la brunitura dei metalli

fonte: wiki

La brunitura è anche detta bronzatura o metallocromia. Si tratta del processo di colorazione superficiale di un metallo ed è praticata principalmente per fornire una protezione contro l’ossidazione, che altera le proprietà del metallo. Tramite la brunitura, il metallo cambia in aspetto assumendo una colorazione più scura.

brun3Escludendo l’acciaio inox, sui metalli ferrosi la brunitura si può eseguire principalmente in due modi:

  1. immergendo il pezzo di ferro o acciaio, ben pulito e sgrassato, in un bagno d’ acqua con acetato di piombo, iposolfito di sodio ed altre sostanze;
  2. immergendo il pezzo, sempre ben pulito e sgrassato, in un bagno a circa 100 °C di acqua in cui vengono disciolti iposolfito di sodio, acetato di piombo e solfato di rame.

Quantità delle sostanze e tempi di immersione possono variare a seconda del metallo da trattare e del colore che questo deve assumere (il metallo assume via via vari colori passando dal blu chiaro, al porpora, al grigio, fino al nero). Ad esempio, una brunitura nera e brillante su ferro ed acciaio si può anche ottenere immergendo il pezzo scaldato fino al colore giallo in olio e, una volta raggiunta la brunitura, scaldandolo ancora lievemente per poi lasciarlo raffreddare a temperatura ambiente. Un altro sistema, assai vecchio (utilizzato generalmente dai restauratori) ma che non altera le proprietà della tempra consiste nel cospargere il pezzo con cera vergine di api alla quale viene successivamente dato fuoco. Una volta che questa è completamente bruciata il pezzo viene lasciato lentamente raffreddare.

brun1Sui metalli non ferrosi come rame, ottone o bronzo si può ottenere una brunitura utilizzando una soluzione di solfuro di potassio (chiamato comunemente “fegato di zolfo“), semplicemente immergendovi i pezzi ben sgrassati anche a freddo e sciacquandoli poi in acqua corrente.

A livelli industriali, uno dei vari metodi di brunitura più utilizzati consiste nel trattamento galvanico. Il pezzo da trattare viene immerso in una soluzione acquosa di solfantimoniato di sodio (o “sale di Schlippe“), di carbonato di sodio anidro e successivamente subisce un’elettrolisi per alcuni minuti (temperatura ambiente, corrente di 0,35 ampere, tensioni comprese tra 2,4 e 4 volt). Questo metodo è impiegato anche nel trattamento di rame ed ottone. Esistono poi in commercio soluzioni brunitrici già pronte, che agiscono anche a freddo e richiedono solo una preventiva accurata pulizia dei pezzi da trattare.

brun2

È sufficiente stendere un velo di prodotto (o più se si desidera un colore più scuro) sul pezzo; una volta raggiunto il colore voluto, il pezzo va sciacquato in acqua ed asciugato accuratamente, quindi lasciato immerso in olio per un certo tempo (onde evitare la possibile ossidazione che potrebbe sopravvenire nelle ore immediatamente successive). Tali soluzioni esistono sia per metalli ferrosi che non, ma generalmente non permettono di ottenere una brunitura omogenea e uniforme su pezzi di grandi dimensioni.

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HARRI, un robot qui veut votre main

du site internet de l’Université Laval

Des chercheurs ont mis au point un système robotisé permettant à deux personnes de se serrer la main à distance

Imaginez le tableau. Barack Obama, dans le bureau ovale de la Maison-Blanche, et Justin Trudeau, dans son cabinet de l’édifice Langevin, doivent discuter d’un sujet urgent par visioconférence. La communication est établie, les deux hommes s’échangent les salutations d’usage et, par l’entremise d’une interface robotisée appelée HARRI, ils se serrent vigoureusement la main, comme ils le feraient en chair et en os. La chose paraît impensable, mais les travaux menés par l’équipe de Clément Gosselin, au Département de génie mécanique, pourraient théoriquement conduire à la concrétisation de cette idée. La demande pour un tel système est très hypothétique, reconnaît d’emblée le professeur Gosselin, mais la science sur laquelle il repose pourrait avoir des applications très concrètes en télésanté.

