In just a couple weeks I’ll be starting my new mechanical engineering job with Electroimpact, a major aerospace tooling and automated machine production supplier (The video above is one of their automated fiber placement machines; it lays down carbon fiber tows on the mandrel barrel sections of the Boeing 787 fuselage).  Since I have limited professional experience with CNC/mechatronics system design (beyond teaching simple design-build-test, kit-based projects to undergrads), I thought it would be appropriate for me to review the major subsystems which comprise the design space of a mechatronics-based engineering project.

Rather than focusing on the technical details of how to specify, design, and select and/or create individual components for each of these systems (which would be a diabolically large and unfeasible blog post), this write-up will just review the major subsystems–the “design building blocks”–of the technology which underpins CNC machinery & robotics–“mechatronics,” in the more general sense.  This write-up will describe and then illustrate these subsystems with some specific examples.  As an entry-level mechanical engineer, the field of mechatronics necessarily incorporates expertise which is beyond the realm of the knowledge which I possess at this point in my career.  And, due to the inherent breadth of disciplines required in mechatronics system design, it will be quite a while–if ever–before I could comfortably write up a thorough explanation on specifying and designing components for all of these subsystems…  Some of these subsystems are much better handled by Electrical Engineers, or engineers with a depth of experience in controls engineering.

Nevertheless, this write-up on mechatronics systems which has been written in broad brush strokes is outlined as follows:

• Mechatronics Defined
• Prime Movers
  • Electrically Powered Actuation
    • Linear Movement
      • Electric Actuators
      • Solenoids
      • Shape-Memory Alloys
    • Rotation
      • Servos
      • Stepper motors
  • Mechanically Powered Actuation
    • Linear Movement
      • Pneumatic cylinders
      • Hydraulic cylinders
    • Rotation
      • Air motors
      • Hydraulic motors
      • Combustion Engine
      • Steam Engine
• Motion Hardware
  • Rolling-Element Bearings
    • Ball Bearings
    • Roller Bearings
    • Needle Bearings
    • Tapered Roller Bearings
    • Spherical Roller Bearings
    • Thrust Bearings
    • Linear Bearings
    • Linear Guide Blocks & Rails
    • Ball Screws & Ball Nuts
  • Plain Bearings
  • Ball Joints
  • Leadscrews
  • Ballscrews
  • Cams
  • Gears
    • Spur Gears
    • Helical Gears
    • Double Helical Gears
    • Bevel Gears
    • Hypoid Gears
    • Worm Gears
    • Rack & Pinion Gears
    • Epicyclic / Sun & Planet Gears
  • Belts & Pulleys; Chain & Sprockets
  • Springs
• Linkages & Structure
  • Foundation
  • Frames & Bedplates
  • Four-Bar Linkages
• The “End Effector” / Tool
• Control Hardware
  • Switches
  • Relays
  • PLCs
  • Power Management Hardware
• Control Software
• Sensors
  • Limit Switches
  • Proximity Sensors
  • Photoelectric Sensors
  • Rotary Encoders
  • Machine Vision
  • Temperature Sensors
  • Pressure Sensors
• References

Mechatronics Defined

Mechatronics is defined as a “combination of the engineering disciplines of mechanical, electronic, computer, control, and systems design.” [1]  With the advent of electrically powered prime movers (motors) and then the development of computers, machines could begin to be controlled much more efficiently with electricity and then accurately and flexibly with computers.  Strictly mechanical systems (pre-electric, industrial revolution era machinery for example) had severe limitations relative to these new types of machines.  Computer-controlled machinery can generally operate much faster, much more efficiently, can be designed to operate usefully in a wider range of situations, and can operate much more independent of human input.  The main value driving this interdisciplinary field of engineering are these advantages which mechatronics systems have over their strictly mechanical forebears.

Prime Movers

Prime movers are those elements of a machine which provide the motive force driving the motion of the device.

Electrically Powered Actuation

Linear Movement

Electric Actuators

Devices called “electric actuators” are technically a combination of a motor and gearbox (which provides torque/rotational movement) coupled to an acme thread and ball-screw assembly to transform the motor’s rotation into extension of the rod (acme thread/ball screws will be described later in the “Motion Hardware” section).  However, they can be bought as electrically powered plug-and-play systems which do provide linear motion.

