Wednesday, March 30, 2016

Linux is my biggest weakness

Sometimes I look job ads. I want to see what kind of engineer companies need. For many embedded software engineer job positions the Linux is necessary. Embedded Linux is most common operating system for embedded devices.

I have never worked with the Linux. Once I installed the Fedora Linux on my desktop PC but I didn’t use it a lot.

Now I plant to learn the Linux. I think this wouldn’t be hard. I will buy the Raspberry Pi Zero. This small computer for 5 dollars can be interesting base for many embedded projects. It has the Linux. I think the Linux on Raspberry Pi Zero would be too much robust for some real time embedded systems. I plan to investigate how this Linux distribution can be reduced and become more reliable for some embedded applications.

Some interesting sites and texts about Linux: 

Tuesday, March 22, 2016

Bike power meter

Fabian Cancellara

As the hardware and triathlon fan I am very interested in a bike power meters. There are lots types of bike power meters, but the principle of operation is similar. 

The equation for the bike power is: P = T * w. T is torque and w is angular velocity

The equation for torque is T = F * r. F is force and r is the distance from the center of rotation to the point where is the force applied. 

So the equation for the bike power is P = F * r * w. On a bike the force and angular velocity can be measured. The hardest part is the force measurement.

The strain gauge is using for measurement the force. This is insulating flexible backing which supports a metallic foil pattern. It is attached to the object by a suitable adhesive. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge

The key of the bike power measurement is to put the strain gauges in some bike part. The strain gauges usually put in bottom bracket, rear freehub, crankset, or pedals. 

When the place of force measuring is known, r distance from the center of rotation to the point where is the force applied is easy to determine. For example, when strain gauges are put in the pedals crank length must be known (170mm, 172.5mm, 175mm, 177.5mmm…). 

If strain gauges are put in bottom bracket, crankset, or pedals w angular velocity is the same as the cadence (number of pedal revolution per minute). If strain gauges are put in rear freehub w angular velocity is the same as the bike wheel angular velocity. The cadence or the bike wheel angular velocity can be measured with magnet (measure the time of revolution) or with gyroscope

I was thinking about the bike power meter project. I was planning to put strain gauges in a bike shoes cleat. This would be interesting for Kickstarter, but someone was faster

Tuesday, March 15, 2016

How works power factor correction

I usually work with a low voltage capacitor banks for power factor correction. Then people ask me how it works? Lots of these people are from other professions and don’t have a knowledge of electrics. It is hard to explain it in a simple language with don’t using electric’s terms and formulas. 

Now I try to explain power factor correction for everyone, don’t depend from professions. 

This isn’t science text and don’t complain if I mix some basic physic terms.

The electric distribution system gives to us the electrical energy through AC voltage and current. In an industry we have two types of energy, one is active (kW), and other is reactive (kvar). Active electrical energy converts in other types of energy like heat, mechanical work, chemicals when charging batteries, light and other. Reactive energy used for creating and maintaining a magnetic field in electrical motors, inductive heaters, some kinds of lights and other. Reactive energy created and maintained a magnetic field on way that it takes energy from the electric distribution system and then returns it to the electric distribution system. This is reason why it calls reactive. Power factor presents ratio of active and reactive energy.

A capacitor needs just reactive energy for creating and maintaining electrical field. It is function just like magnetic field it terms of take and back energy from the electrical distribution system. One thing is different, the timing of take and back energy. When magnetic field take energy, electrical field would back energy and conversely. When we add a capacitor load reactive energy from magnetic field and reactive energy from electrical field added energy to each other and don’t take energy from electric distribution system. Consequence is low energy form the electrical distribution system, low main current, and low electrical power consumption bill. This is the way how works power factor correction. This is some kinds of paradox, we add capacitors, more loads to the electrical distribution system and electrical power consumption goes down.

In industry we use low voltage capacitor banks in transformer stations. There is power factor regulator which power on or off capacitors depends of reactive power consumptions. 

If someone has better idea how to explain power factor correction, please contact me.

Sunday, March 6, 2016

Tempo calculator

When I was active runner, I had written a blog about distance running. Then, I was playing with web programming. For that blog I made tempo calculator using HTML and Java Script.

This calculator has data about time, distance and velocity. Distance and velocity data are shown in different units. The tempo calculator calculates one of three data from other two data.

Below is my tempo calculator. Interface is on the Serbian. If someone needs HTML, Java Scrip file, please contact me.

Tempo kalulator

h min sek

km m milje broj krugova na atletskoj stazi

min/km min/milja km/h min/krugu na atletskoj stazi

Thursday, March 3, 2016

More efficiency induction motor

These days at my job there are talks about more efficiency induction electrical motor. Some people say that induction electrical motor can be different rewinding and then become a more efficiency. A little about induction motor:

Lots of people have made mistakes when talking about efficiency because they mention just power factor. Power factor isn’t always connects with efficiency. Nice text about it: 

A little more about power factor:

One of new more efficiency winding is YY/D which explained in these two texts: