuring almost anything, from environmental conditions (distance, light, sound, temperature) to angular and linear acceleration, forces and distances. The first sensor often incorporated into a mobile robot is a distance sensor, which is usually in the form of an infrared or ultrasonic sensor. In both cases, a pulse (of light or sound) is sent and its reflection is timed to get a sense of distance. Usually these values are sent to the controller many times each second. RobotShop offers a wide variety of sensors applicable to almost any robotics project. If you are looking for a distance sensor, we offer them in a variety of configurations and optimal distances to suit almost any budget. If you are looking for a more professional solution for measuring distances, take a look at our selection of scanning laser rangefinders, which are able to scan over >180 degrees (and less than 1 degree of accuracy) in well under 1 second! Categories include:
Our intuitive site layout groups similar products making searching for and comparing products easier than ever before. Whether you are looking for a specific sensors or just browsing to see what is available, the “Robot Sensors” category within “Robot Parts” has everything you need, and many products you never knew existed. If there are any products you don’t see, feel free to send us a message. Our goal is to be the leading provider of domestic and professional robot technology by putting robotics and your service. |
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Photo Voltaic Cells
Photo Voltaic Cells or solar cells are well known for their use as power sources for satelites, enviromentalist green energy campaigns and pocket calculators. In robotics solar cells are used mainly in BEAM robots. Commonly these consist of a solar cell which charges a capacitor and a small circuit which allows the capacitor to be charged up to a set voltage level and then be discharged through the motor(s) making it move.
For a larger robot solar cells can be used to charge its batteries. Such robots have to be designed around energy efficiency as they have little energy to spare.
Batteries
Batteries are an essential component of the majority of robot designs. Many types of batteries can be used. Batteries can be grouped by whether or not they are rechargeable.
Batteries that are not rechargeable usually deliver more power for their size, and are thus desirable for certain applications. Various types of alkaline and lithium batteries can be used. Alkaline batteries are much cheaper and sufficient for most uses, but lithium batteries offer better performance and a longer shelf life.
Common rechargeable batteries include lead acid, nickel-cadmium (NiCd)and the newer nickel metal-hydride (Ni-MH). NiCd & Ni-MH batteries come in common sizes such as AA, but deliver a smaller voltage than alkaline batteries (1.2V instead of 1.5V). They also can be found in battery packs with specialized power connectors. These are commonly called race packs and are used in the more expensive RC race cars. They will last for some time if used properly. Ni-MH batteries are currently more expensive than NiCd, but are less affected by memory effect.
Lead acid batteries are relatively cheap and carry quite a lot of power, although they are quite heavy and can be damaged when they are discharged below a certain voltage. These batteries are commonly used as backup power supply in alarm systems and UPS.
An extremely common problem in robots is the "the microcontroller resets when I turn the motor on" problem[1]. When the motor turns on, it briefly pulls the battery voltage low enough to reset the microcontroller. The simplest solution[2] [3] [4] [5] is to run the microcontroller on a separate set of batteries.
HISTORY OF THE BATTERY:
The first evidence of batteries comes from discoveries in Sumerian ruins dating around 250 B.C.E. Archaeological digs in Baghdad, Iraq [6]. But the man most credited for the creation of the battery was named Alessandro Volta, who created his battery in the year 1800 C.E. called the voltaic pile. The voltaic pile was constructed from discs of zinc and copper with pieces of cardboard soaked in saltwater between the metal discs. The unit of electric force, the volt, was named to honor Alessandro Volta [7]. A time line of breakthroughs and developments of the battery can be seen here [8].
HOW A BATTERY WORKS:
Most batteries have two terminals on the exterior, one end is a positive end marked “+” and the other end is the negative marked “-”. Once a load, any electronic device, a flashlight, a clock, etc., is connected to the battery the circuit being completed, electrons begin flowing from the negative to positive end, producing a current. Electrons will keep flowing as fast as possible until the chemical reaction on the interior of the battery lasts. Inside the battery there is a chemical reaction going on producing the electrons to flow, the speed of production depends on the battery’s internal resistance. Electrons travel from the negative to positive end fueling the chemical reaction, if the battery isn’t connected then there is no chemical reaction taking place. That is why a battery (except Lithium batteries) can sit on the shelves for a year and there will still be most of the capacity to use. Once the battery is connected from positive to negative pole, the reaction starts, that explains the reason why people have gotten a burn when a 9-volt battery in their pocket touches a coin or something else metallic to connect the two ends, shorting the battery making electrons flow without any resistance, making it very, very hot. [9]
MAIN CONCERNS CHOOSING A BATTERY:
- Geometry of the batteries. The shape of the batteries can be an important characteristic according to the form of the robots.
- Durability. Primary(disposable) or secondary (rechargeable)
- Capacity. The capacity of the battery pack in milliamperes-hour is important. It determines how long the robot will run until a new charge is needed.
- Initial cost. This is an important parameter, but a higher initial cost can be offset by a longer expected life.