Solenoids

Solenoids are an induction-based technology which ejects a ferrous pin from the inductor barrel/coil when current is applied to the coil.  I couldn’t find a video of a solenoid in action which wasn’t part of a valve demonstration, but this video does show the guts of the valve (the actual solenoid), as well as a particularly dorky looking fellow in the role of narrator:

Controlling flow in valves is a very common application of this technology.  I employed these valves in my senior capstone design project to create a pneumatic t-shirt launcher mounted on a R.C. mobile platform for Michigan State sporting events.

Shape-Memory Alloys

Shape-memory alloys are applied in technology relatively rarely, but they commonly function as electrically powered actuators which provide linear motion.  Shape-memory alloys are special metals (or other materials) which, when heated (typically through resistance to electric current running through them), return to a shape which they had prior to being deformed by an external force.  Demonstrating that heat (a warm cup of water) can return the shape memory alloy to its original shape:

In an actual mechatronics application, a spring force opposing the memory allocy can be used to stretch the shape memory alloy, which achieves machine motion in one direction.  A current applied to the alloy causes the alloy to contract, which returns the device to its original shape or position.  A common shape memory material is Nitinol.

Rotation

Electric motors are one of the major technologies which makes the field of mechatronics a viable field worth pursuing.  There are many different types and classifications of electric motors, and a thorough listing of all types of motors would be the subject of another blog post.  However, the two most relevant types of motors with respect to precision mechatronics are listed below.

Servomotors

Servomotors are one dominant type of motor used in mechatronics.  Their electrical design (synchronous, asynchronous, AC or DC) is not limited to a specific design, but all servomotors share the feature of feedback control.  This relies on a controller, which can be a component separate from the motor itself known as the “servo drive,” which monitors the state of the motor or mechanical system with a sensor.  The controller then compares this sensor reading to a command input.  The servo drive then amplifies the differential electrical reading to power the motor towards the command input value.  A commonly used type of sensor is a rotary encoder (explained in the “Sensors” section), which measures rotational position of the motor shaft.

Stepper Motors

A stepper motor on the other hand–instead of relying entirely on external control hardware and a sensor–also relies on a specific geometric shape of the rotor and stator.  This type of rotor and stator design allows it to be rotated by a specific angle.  The stator consists of several toothed electromagnets placed around the periphery of a similarly toothed wheel on the rotor.  The electromagnets are energized sequentially such that torque is optimally generated based on the current position of the electromagnet teeth, relative to the position of the teeth on the rotor (a very good animation of stepper motor design is available at the wikipedia article on the subject).  Stepper motors have high starting torque and, apparently, can be used for more than just accurate positioning of CNC machines:

Mechanically Powered Actuation

Linear Movement

Pneumatic Cylinders

Pneumatic (compressed air) power is used in mechatronics applications requiring the application of a liner force.  Compressed air is already such a common thing in factories where impact tools are used, that it can be a natural extension to use equipment that runs on compressed air.

You can see in this video that the cylinder is double acting: some cylinders rely on a return spring to draw the cylinder back into place after extending it (the latter are called “single acting”).

Hydraulic Cylinders

Where high powered linear motion is required, hydraulic cylinders (compressed fluid) do the job.  Hydraulic cylinders require heavy duty pumps for anything require high throughput, but their power density allows them to perform even the largest of lifting operations: watch the “A heavy weight on stilts” video at Mammoet’s website to get a sense of just how large the loads can be, which can be lifted by hydraulics.

The tensile testing frames I used during my master’s thesis to test carbon fiber composite coupons were hydraulic cylinders, controlled by feedback control.  They could apply peak loads of approximately 1,000 lbs at 100 Hz (1oo cycles per second!).

Rotation

Air Motors

Less common than using compressed air to power pneumatic cylinders is using compressed air to power an air motor, which rotates the shaft of a ‘reversed blower’ type device.

Wind turbines too can be considered a mechatronics system relying on air to drive the system.  Wind turbines themselves are certainly mechatronics machines (being automatically operated, controlled, and monitored by a PLC system).  I spent about a year and a half working on these devices at General Electric.

Hydraulic Motors

Where a supply of compressed fluid exists, or electricity cannot be used, hydraulic motors may make sense, where a torque is required.  Hydraulic motors drive a shaft off of a set of vanes or gears which are propelled by hydraulic fluid rushing through the motor housing.  They supply much more torque than air motors.

