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Robotics Technology - Actuators

Actuators, also known as drives,  are mechanisms for getting robots to move. Most actuators are powered by pneumatics (air pressure), hydraulics (fluid pressure), or motors (electric current). Most actuation uses electromagnetic motors and gears but there have been frequent uses of other forms of actuation including NiTinOL"muscle-wires" and inexpensive Radio Control servos. To get a motor under computer control, different motor types and actuator types are used. Some of the motor types are Synchronous, Stepper, AC servo, Brushless DC servo, and Brushed DC servo.  Radio Control servos for model airplanes, cars and other vehicles are light, rugged, cheap and fairly easy to interface. Some of the units can provide very high torque speed. A Radio Control servo can be controlled from a parallel port.  With one of the PC’s internal timers cranked up, it is possible to control eight servos from a common parallel port with nothing but a simple interrupt service routine and a cable. In fact, power can be pulled from the disk drive power connector and the PC can run all servos directly with no additional hardware. The only down side is that the PC wastes some processing power servicing the interrupt handler. 

DC Motors

    The most common actuator you will use (and the most common in mobile robotics in general) is the direct current (DC) motor. They are simple, cheap, and easy to use. Also, they come in a great variety of sizes, to accommodate different robots and tasks.  DC motors convert electrical into mechanical energy. They consist of permanent magnets and loops of wire inside. When current is applied, the wire loops generate a magnetic field, which reacts against the outside field of the static magnets. The interaction of the fields produces the movement of the shaft/armature. Thus, electromagnetic energy becomes motion.  As with any physical system, DC motors are not perfectly efficient, meaning that the energy is not converted perfectly, without any waste. Some energy is wasted as heat generated by friction of mechanical parts.  Inefficiencies are minimized in well-designed (and more expensive) motors, and their performance can be brought up to the 90th percentile, but cheap motors (such as the ones you may use) can be as low as 50%. (In case you think this is very inefficient, remember that other types of effectors, such as miniature electrostatic motors, may have much lower efficiencies still.)  A motor requires a power source within its operating voltage, i.e., the recommended voltage range for best efficiency of the motor. Lower voltages will usually turn the motor (but provide less power). Higher voltages are more tricky: in some cases they can increase the power output but almost always at the expense of the operating life of the motor. E.g., the more you rev your car engine, the sooner it will die.  When constant voltage is applied, a DC motor draws current in the amount proportional to the work it is doing. For example, if a robot is pushing against a wall, it is drawing more current (and draining more of its batteries) than when it is moving freely in open space. 

    The reason is the resistance to the motor motion introduced by the wall.  If the resistance is very high (i.e., the wall just won't move no matter how much the robot pushes against it), the motor draws a maximum amount of power, and stalls. This is defined as the stall current of the motor: the most current it can draw at its specified voltage.  Within a motor's operating current range, the more current is used, the more torque or rotational force is produced at the shaft. In general, the strengths of the magnetic field generated in the wire loops is directly proportional to the applied current and thus the produced torque at the shaft.  Besides stall current, a motor also has its stall torque, the amount of rotational force produced when the motor is stalled at its operating voltage.  Finally, the amount of power a motor generates is the product of its shaft's rotational velocity and its torque. If there is no load on the shaft, i.e., the motor is spinning freely, then the rotational velocity is the highest, but the torque is 0, since no mechanism is being driven by the motor. The output power, then, is 0 also.  In contrast, when the motor is stalled, it is producing maximum torque, but the rotational velocity is 0, so the output power is 0 again.  

    Between free spinning and stalling, the motor does useful work, and the produced power has a characteristic parabolic relationship  demonstrating that the motor produces the most power in the middle of its performance range.  Most DC motors have unloaded speeds in the range of 3,000 to 9,000 RPM (revolutions per minute), or 50 to 150 RPS (revolutions per second).  That turns out to put them in the high-speed but low-torque category (compared to some other alternatives). For example, how often do you need to drive something very light that rotates very fast (besides a fan)? Yet that is what DC motors are naturally best at.  In contrast, robots need to pull loads (i.e., move their bodies and manipulators, all of which have significant mass), thus requiring more torque and less speed. As a result, the performance of a DC motor typically needs to be adjusted in that direction, through the use of gears.

Gearing

    The force generated at the edge of a gear is equal to the product of the radius of the gear and its torque (F = r t), in the line tangential to its circumference.  By combining gears with different radii, we can manipulate the amount of force/torque the mechanism generates. The relationship between the radii and the resulting torque is well defined, as follows:  Suppose Gear1 with radius r1 turns with torque t1, generating a force of t1/r1 perpendicular to its circumference. Now if we mesh it with Gear2, with r2, which generates t2/r2, then t1/r1 = t2/r2.  To get the torque generated by Gear2, we get: t2 = t1 r2/r1.  Intuitively, this means: the torque generated at the output gear is proportional to the torque on the input gear and the ratio of the two gear's radii. If r2 > r1, we get a bigger number, if r1 > r2, we get a smaller number. 