Au fil des ans, Clément Gosselin et ses collaborateurs du Laboratoire de robotique ont développé une expertise dans les préhenseurs intelligents. Ils ont notamment mis au point la main SARAH, dotée de trois doigts mobiles dont l’action mécanique s’adapte avec souplesse à la forme des objets qu’elle saisit. SARAH est actionnée par deux moteurs électriques et elle peut ajuster sa force de préhension à la nature de l’objet manipulé, qu’il s’agisse d’un madrier, d’une bague, d’une balle de tennis ou d’une éponge. Une version modifiée de cette main a été conçue pour participer au démantèlement d’un centre de recherche nucléaire en Grande-Bretagne. De plus, une version humanisée de ce préhenseur, destinée aux personnes amputées, a récemment fait l’objet de tests cliniques.

harriPlusieurs composantes de ces mains robotisées ont été utilisées pour créer HARRI, signale le professeur Gosselin. “Le but du projet est de concevoir un système permettant à un thérapeute de guider à distance les mouvements d’un patient. Il fallait donc développer une interface capable de communiquer de façon bidirectionnelle les mouvements de deux personnes. Dans un premier temps, nous nous sommes donné le défi d’y arriver avec l’un des gestes bidirectionnels les plus courants chez les humains: une poignée de main.

Dans un article du récent numéro du Journal of Mechanisms and Robotics, Nicolò Pedemonte, Thierry Laliberté et Clément Gosselin expliquent comment ils sont parvenus à concevoir HARRI, un acronyme pour Handshaking Anthropomorphic Reactive Robotic Interface. “La principale difficulté consistait à rendre le geste réaliste, précise le professeur Gosselin. Il faut que la fermeté de la main ainsi que la dynamique du mouvement imitent ce qui se produit lorsqu’on serre la main d’un être humain.” HARRI comporte des senseurs, deux moteurs et une interface qui transmet des informations à une autre main robotisée située à distance. Un système miroir fait la même chose à l’autre extrémité. Chaque doigt de HARRI est articulé, mobile et indépendant des autres, ce qui donne une bonne dose de flexibilité et de réalisme à l’ensemble. Les mains peuvent être montées sur un robot ou sur des rails verticaux.

Aux dires des personnes qui ont serré la pince de HARRI, les résultats sont étonnants. “C’est très réaliste, confirme Clément Gosselin. Le système transmet fidèlement le style de poignée de main de chaque personne.” Advenant le cas où une grande distance séparerait les deux interlocuteurs, il faudrait tenir compte du décalage dans la transmission des signaux, mais il existe déjà des algorithmes qui permettent de tels ajustements.

L’idée de serrer la main d’une personne par l’intermédiaire d’un robot fait sourire, mais y a-t-il vraiment une demande pour un tel système? “Pour l’instant, non, répond le chercheur, mais dans un univers où les rapports humains sont de plus en plus virtuels, ce geste pourrait être apprécié dans certaines circonstances. Les applications possibles dans le domaine de la télésanté sont plus évidentes. Un thérapeute pourrait guider à distance les mouvements d’une personne en réadaptation et s’assurer qu’elle les exécute correctement, qu’il s’agisse de mobiliser certaines articulations ou encore de lui réapprendre à écrire.

Solenoid actuators: some hints

source: this website

A solenoid actuator can be defined as an electromagnetic device that converts an electrical signal into a magnetic field. Solenoids are available in a variety of formats; the  two more common types are the linear solenoid and the rotary solenoid.

io15In a Linear Solenoid (LS), electrical energy is converted into a mechanical pushing or pulling force or motion. Inside the LS, an electrical coil is wound around a cylindrical tube with a ferromagnetic actuator, called plunger. When electrical current flows through the coil, a magnetic field is instantaneously generated. The direction of this magnetic field is determined by the direction of the current flow within the wire. Thus, the coil becomes an electromagnet with its own north and south poles: in such a configuration, the coil behaves exactly as a permanent type magnet. The strength of the magnetic field can be increased or decreased by either controlling the amount of current flowing through the coil or by changing the number of turns (loops) of the coil windings.