And similar to wind turbines, river dams use much larger turbines relying on fluid to drive the mechatronics system / generate electricity.

Combustion Engine

Internal combustion engines which convert expanding combustion gases directly to mechanical work include variations of piston-based or turbine engines.  Obviously, a wide range of power can be derived from combustion engines–from small handheld portable generators to 600 MW gas turbines at power plants.  Modern controls applied to combustion engines has rendered them mechatronics systems in themselves–even before considering the possibility of using them to directly power another mechatronics system.

1,000 Watts of piston power by Honda

520,000,000 Watts of turbine power by General Electric

Steam Engine

The steam engine–either piston-powered or turbine-based–is another mechanical source of torque.  Steam piston engines were common during the industrial revolution but are rare nowadays (However, the floating relic known as the SS Badger–a coal-fired carferry I lived and worked on one summer on Lake Michigan–still relies on them.  We had a breakdown one night and I observed the ship engineers swapping a new two-foot diameter piston into the suffering steam-powered cylinder).  Modern steam-powered systems burn fossil fuels (coal or gas typically) to generate steam in boilers, which is then run through a steam turbine.  These are large complex systems which are usually attached to generators whose electricity is subsequently used to power smaller machines.  As with many combustion engines, steam engines are typically complex mechatronics systems in their own right.

Motion Hardware

“Motion hardware” are the machine elements which promote system motion, and enable it to move in a reliable manner.  They often also serve to prevent wear of other components (the linkages and structure).

Rolling-Element Bearings

Rolling-element bearings come in many forms, but greatly reduce frictional wear at a pivot point by replacing material-on-material sliding with elements rolling across each other.  There are many types of rolling element bearings.  The images below are taken either from the wikipedia article on rolling-element bearings, or from the McMaster-Carr catalog offerings on bearings.

Ball Bearings

Ball bearings are good for high speed applications and can be made fairly economically relative to other bearing types.  However, the high point-load contact with the race can limit the load they are able to bear.  The inner and outer races can tolerate some misalignment and ball bearings can tolerate axial and radial loads.

Roller Bearings

Roller bearings rely on cylindrical rolling elements.  Due to the line-contact of the cylindrical rolling element, they can bear a higher load than an equivalently sized ball bearing of similar material.  However, they do not tolerate race misalignment or axial loading very well.

Needle Bearings

Whereas roller bearings use rolling elements with a length only slightly greater than their diamter, needle bearings rely on elements whose length greatly exceeds their diameter.  This also incrases the number of elements while permitting the bearing to fit much more closely around the shaft.

Tapered Roller Bearing

Tapered roller bearings rely on conical races and conically shaped rolling elements.  Their large contact surface and ability to bear high radial and axial loads are beneficial, but their unique, conically based geometry makes them more difficult to manufacture than simple ball bearings.  The outer ring, or “cup,” can be separated from the inner roller cage and race.

Spherical Roller Bearing

Spherical roller bearings are designed with rolling elements whose outer surface is some portion of a sphere, with a race to run to match.  The primary benefit of these bearings is that they can run with misalignment of the inner and out race, but they are difficult to manufacture and run hot because different points along the contact line run at different speeds as the elements rotate, causing wear.

Thrust Bearings

These bearimgs can have cylindrical, conical, or spherical rolling elements, and can also be bought as “turntables.”  Obviously, they are designed for where the primary load is axial thrust.  Fluid and magnetic thrust bearings are also available, which do away with rolling elements altogether.

Linear Bearings

Linear bearings are used to promote sliding along the axis of a rod–as opposed to promoting rotation of the rod within the bearing (though linear bearings are available which permit both, if that type of unique application is what you are dealing with).

Linear Guide Blocks & Rails

Linear guide blocks and rails are co-engineered, precision-made units.  The rolling elements in the guide block are designed to fit into features on the rails with minimal clearance.  Due to the fact that the rolling elements ride in grooves on the rails, linear guide blocks typically can bear more load than simple linear bearings, and can handle offset loads better than simple linear bearings.