     If the output gear is larger than the input gear, the torque increases. If the output gear is smaller than the input gear, the torque decreases.  Besides the change in torque that takes place when gears are combined, there is also a corresponding change in speed. To measure speed we are interested in the circumference of the gear, C= 2 * pi * r. Simply put, if the circumference of Gear1 is twice that of Gear2, then Gear2 must turn twice for each full rotation of Gear1.  If the output gear is larger than the input gear, the speed decreases. If the output gear is smaller than the input gear, the speed increases.  In summary, when a small gear drives a large one, torque is increased and speed is decreased. Analogously, when a large gear drives a small one, torque is decreased and speed is increased. Thus, gears are used in DC motors (which we said are fast and low torque) to trade off extra speed for additional torque.  Gears are combined using their teeth. The number of teeth is not arbitrary, since it is the key means of proper reduction.  Gear teeth require special design so that they mesh properly. If there is any looseness between meshing gears, this is called backlash, the ability for a mechanism to move back \& forth within the teeth, without turning the whole gear.  

    Reducing backlash requires tight meshing between the gear teeth, but that, in turn, increases friction. As you can imagine, proper gear design and manufacturing is complicated.  To achieve "three to one gear reduction (3:1)", we apply power to a small gear (say one with 8-teeth) meshed with a large one (with 3 * 8 = 24 teeth). As a result, we have slowed down the large gear by 3 and have tripled its torque.  Gears can be organized in series ("ganged"), in order to multiply their effect. For example, 2 3:1 gears in series result in a 9:1 reduction. This requires a clever arrangement of gears. Or three 3:1 gears in series can produce a 27:1 reduction. This method of multiplying reduction is the underlying mechanism that makes DC motors useful and ubiquitous.

Electronic Control of Motors

    It should come as no surprise that motors require more battery power (i.e., more current) than electronics (e.g., 5 milliamps for the 68HC11 processor v. 100 milliamps - 1 amp for a small DC motor). Typically, specialized circuitry is required. You need to learn about H-bridges and pulse-width modulation there.

Servo Motors

    It is sometimes necessary to be able to move a motor to a specific position. If you consider your basic DC motor, it is not built for this purpose. Motors that can turn to a specific position are called servo motors and are in fact constructed out of basic DC motors, by adding:

  • some gear reduction

  • a position sensor for the motor shaft

  • an electronic circuit that controls the motor's operation

    Servos are used in toys a great deal, to adjust steering on steering in RC cars and wing position in RC airplanes.  Since positioning of the shaft is what servo motors are all about, most have their movement reduced to 180 degrees. The motor is driven with a waveform that specifies the desired angular position of the shaft within that range.  The waveform is given as a series of pulses, within a pulse-width modulatedsignal. Thus, the width (i.e., length) of the pulse specifies the control value for the motor, i.e., how the shaft should turn.  Therefore, the exact width/length of the pulse is critical, and cannot be sloppy. There are no milliseconds or even microseconds to be wasted here, or the motor will behave very badly, jitter, and go beyond its mechanical limit. This limit should be checked empirically, and avoided.  In contrast, the duration between the pulses is not critical at all. It should be consistent, but there can be noise on the order of milliseconds without any problems for the motor. This is intuitive: when no pulse arrives, the motor does not move, so it simply stops. As long as the pulse gives the motor sufficient time to turn to the proper position, additional time does not hurt it.

Continuous Rotation Motors

A regular DC motor can be used for continuous rotation. Furthermore, servo motors can also be retrofitted to provide continuous rotation (remember, they only to 180 otherwise), like this:

  • remove mechanical limit (revert back to DC motor shaft)

  • remove pot position sensor (no need to tell position)

  • apply 2 resistors to fool the servo to think it is fully turning

Related Products For Drives and Actuators

    Research into shape memory alloys, polymer gels and micro-mechanism devices is ongoing, and changing often. Nickel-titanium alloys were first discovered by the Naval Ordinance Laboratory decades ago and the material was termed NiTinOL. These materials have the intriguing property that they provide actuation through cycling of current through the materials. It undergoes a ‘phase change’ exhibited as force and motion in the wire. At room temperature Muscle Wires are easily stretched by a small force. However, when conducting an electric current, the wire heats and changes to a much harder form that returns to the "unstretched" shape -- the wire shortens in length with a usable amount of force. Nitinol can be stretched by up to eight percent of their length and will recover fully, but only for a few cycles. However when used in the three to five percent range, Muscle Wires can run for millions of cycles with very consistent and reliable performance.

 


 

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