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.


Hubo II, a new humanoid robot !

Hubo II has been developed by Professor Jun Ho Oh and his colleagues at the Korea Advanced Institute of Science and Technology’s Humanoid Robot Research Center, aka Hubo Lab. The original Hubo , built in 2004, was one of the first advanced full-body humanoid robots developed outside Japan. Hubo II is lighter and faster than its older brother, weighing 45 kilograms, or a third less, and capable of walking two times faster.

A major improvement over early humanoid designs is Hubo II’s gait. Most humanoid robots walk with their knees bent, which is dynamically more stable but not natural compared to human walking. Hubo II performs straight leg walking. It consumes less energy and allows for faster walking. The robot has more than 40 motors and dozens of sensors, cameras, and controllers. It carries a lithium polymer battery with a 480 watt-hour capacity, which keeps the robot running up 2 hours with movement and up to 7 hours without movement.

Another improvement is the hand design. It weighs only 380 grams and has five motors and a torque sensor. It can handle any object that fits on its palm, and its wrist can rotate in a humanlike way. In humanoid robot projects, the main challenge is not just cramming all the hardware into a tight space, but also making sure everything works together. Cables can unexpectedly restrict joint movements; power and control boards interfere with each other; modules end up too heavy and create instability. Professor Oh wants to make a robust design to avoid such catastrophic failures. He believes Hubo II is a big step in that direction.

source: this website

Let’s do it Wireless!

Someone pointed me to this very interesting article from . It talks about a new technology for wireless energy transfer. Since it’s actually interesting, I just copy it here (contents belong to Scott Davidson).

Wireless Power Charging Systems Span Time and Frequency Domains

The use of magnetic induction as a way to move power from a source to a device is nothing new. It works, it’s efficient, and it’s safe. So why aren’t we all wirelessly charging our mobile phone, tablet, and laptop batteries? Certainly consumers would appreciate the convenience of just placing their smart phone or tablet down somewhere and watching it automatically charge up.
Like most things in the electronics industry, technologies require standards to achieve widespread adoption. For consumers, standards provide the confidence that the technology works and they won’t be locked into a dead end. For the industry, standards lead to the critical mass needed to drive down component costs. Without standards, even promising technologies fail to gain much traction.

At long last, wireless charging appears to be on its way thanks to the Qi (pronounced “chee”) standard published by the Wireless Power Consortium (WPC). Its more than 100 members range from phone vendors to chipmakers to wireless carriers. At the 2012 International CES, more than 75 products and prototypes included Qi technology. A few other wireless power efforts, most notably WiTricity, are in the works. But with its broad industry backing, Qi seems to be the most probable winner, at least in the short term.
For the embedded or systems engineer, this momentum likely means that you should plan on having to implement wireless charging in your designs at some point in the next few years. Initially, the charging of mobile electronic devices will be the most popular application area for wireless power. But there are many other potential applications in medical devices and the industrial and transportation segments. For instance, instead of plugging in electric vehicles, drivers could simply drive into wireless charging bays when they need to juice up.

Unlike wireless telecommunication systems such as radio or cellular phones, wireless power transmission depends more on the efficiency of transfer than signal-to-noise ratio. From a measurement perspective, the chargers present many design challenges.
In its current spec, a Qi wireless charger is designed to produce 5 W of charging power. The efficiency of power transfer depends on system design including both transmitter and receiver, specifically the interaction between each one. Designs typically target greater than 70% efficiency for a 5-W system. The selection of coils, shielding, components, and physical design influence the overall system efficiency.
This is more complicated in a wireless charging system than in a typical charger, since the wireless system requires both a transmitter and a receiver. Other complications exist due to the shielding requirements, which are necessary to protect sensitive electronics and the battery from the RF fields. And, the system must be able to detect foreign objects so they don’t get hot or reduce the system’s efficiency.

A Qi system includes low-frequency modulated RF, digital, and analog circuits all on a single board (see the figure). The charging system uses digital communication for JTAG debugging and to transfer data between the secondary and primary circuits across the resonant link. A secondary-side microcontroller monitors the charger’s output voltage, generates signals, and uses modulation techniques to transfer information to the primary side.

The information is demodulated on the primary side, where the primary-side microcontroller interprets it. The modulated information is organized into information packets that have preamble bytes, header bytes, message bytes ,and checksum bytes. Per the WPC specification, information packets can be related to Identification, Configuration, Control Error, Rectified Power, Charge Status, and End of Power Transfer information.
The emergence of wireless power dovetails with another trend in embedded systems: the use of wireless everywhere. More than 60% of oscilloscope users also use a spectrum analyzer. These engineers are troubleshooting embedded system designs with integrated wireless modules, requiring them to work in both the time and frequency domain. This has led to the need for more capable oscilloscopes such as the Tektronix MDO4000 that can provide time-correlated views of analog, data, and RF signals. The MDO4000 is the world’s first oscilloscope with this ability.

Qi designs illustrate the importance of mixed-domain capabilities. When paired with appropriate accessories such as near-field probes and bench power supplies, the mixed-domain oscilloscope (MDO) can monitor digital control signals, track the RF received output with a spectrum view, and show RF amplitude versus time. This allows the designer to see a signal at its point of origin, within the RF link signal, and at the point of receipt across the transmitter winding. It can also measure the analog step load performance of output regulators and evaluate electromagnetic interference (EMI) emissions.
With a strong push from the WPC, it’s a solid bet that cable clutter will be a thing of the past before too long. For engineers tasked with adding wireless battery charging to their designs, the ability to look at time-correlated analog, digital, and RF signals will be critical to efficient troubleshooting of system-level issues. By integrating a mixed-signal oscilloscope with a modern spectrum analyzer, the MDO4000 for the first time delivers that capability. Goodbye wires.