Ball Screws & Ball Nuts

Another type of rolling-element bearing which promotes linear translation is the “ball screw and ball nut.”  This is a bearing with a large number of spherical ball bearings that are captive, internal to the ball nut.  The ball nut itself is threaded on a ball screw threaded rod, and when the ball screw is rotated, the ball nut translates up or down the ball screw rod.  The ball bearings roll along inside the grooves in the threaded rod, and are recirculated to begin the process again, through a ball passageway that is external to the race.  Ball screws have a much higher efficiency than leadscrews (described later) because they rely on rolling elements, instead of frictional sliding.  They are fairly expensive to make, but are great where precise linear positioning is required.

Plain Bearings

Also known as a “sleeve bearings” or “journal bearings”…  where loads are not too high, it can be practical and much less expensive to install a sleeve bearing to promote sliding (along a shaft) or rotation (within the bearing).  Sleeve bearings come in many types of durable plastic, but also in porous metal which has been impregnated with oil to “self-lubricate” itself; some even have graphite cores embedded in them to promote lubrication.

Plain linear bearings are also available for promoting linear sliding along a shaft:

Plain bearing versions of the “guide block” format of linear bearing are also available:

Ball Joints

Ball joints are a type of bearing which “captures” a spherical metal ball in a housing, and allows rotation within a limited range of motion.  They are not technically rolling-element bearings because the sphere slides within the housing–it does not roll.  There are many different geometries of ball joints available.

Leadscrews

Leadscrews (also known as “power screws“) are used to translate rotational motion into linear translation.  Acme leadscrew threaded rod can be purchased and matched to internally threaded components which can be mounted to the device which requires linear translation.

Acme threaded rod:

Acme threaded nuts and flanges:

Leadscrews can also be bought as a prepackaged devices with a “slide” table mounted onto a component with internal threading matched to the leadscrew.

Leadscrews are commonly used in the  x and y axis positioning of milling machines, and anywhere else precise positioning is needed and a feasible length of threaded rod can be installed.

Cams

Cams are mechanical timing elements which are carefully shaped to cause a predicted motion output for a cam-follower that rides on the cam.  The most familiar application of cams is in internal combustion piston-based engines–they are used to lift the intake and exhaust valves at known positions of the engine crankshaft, preventing the piston from colliding with the valves.  Much more complicated motion can be accomplished than just a simple linear valve lift:

Gears

Gears transfer rotational motion accurately between two shafts and can increase/decrease torque while decreasing/increasing shaft rotational speed, respectively.  Gearboxes are combinations of multiple sets of gears which can achieve higher gear ratios in more reasonably sized and feasibly produced sizes, than if just one gear stage were employed.  There are many different types of gears which allow the engineer to achieve different ends.  Again, many of the images below are from a wikipedia article on gearing.

Spur Gears

This is the typical hob-cut, simple gear external gear.  Axes must be parallel for these types of gears since the teeth are parallel to the shaft axis; and they exhibit noise if running in high speed applications because the teeth do not engage gradually.

Helical Gears

Helical gears, while being more complex to make than spur gears, are much better suited for high-speed applications since the teeth engage more gradually than spur gears.  Spur gears engage instantaneously along a line across the whole tooth, but helical gears engage at a point, develop into a line of contact, then separate at a single point of contact.  This causes less noise / mechanical shock in the tooth.  The tooth shape of a helical gear is a segment of a helix, and their shaft axes can be oriented at any angle, though the contact area tends to be optimal when the shafts are parallel [2].  Thrust is generated along the shaft axis in helical gears, which has to be counteracted with thrust, taper, or spherical bearings.

Double Helical Gears

Double helicals (also known as “herringbone” gears) prevent the problem of axial thrust which is present in helical gears.  However, they are even more complex to manufacture.

Bevel Gears

Bevel gears consist of two conically profiled gears (in cross-section) whose hypothetical vertices intersect.  Teeth can be straight-cut or spiral-cut (which are analagous in functionality to straight hob-cut spur gears and helical gears).

Hypoid Gears

Similar to bevel gears except the shaft axes do not lie in the same plane.

Worm Gears

Worm gears function similar to leadscrews.  The worm can always drive the gear, but they can be designed such that the gear cannot backdrive the worm.

Rack and Pinion

When the teeth on the periphery of a helical or straight-cut gear are instead placed along a single line instead of the circumference of a circle, a “rack and pinion” is formed.  In this way, finite linear motion can be achieved directly from a rotating gear.