When an electrical current is passed through the coil windings, the plunger, which is is free to move (to slide) in and out of the coil body, is attracted by the magnetic flux. Accordingly, the plunger translates towards the center of the coil body. Such translation results in the mechanical pushing or pulling force provided by the LS. The force and speed of the plunger movements is determined by the strength of the magnetic flux generated within the coil (which depends, as mentioned above, on the amount of electrical current). When the supply current is turned off (de-energized) the electromagnetic field collapses and the plunger is allowed to go back to its original rest position. This is usually achieved passively, by means of a return spring (a small compression spring attached to one end of the plunger itself).  Both push- and pull-LS types are generally constructed the same with the difference being in the location of the return spring and design of the plunger. The back and forth movement of the plunger is known as the LS’s stroke, in other words the maximum distance the plunger can travel pull-LS.gifin either in or out direction. LSs can be used to electrically open doors and latches, open or close valves, move and operate robotic limbs and mechanisms, and even actuate electrical switches just by energizing its coil. LSs can also be designed for proportional motion control, were the plunger position is proportional to the input power.

Analogously, a Rotary Solenoid (RS) provides the rotational movement of the plunger in either clockwise, anti-clockwise or in both directions. The coil is wound around a steel frame with a magnetic disk connected to an output shaft. When the coil is energized, the electromagnetic field generates multiple north and south poles. These poles repel the adjacent permanent magnetic poles of the disk, causing the latter to rotate at an angle determined by the mechanical construction of the rotary solenoid. Therefore, the shaft rotation can be controlled by either energizing or de-energizing the RS, or by altering the position of the permanent magnet rotor.

Commonly available RSs have strokes of 25, 35, 45, 60 and 90 degrees, as well as multiple movements to and from a certain angle. RSs can be used to replace small DC motors or stepper motors were the angular movement is very small, with the angle of rotation being the angle moved from a desired start position to a specific end position. RSs are used in vending or gaming machines, valve control, camera shutter with special high speed.

solenoids

Check out this website for many other useful hints about solenoid switching, energy consumption and duty cycle 🙂

step motors: some hints

source: this website

A step motor is a constant output power transducer, where power is defined as torque multiplied by speed. This means motor torque is the inverse of motor speed. An interesting aspect of a step motor is that its power is independent of speed.

In general terms, torque is proportional to ampere-turns (current * the number of turns of wire in the winding). Since current is the inverse of speed, torque also has to be the inverse of speed. In an ideal step motor, as speed approaches zero, its torque would approach infinity while at infinite speed torque would be zero. Because current is proportional to torque, motor current would be infinite at zero as well. Electrically, a real motor differs from an ideal one primarily by having a non-zero winding resistance. Also, the iron in the motor is subject to magnetic saturation, as well as having eddy current and hysteresis losses. Magnetic saturation sets a limit on current to torque proportionally while eddy current and hysteresis (iron losses) along with winding resistance (copper losses) cause motor heating.

TScurveThe first figure here shows an ideal motor’s natural speed-torque curve. Below a certain speed, called the corner speed, current would rise above the motor’s rated current, ultimately to destructive levels as the motor’s speed is reduced further. To prevent this, the drive must be set to limit the motor current to its rated value. Because torque is proportional to current, motor torque is constant from zero speed to the corner speed. Above the corner speed, motor current is limited by the motor’s inductive reactance. The result is a two-part speed-torque curve which features constant torque from zero speed until it intersects the motor’s natural load line, called the corner speed, beyond which the motor is in the constant power region.