- Environmental factors. Used batteries have to be disposed of and some of them contain toxic materials. [10]
PRIMARY (DISPOSABLE) BATTERY TYPES
• Zinc-carbon battery - mid cost - used in light drain applications
• Zinc-chloride battery - similar to zinc carbon but slightly longer life
• Alkaline battery - alkaline/manganese "long life" batteries widely used in both light drain and heavy drain applications
• Silver-oxide battery - commonly used in hearing aids
• Lithium Iron Disulphide battery - commonly used in digital cameras. Sometimes used in watches and computer clocks. Very long life (up to ten years in wristwatches) and capable of delivering high currents but expensive. Will operate in sub-zero temperatures.
• Lithium-Thionyl Chloride battery - used in industrial applications, including computers and electric meters. Other applications include providing power for wireless gas and water meters. The cells are rated at 3.6 Volts and come in 1/2AA, AA, 2/3A, A, C, D & DD sizes. They are relatively expensive, but have a proven ten year shelf life.
• Mercury battery - formerly used in digital watches, radio communications, and portable electronic instruments, manufactured only for specialist applications due to toxicity [11]
Helpful link comparing the most popular types of batteries in many different types of categories [12] [13]
SECONDARY (RECHARGEABLE):
(Will be discussing the two most popular secondary batteries)
Lithium-ion Batteries:
Advantages:
These batteries are much lighter than non-lithium batteries of the same size. Made of Lithium (obviously) and Carbon. The element Lithium is highly reactive meaning a lot of energy can be stored there. A typical lithium-ion battery can store 150 watt-hours of electricity in 1 kilogram of battery. A NiMH (nickel-metal hydride) battery pack can store perhaps 100 watt-hours per kilogram, although 60 to 70 watt-hours might be more typical. A lead-acid battery can store only 25 watt-hours per kilogram. Using lead-acid technology, it takes 6 kilograms to store the same amount of energy that a 1 kilogram lithium-ion battery can handle. Huge difference!
Disadvantages:
Begin degrading once they are created, lasting only two or three years tops, used or not. Extremely sensitive to high temperatures, heat degrades battery even faster. If a lithium battery is completely discharged, it is ruined and a new one will be needed. Because of size and ability to discharge and recharge hundreds of times it is one of the most expensive rechargeable batteries. And a SMALL chance they could burst into flames (internal short, separator sheet inside battery keeping the positive and negative ends apart gets punctured). [14]
Alkaline Batteries:
The anode, the positive end, is made of zinc powder because the granules have a high surface area, increasing the rate of reaction and higher electron flows. It also helps limit the rate of corrosion. Manganese dioxide is use on the cathode, or the negative side, in powder form as well. And potassium hydroxide is the electrolyte in an alkaline battery. There is a separator inside the battery to separate the electrolyte between the positive and negative electrodes. [15]
Fuel Cells
Fuel cells are a possible future replacement for chemical cells (batteries). They generate electricity by recombining hydrogen gas and oxygen. (commercial fuel cells will probably use methanol or other simple alcohols instead of hydrogen). Currently these are very expensive, but this might change in the near future when these cells are more commonly used as a replacement for laptop batteries.
Note: since fuel cells use flammable products you should be extra careful when you build a power source with these. Hydrogen has no odor like natural gas and is flammable and in some conditions explosive.
Pressurized canisters have their own set of risks. Make sure you really know how to handle these. Or at least allow other people enough time to get behind something thick and heavy before experimenting with these.
Mechanical
Another way to store energy in a robot is mechanical means. Best known method is the wind-up spring, commonly used in toys, radios or clocks.
Another example of mechanical energy storage is the flywheel. A heavy wheel used to store kinetic energy.
Air Pressure
Some robots use pneumatic cylinders to move their body. These robots can use either a bottle of pressurized air or have a compressor on board. Only the first one is a power source. The latter power source is the batteries powering the compressor. Pneumatic cylinders can deliver very large forces and can be a very good choice for larger walkers or grippers.
Note: Pressurized canisters and pneumatic components can be dangerous when they are handled wrongly. Failing pressurized components can shoot metal pieces around. Although these aren't necessarily life threatening, they can cause serious injuries even at low pressures.
Canisters on their own pose additional risks: Air escaping from a pressurized canister can freeze whatever happens to be in its way. Don't hold any body parts in front of it.
Pneumatic and hydraulic cylinders can deliver large forces. Your body parts can't handle large forces. You get the picture.
Chemical Fuel
For model airplanes there exist small internal combustion engines. These engines can be used to power robots either directly for propulsion or indirectly by driving an alternator or dynamo. A well designed system can power a robot for a very long time, but it's not advisable to use this power system indoors.
Note: This is another dangerous way of doing things. Fuel burns and is toxic. Small amounts of fuel in a open container can explode when ignited. Exhaust is toxic and a suffocation risk. Make sure of that you know what you're doing or get good life insurance.
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