Epicylic / Sun & Planet Gears

Gears can be paired to create “epicyclic” gear sets, in which one or more of the gear axes rotates during gearing operation.  By fixing one or more sets of these axes, different output gear ratios can be obtained–and this is the principle and type of gearing in the modern automotive gearbox transmission (a great interactive illustration, in which various axes can be held fixed by the user, is available here).

Belts & Pulleys; Chain & Sprockets

Belts & pulleys (and cables) / chain & sprockets transfer force and motion from one shaft to another, and similar to gears, they can increase/decrease torque while decreasing/increasing rotational speed respectively.  Pulleys combined with a non-toothed belt or cable, such as a V-belt, provide a useful method of transferring power where an approximate (but not absolute) ratio of input speed to output speed is necessary.

Timing belts are pulleys with teeth features on the belt and pulley, which provide a much more exacting ratio of input-to-output rotational speed.

Of course, compound pulleys (a “block and tackle” arrangement) can be used to magnify mechanical force–trading off the distance a load is moved for the amount of force applied to it.

Timing chains and sprockets are more expensive, (can be) noisier than belting, and require lubrication, but they can transfer more power within a given space and have a direct and exact ratio of input-to-output shaft speed.  They also require exacting location of the shafts.

Springs

Springs store mechanical energy in the form of strain energy of the spring material.  They can supply a linear reactive force or a torque when they are deformed from their resting state (either compressed or extended).  A typical helical spring for linear tension force:

“Clock springs” can supply a reactive torque when deformed from their initial state:

Leaf springs and recurve bows function on the same basic geometry–applying a bending moment to a strip of material, applying tension between the two points of the spring element’s attachment.  They can also apply a force transverse to a line through the mounting points, if deformed from their initial static state.  This is what provides suspension force in the case of leaf springs, and the force to launch an arrow in the case of a recurve bow.

Linkages & Structure

The “Linkages & Structure” serve as the frame (dynamic or static) or platform/base upon which all the other elements are installed or work from.  These are often custom designed for unique process machinery.  Basic elements of mechtronics linkage and structure include the following categories.

Foundation

Some mechatronics systems are meant to be stationary, but involve very large forces–just from the sheer mass of the machine or what it is manipulating, or from dynamic loading from the machine’s motion.  This is especially true for process and assembly machinery and the design of the foundation for the machine can be critical to ensuring the machine will operate effectively–appropriately supporting and damping out any vibrations.  Companies such as Unisorb specialize in installation and foundation design specifically.

Frames & Bedplates

Generally, major mechatronics systems that require a serious foundation will also have machine components which could be described as the “machine frame” in order to do useful work.  Frames & bedplates serve as structures which all the other system components mount to, and serve as the “rigid base” which counteract any forces generated by the prime movers to move the other kinematic elements of the system.  A diagram from a GE turbine:

In that illustration, the “Main Frame” (labeled as number 5), as well as the “bedplate” (directly under the Gearbox-13), and the tower serve as the “frame” to which the dynamic elements are mounted–the rotor assembly, rotor shaft, (internal gears of the) gearbox, and the generator’s rotor (Though it should be noted that the machine head itself can rotate on top of the tower).

Four-Bar Linkages

A unique machine element which can effect complex machine motion, while still serving as part of the machine structure is the “four-bar linkage.”  Four-bars are exploited in robotics, and anywhere a unique but consistently repeatable path is required, or a mechanical advantage needs to be derived from a simple assembly of linkages in series:

Applications are numerous in mechatronics.  A parallel manipulator is shown in the video below–very similar to the ABB Flexpicker featured in an earlier writeup–and each of the three arms is a parallelogram of linkages with ball joints at their vertices:

Each of the kinematic parallelograms is a four-bar linkage.  A four-bar is also what enables the vise-grip to do such great work for us in the shop:

Four-bar linkages can be thought of as a means to do one of three things.  The first is to achieve a unique but repeatable tool path (the position of elements in the four-bar system are an exercise in advanced geometry).  The second is to increase force input to a system: by rotating one of the larger linkages in the system, input force can be multiplied in the motion of one of the smaller links (this is what vise-grip pliers do).  Four-bar linkages can also help you achieve the opposite process: by manipulating one of the shorter linkages in the system, the output speeds and distances translated at a larger linkage in the system can be greatly increased relative to the input.  Mounting a motor at one of the joints–driving one of the linkages to rotate–is one way in which a four-bar could be incorporated into a mechatronics system.