TScurveREALA real step motor has losses that modify the ideal speed-torque curve, as shown in the second figure here. The most important effect is the contribution of detent torque. Detent torque is usually specified in the motor datasheet. It is always a loss when the motor is turning and the power consumed to overcome it is proportional to speed. In other words, the faster the motor turns the greater the detent torque contributes power loss at the motor’s output shaft. This power loss is proportional to speed and must be subtracted from the ideal, flat output power curve past the corner speed. Notice how the power output decreases with speed because of the constant-torque loss due to detent torque and other losses. The same effect causes a slight decrease in torque with speed in the constant torque region as well. Finally, there is a rounding of the torque curve at the corner speed because the drive gradually transitions from being a current source to being a voltage source.

TScurve2XFinally, the motor power output (speed * torque) is determined by the power supply voltage and the motor’s inductance. The motor’s output power is proportional to the power supply voltage divided by the square root of the motor inductance. As illustrated in the third figure here, if one changes the power supply voltage, then a new family of speed-torque curves results. As an example, if the power supply voltage is doubled then the curve has twice the torque at any given speed in the constant torque region. Since power equals torque times speed, the motor now generates twice as much power as well.

 

Prosthetic Knee Systems Overview

source: Bill Dupes’s original post published on this website

OF ALL PROSTHETIC COMPONENTS, THE KNEE SYSTEM IS ARGUABLY THE MOST COMPLEX. IT MUST PROVIDE RELIABLE SUPPORT WHEN STANDING, ALLOW SMOOTH, CONTROLLED MOTION WHEN WALKING, AND PERMIT UNRESTRICTED MOVEMENT FOR SITTING, BENDING AND KNEELING.

Prosthetic knees have evolved greatly over time, from the simple pendulum of the 1600s to those regulated by rubber knees01bands and springs or pneumatic or hydraulic components. Now, some knee units have advanced motion control modulated through microprocessors. For the transfemoral (above-knee, including hip and knee disarticulation) amputee, successful function depends on selecting the correct knee to fit the person’s age, health, activity level and lifestyle. The latest or advanced knee is not necessarily the best choice for everyone. For some amputees, safety and stability are more important than functional performance. Active amputees, on the other hand, prefer a knee that will give them a higher level of function even if it requires greater control.

Given the wide variety of choices and consumer needs, prosthetists and rehabilitation specialists can help amputees choose the best prosthetic knees for their individual requirements. They can also teach amputees how to use their new knees properly, which is critical for avoiding discomfort, stumbling and falling. A key way to evaluate an individual’s prosthetic needs is to observe his or her walking cycle, which can be divided into two parts: the “stance phase” (when the leg is on the ground supporting the body) and the “swing phase” (when the leg is off the ground, also referred to as “extension”). The happy medium between these two extremes (stance, or stability, versus ease of swing, or flexion) is different for each individual.

Although over 100 individual knee mechanisms are commercially available, they can be divided into two major classifications: mechanical and computerized. Mechanical knees can be further separated into two groups: single-axis knees and polycentric, or multiaxis, knees. All knee units, regardless of their level of complexity, require additional mechanisms for stability (manual or weight-activated locking systems) and additional mechanisms for control of motion (constant or variable friction and “fluid” pneumatic or hydraulic control).

Single-Axis Vs. Polycentric Knees

The single-axis knee, essentially a simple hinge, is generally considered the “workhorse” of the basic knee classes due to its knees02relative simplicity, which makes it the most economical, most durable, and lightest option available. Single-axis knees do have limitations, however. By virtue of their simplicity, amputees must use their own muscle power to keep them stable when standing. To compensate for this, the single-axis knee often incorporates a constant friction control and a manual lock. The friction keeps the leg from swinging forward too quickly as it swings through to the next step.

