Chico Mendes - Environmental robot powered by DC motors.
maxon DC motors drive environmental robot.On hilly terrain, in winding waterways, in muddy swamps – “Chico Mendes” feels at home in even the most inhospitable surroundings. Standing 1.20 meters tall, with four agile motor driven legs and outstanding analytical skills, he is perfectly equipped to cope with life in the Amazon’s 370 square kilometre large rain forest.
No, this is not an exotic animal, but the latest environmental robot belonging to Brazilian energy firm Petrobras. The robot was developed with the help of the latest submarine and drive technology in a joint project between the sociological research institution Piatam (Potential Impacts and Environmental Risks of the Petroleum and Gas Industry in the Amazon), the Brazilian state university and Petrobras.
“Chico Mendes”, named after the rainforest activist who was killed in 1988, has four twin brothers. They and he will be testing the quality of land and water along the new 420 kilometre gas pipeline on the Solimões and Negro rivers. The banks of these two Amazonian rivers are part of the world’s largest and most enigmatic eco-systems. The five environmental robots have been built to analyze physiochemical parameters, such as oxygen content or the PH value of water. The on-board equipment is also designed for photographing mosquito larvae in high-resolution or capturing authentic primeval forest and riverbed sounds. Piatam promises to deliver a representative picture of the ecological environment around the new pipeline using all the data being transmitted electronically on a continuous basis.
In developing “Chico Mendes”, particular attention was paid to making the hybrid robot adapt to the rainforest landscape as much as possible, minimizing any damage to the very nature it is supposed to be protecting. Such specifications posed a particularly difficult challenge in drive technology terms.
“Chico Mendes” has to battle through mud, water and swamps with the help of electric DC microdrives and minimotors. This involves eight of maxon motor’s DC motors which, with an exceptionally long service life, deliver a high performance in a tiny space. Apart from the above characteristics, their low energy consumption is another important reason why they were chosen for the Piatam project. The environmental robots are driven by solar power in an initial phase. With an efficiency of over 90%, maxon’s motors are ideally suited to this type of application.
maxon RE 40 DC motors feature in all four of the robot’s wheels. The wheels are made of fibre glass and measure 66 centimetres in diameter. Another four maxon RE 35 DC motors are housed in the adaptive suspension, and automatically adapt to conditions depending on the terrain and water conditions in the rain forest. Two planetary gearheads, measuring 42 and 52 millimetre across, provide the requisite torque for the vehicle which is 1.5 meters long, 2.2 meters wide and 1.2 meters high.
The environmental robot's high dynamism requires equally dynamic electronics to control the DC motors. All operating DC motors can be accurately controlled using maxon motor's digital positioning controller EPOS 70/10. EPOS stands for “easy-to-use positioning system”. All EPOS controllers can be networked according to CANopen standards. The drives are programmed according to IEC 61131-3 standard using a simple software tool with graphic user interface.
Key positioning of DC servo motors
High-tech servo drives are in demand everywhere, from medical, robotic and handling technology, car building, medicine and surgery, cash machines to the computer industry. Systems have to be space-saving, have fast reactions, be controllable and meet long service life requirements. maxon motor has a dominant position in this field with innovative, practical solutions and a wide product range.
maxon motor’s DC servo motors are increasingly used in mechanical engineering and tool building and here are some of the reasons why:
• smaller and more powerful motors, the smallest of which has a diameter of just 6 mm with an output of at least 1.2 watts. The output of the ten-times larger 60 mm EC motor is 400 watts.
• the modular system with tachos, encoders and controllers as well as flexibility for specific requirements.
• the current trend towards decentralizing drives and universal application options.
• the special motor technology with related benefits.The ‘heart’ of the DC motors’ drive, as described below, is the worldwide patented ironless winding, System maxon.
By comparison, (Figure 5) the rotor of a conventional DC motor comprises an iron cylinder (iron core or armature iron) with inserted spool segments. This rotor is encumbered with a large moment of inertia, high inductance and therefore a long start-up time. In the maxon DC motor however, a pure copper winding rotates in the magnetic field. The special design means that the winding is self-supporting, also known as squirrel cage. Every motor type has numerous windings available for optimum adjustment to the required nominal voltages. This differentiation is achieved through the wire cross-section and number of windings, resulting in different types of terminal resistance from the motors. The technical benefits of ironless winding are:
• no magnetic detent, armature iron missing. The rotor can be stopped in any position.