The “End Effector” / Tool

The end effector is the device which carries out the task which the machine is designed to perform.  This is typically a custom-designed device, and is specific to whatever the mechatronics machine has been designed to do.  In the first video at the very top of this write-up, the end effector is the head which contains multiple spools of carbon fiber tows, the motors and hardware for paying out the tows and guiding them onto the mandrel, and the shears used to trim the tows.  In this robot however, the end effector is a painting head:

In this robot, it’s a spot welder:

The end effector is specific to whatever the task of interest is.

Control Hardware

There is electronics or controls-related hardware associated with any mechatronics system.

Switches

A switch is typically defined as a component which can interrupt an electric circuit.  It usually also refers to an electromechanical device that is manually thrown by an operator.  Wide-scale use of switches to operate mechatronics systems is not common today–good system design and the use of PLCs and sensors can remove the need for an operator to “approve” every step of a machine’s operation by throwing a swtich.

Switches can be designed to be “thrown” by almost any sort of stimuli the engineer can think of, which then makes the “switch” then defined as a “sensor” (a binary, on-or-off, mechanical state detector): vibration, turning a key, the orientation of the switch, presence of a magnetic field, rotation, presence of a mechanical stop–the possibilities are numerous.  For machinery incorporating pneumatic and hydraulic power, switches (valves) are available for these systems as well.

Relays

Relays are a step up in terms of automation, relative to switches.  Instead of manually throwing a switch, relays close a circuit when current is provided to the relay from another circuit.  In this way, a sensor can close a relay when it detects a certain machine state, and the closed relay then powers some other device, reacting to the machine state which triggered the sensor.  For example, an optical switch monitoring a conveyor belt can detect the presence of a package and then send a low voltage signal to the relay wired to the conveyor belt’s motor.  When the optical switch detects a package on the belt, it closes the relay with a low voltage sensor reading; the closed relay supplies high-power electricity to the motor, which then drives the conveyor forward.

Electromechanical relays are based on induction: copper windings energized by the low power circuit drive an iron core out of the coil, which closes or opens the high-power relay contacts.

Relays do not have to be mechanical–physically closing a contact; they are available as solid-state devices.  Based entirely on circuitry and with no moving parts, these can be particularly useful where vibration can risk closing an electromechanical relay when the circuit is not actually energized.  For high-current applications or where frequent switching occurs, heatsinks are required to dissipate the power which is consumed during their operation.

Programmable Logic Controller (PLC)

Many, if not most, modern mechatronics devices are controlled by programmable logic controllers.  PLCs are simple computers which can continuously monitor multiple inputs, compare these inputs to user defined settings or operator commands, and then command govern the behavior of multiple components in the system as outputs–turning them on or off as appropriate.  In this way, PLCs are the “brain” of many mechatronics systems you see today.

PLCs were designed to replace the increasingly complex  control systems comprised entirely of huge systems of logic-based relay circuitry.  Modern PLCs commonly have the code which runs them written on and debugged on a separate personal computer.  This code is then downloaded to the PLC via ethernet or RS-232 cabling and stored on RAM or flash memory.  To control high-powered devices, PLCs often energize relays to close high-power circuits.  PLCs need not interact and control their mechatronic systems according to digital on/off inputs and outputs; analog inputs and outputs can be incorporated because PLCs can monitor the intensity of a voltage or current being sent to it (this enables quantifiable monitoring of things like pressure, temperature, or weight).  Additionally, PID control can be implemented to prevent overshooting system targets.

Power Management Hardware

With so many devices requiring different power and voltage levels (sensors, prime movers, end-effector machinery), it’s common for mechatronics systems to need power management hardware.  To be honest, I have very little experience with this hardware at this point in my career, but it’s worth mentioning that it can be necessary.

Control Software

The software controlling the PLC depends on the PLC which is selected for the job.  The logic controller’s manufacturer will have their own code language for their hardware.  For example, Fanuc has its own software for the robots it sells, I’ve worked with and taught the use of Parallax pbasic to students, and Arduino microcontrollers have a language that is similar to C++.

The details of coding are a topic way outside the purpose of this blog post, but logic commands like “IF,” “THEN,” “ELSE,” “FOR,” “WHILE,” and “COUNT,” were some of the common commands from pbasic code.  There are analogous commands between different manufacturer’s coding languages, and computer programming in general.  For example, lot of my training as an undergrad in MatLab code writing carried over pretty directly into coding in pbasic.