knees03Polycentric knees, also referred to as “fourbar” knees, are more complex in design and have multiple axes of rotation. Their versatility is the primary reason for their popularity. They can be set up to be very stable during early stance phase, yet easy to bend to initiate the swing phase or to sit down. Another popular feature of the knee’s design is that the leg’s overall length shortens when a step is initiated, reducing the risk of stumbling. Polycentric knees are suitable for a wide range of amputees. Various versions are ideal for amputees who can’t walk securely with other knees, have knee disarticulation or bilateral leg amputations, or have long residual limbs. A standard polycentric knee has a simple mechanical swing control that provides an optimal single walking speed; however, many polycentric knees incorporate fluid (pneumatic or hydraulic) swing control to permit variable walking speeds. The most common limitation of the polycentric design is that the range of motion about the knee may be restricted to some degree, though usually not enough to pose a significant problem. Polycentric knees are also heavier and contain parts that may need to be serviced or replaced more often than those of other types of prosthetic knees.

Microprocessor Knees

Microprocessor knees are a relatively new development in prosthetic technology. Onboard sensors detect movement and timing and then knees04adjust a fluid /air control cylinder accordingly. These microprocessor-controlled knees lower the amount of effort amputees must use to control their timing, resulting in a more natural gait. In spite of all of the amazing inventions and constant tweaks and improvements, the perfect prosthetic knee has yet to be invented; otherwise, there wouldn’t be over 100 different designs on the market. As advanced as the technology seems today compared to the earliest designs of the 1600s, one can only imagine the developments that will eventually result as researchers further explore the potential of mechanical, hydraulic, computerized and “bionic,” or neuroprosthetic, technology.

– by the way, my CV is finally up-to-date! –

assistive devices comparison

what-are-crutch-alternativesTwo weeks ago I stumbled upon this interesting website, which provides useful information about different assistive devices. I tried to make a list of Pros & Cons of crutches, walkers and exoskeletons (as for the latter category, in very general terms) in order to compare them from a more global point of view. The result is the table below here, that you can zoom by a simple click.

Would you help me complete it? For sure it contains mistakes and inaccuracies, and further details should be added 🙂

tableAC

bike, brakes, forces, moments, friction…

brutally copy-pasted from this website, to which all rights belong

What happens on bikes when we brake? To make a simple model, consider a bike that contacts the ground in two places and has a center of mass. The bike is gray, the ground is black, and the forces on the bike are red. The front is to the left; the bicycle is moving left.

01

As the bike goes along, the normal forces on the wheels counter the force of gravity on the center of mass. We’ve drawn the center of mass equidistant from the supports, so to make the net torque zero, the two normal forces are equal. What happens when we brake using the front wheel?

02

We’ve added in the horizontal braking force slowing the bike down. There is now a torque about the center of mass. This torque acts to rotate the bicycle up.
However, as long as the braking force is fairly small, we don’t actually lift the bicycle up off the ground. Instead, it will rise a very small distance. As it rises, the normal forces re-adjust to cancel the torque about the center of mass, like this:

03

There is no longer torque about the center of mass, so the bike no longer rotates. It has gained some very small gravitational potential energy, but too slight to notice. However, the weight on the front wheel is now much greater. Since there is more weight on that wheel, we can apply even more braking force if we want. The braking force is limited by a constant coefficient of friction times the normal force, so a bigger normal force allows a bigger braking force.

If we brake with the back wheel, the braking forces causes precisely the same torque about the center of mass. The normal force on the front wheel will still increase and the normal force on the rear wheel will still decrease. That means we can’t brake as well because we’re using the wheel with less weight on it. We can’t flip over because as the bike starts to rise (and it will, even using the back brake), the braking force gets weaker and weaker and ceases to provide enough torque to continue rotating the bike.

The condition for the bike to stay on the ground is that the torque from the braking force (about the center of mass) needs to be less than that of the maximum normal force on the front wheel. This gives:

( Fb / m )  <  ( g * d / h )

where Fb is the braking force, m the mass, g gravitational acceleration, d the horizontal distance from the front wheel to the center of mass, and h the height of the center of mass. This puts a limiting acceleration on the bicycle while braking. To be able to brake harder, get lower and further back.