• fast acceleration thanks to the low mass moment of inertia
• low inductance
• high efficiency – up to 90% which is significantly higher than conventional motors
• linearity between voltage and speed as well as between load and speed
• small dimensions thanks to compact design
• Commutation:
precious metal brushes and commutators guarantee high constancy of the low contact resistance, even after a prolonged standstill. Other benefits include very low start-up voltages and minimal electrical interference. The use of the CLL system (Capacitor Long Life) curbs sparking and significantly increases the commutation system’s service life. Most of the motors are intended for use in high-tech applications in battery-powered appliances.
When combined with copper commutators, graphite brushes are suitable for robust use in reverse and start/stop operation, where tens of millions of cycles can be achieved.
Brushless DC servomotors stand out in particular because of their favourable torque behaviour, high dynamism, extremely wide speed range and their service life. Together with ‘Y’ and ‘Δ’ wiring patterns, the winding technology allows for a range of designs.
Below is a list of some of the technical benefits of maxon EC motors:
• service life only limited by ball bearings
• no magnetic detent / cogging
• excellent control features
• highly efficient
Gearheads
Gearing must be used if performance is required at a sharply increased torque and correspondingly lower speed. The most commonly used ones are spur and planetary. Spur gearheads are reasonably priced.
Planetary gearheads are particularly suitable for transmitting high torque. The use of ceramic components significantly boosts the performance of the gearheads. Today, planet carriers and cog parts are also made of high-tech ceramic materials in addition to axes. Ceramic powder injection moulding is a core process for maxon that is increasing in importance.
Tacho / encoder
In principle, any maxon DC motor can be used as a DC tacho. A DC tacho is preferably used for controlling speed, as the given voltage is proportionate to the speed. The ideal DC tacho has, a) a permanently low internal resistance, b) a very high specific voltage, c) no ripple, d) no temperature dependency, e) no linearity error and f) no reverse error. As some of these requirements are contrary to what is needed, suppliers have to restrict themselves to a compromise and set their own priorities in terms of individual parameters (e.g. dimensions, temperature dependency and mass inertia). maxon offers a DC tacho with ironless rotor, AlNiCo permanent magnet and a casing 22 mm in diameter. This creates the greatest linearity between speed and output voltage. The tacho rotor is firmly connected to the motor shaft for resonance reasons.
An encoder is required for positioning. The output signal is intermittently dependent on the rotor angle position and has a rectangular pattern. Under the GLS optical principle, an LED sends light through a finely screened pulse generating disc that is firmly mounted on the motor shaft. The receiver (photo transistor) changes light/dark signals into corresponding electrical impulses which are amplified and processed in the corresponding electronics.
Other systems operate on the magnetic principle. Key benefits are miniaturization, high resolution and low cost.
One special feature is the combination of DC tacho and encoder, with the speed-proportionate DC voltage signal allowing speed control and direction recognition. The incremental encoder signals allow precise positioning.
Control and regulation
If a DC motor is connected to a fixed voltage, the speed follows the speed/torque gradient whenever the load changes. If speed changes too quickly in an application under various loads, either a more powerful motor must be used or operating voltage changed (U1..2..3). Speed remains constant. The gradient shifts in parallel and this can be automated electronically. The electronic controller adjusts the voltage to the motor's relevant load case. The complete system comprising controller and motor now seems to have a flat characteristic. The controller needs two pieces of information to adjust the speed: the speed that has to be controlled (set value) and the speed that has already been reached (actual value). Comparing these two values gives a positive or negative variation. The controller then changes the motor voltage until the variation is zero.
maxon’s servo-amplifiers work on one of the following two principles to control the output stage transistors:
a) linear: the supply voltage is divided between motor and output stage. The controller changes the voltage on the motor (UM) proportionately (linear). The voltage declining at the output stage (UT) causes power loss. The simple and low-cost structure of the output stage is advantageous, but the drawback is the power loss caused by low motor voltages and high currents. maxon motor’s Linear Servo Control LSC is suitable for controlling DC motors up to 30 watts.
b) pulse control PWM : the controller switches the motor ON and OFF at short intervals (pulses/meters). If the OFF interval is longer, the motor slows down the acceleration. The crucial average value of the voltage changes in relation to the ON to OFF time. Very little energy is converted into heat. In the modular system, maxon motor’s PWM servocontrollers PSC are designed for four different operating modes and DC motors of up to 500 watts.
Electronically commutated EC motors require special controllers. Simple speed control is possible with or without Hall sensors (in the motor). Speed is recorded using encoder signals for high-performance controllers. There is a distinction between unregulated and regulated drive – closed-loop control.
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| Chico mendes powered by DC motors. |
Standard DC rotor |
Chico mendes in water |
Author: mmau edit/22.09.2009