Sensors

Sensors monitor features of the system which the engineer deems important to the behavior of the system–either with respect to function, safety, or preventing system damage.  Some common sensors are presented below.

Limit Switch

These are mechanical switches with a toggle or button which, when depressed by an object moving past them, either activate or deactivate a circuit.  These provide true/false information about the machine status–has the machine reached this position–yes or no?  They don’t provide information about the absolute current position of the machine beyond whether it has reached the limit defined by the switch position itself.  They are useful in shutting off or slowing down machinery which are reaching a position which could be dangerous to an operator, or to damaging the system itself.  Limit switches come in many forms.

Proximity Sensor

Proximity sensors come in various forms.  They can detect the presence of a metal object (through the hall effect), they can detect the presence of liquid–some liquid level sensors close their circuit through the liquid itself.  Noncontact liquid level sensors can measure the presence of liquid by reflecting ultrasonic waves off the surface of the liquid.  A metal-sensing proximity switch looks like this:

While working as a mechanical engineering co-op for GE Wind Energy, I saw the switches in this image used as redundant measures of wind turbine tower rotation…  In addition to encoder measurement of yaw motor rotation, these prox switches measured the passage of gear teeth on the yaw bearing, relate to the bedplate’s motion.

Photolelectric Sensors

Photoelectric sensors are another sensor which provides information in a “yes or no” format–they provide a signal to the controller when an object is present or absent, when it interrupts a light beam.

Rotary Encoders

Most machine motion is facilitated by torque-providing prime movers–electric motors, or perhaps engines rotating a shaft.  If this is isn’t the case (for example, if hydraulic cylinders are being used), most machine motion can still be measured fairly easily as a function of rotation, and thus rotary encoders are the devices which are commonly used to measure the absolute position of a machine.  Rotary encoders measure the position of a rotating shaft.  Some encoders also have the ability to count the number of complete revolutions the shaft has turned,  which gives the engineer the ability to know how far the machine has moved from a specified datum.

If an encoder is being integrated with a PLC system, an analog to digital converter (ADC) will be required to convert the analog voltage or current signal to a digital signal that the PLC can read–if it’s not already integrated into the encoder itself.  Encoders optically measure a pattern etched into or through a surface with a photo detector, the photo detector yields a current/voltage reading, and this reading is converted to a digital signal.  The encoder etching pattern determines the shaft position which corresponds to the analog/digital.

It can be observed in the image above that each of the eight, 45-degree arc sections has a unique pattern in it, which then uniquely (in 45-degree increments) determines the position of the shaft.  However, encoders are available with more than 10,000 counts per revolution!  Obviously, very precise positioning can be achieved with encoders.  Precision can (typically) be further enhanced by measuring motor rotation instead of machine component rotation–this is true because motor rotation speeds are (typically) kicked down by a gearbox in order to attain speeds which are useful for most machinery.

In the days of yore–when the computer mouse relied on a trackball instead of optical surface sensing–encoders were what measured the movement of the trackball.

Machine Vision

Machine vision is a large topic in itself, but it is like a photoelectric switch on steroids…  Instead of measuring the presence or absence of light on a single sensor, an entire array of pixels on a CCD can be monitored for the presence or absence of specified light intensity levels.  In this way, objects of interest can be differentiated from their background, and these objects can then be reacted to by the controller system that’s monitoring them.  For example, if you fast forward to 1:50 in the video below, the ABB Flexpicker control system can be seen locating the presence (AND type!) of muffins on the conveyor belt:

Machine vision is a new but rapidly expanding field.

Temperature Sensors

Temperature can be measured as an analog signal by a temperature sensor, converted to a digital signal, and read and reacted to by a microcontroller.  Thermocouples can serve this purpose; infrared thermometers are also available.

Pressure Sensors

Similar to temperature sensors, pressure transducers can measure pressure–measured as an analog signal, converted to digital by an ADC, which is then read and reacted to by a PLC.

There are few limits on how many types of machine conditions could potentially be read by a sensor of some type, as either a digital or analog signal.

References

[1] http://en.wikipedia.org/wiki/Mechatronics

[2] http://www.roymech.co.uk/Useful_Tables/Drive/Hellical_Gears.html