The other Great Wall

-

Being in China for a few days is an eye-opening experience: both the future and the past are mixed together here in a curious combination. There are many good things beyond the food: A taxi driver may return your missing phone and the waiter will patiently wait for you to order. Other things are not so nice: don't expect queues to be respected nor all cars to stop at pedestrian crossings, even if there is a red light.

Internet experience is also special: Many sites just don't work. Others work like if something was wrong under the hood and finally, the rest work like in the rest of the world. It is not a problem of network speed (though some links could use some extra speed) but of the control the government imposes upon what can or cannot be browsed by citizens and visitors of China.

For some visitors like me, showing some pictures on Facebook for family and friends is a common activity when we are abroad. Not being able to even open Facebook or to make a blog entry (like this one) feels odd. Of course the reason is the so-called Great Firewall of China.

But this did not come as a surprise as I was expecting to have to deal with this problem. The common answer to this problem is to use a VPN connection to a server outside of China and to use that VPN server as the default gateway for all your IP traffic. Performance may degrade but you are free from the limitations of the firewall. However, what was a surprise was that the campus network did not allow VPN connections. Without a VPN my attempts to use this as a way to overcome the firewall limitations were gone.

Given that my original plan was a no-go, I had another back-up plan: to use a ssh connection to forward web traffic to a proxy outside of China. This is the plan that is working nicely now. I guess that other providers in China do not use to block VPN connections though.

Comments

Post a Comment

Popular posts from this blog

VFD control with Arduino using RS485 link

-
Image
Variable Frequency Drives ( VFD ) allow the control of spindles so speed can accurately be controlled and a detailed acceleration profile for the spindle and reverse rotation can all be handled. In essence a VFD is an three phase inverter for three-phase AC motor. I am using a popular (I mean cheap) Chinese VFD and though the reference manual is not great, I could see there is a built-in RS-485 port. I usually control the start, stop and speed selection using the keyboard on the unit but I thought it will be more useful if I could control everything from the same Arduino is doing our CNC table control. Some cheap RS-485 off eBay and some lines of code later, I can start, stop and change the speed from an Arduino. What a cool thing to have! Some configuration of your VFD are needed before you can use it like that.  You need to set PD163=1 (I am using address 1 in the code). PD=164=1 (for setting serial to 9600bps) and PD165=0 (for using ASCII and 8N1 character format)
Read more

How to get sinusoidal s-curve for a stepper motor

-
Image
After some experimentation, and delving a bit into the math of motion, I realized there can be a significant difference on the motion of a machine by just making small changes. A common and popular approach to smooth one axis of motion is to use a trapezoidal speed pattern, where acceleration and deceleration are uniformly accelerated movements (aka constant acceleration motion) separated by a portion happening at cruising speed. The problem comes with the fact that, while that approach is simple to understand and to code, it does contain sudden changes in the acceleration, that show as spikes on the acceleration derivative, called jerk. These sudden changes in the acceleration translate into undesired vibration in our machines. Most of that can be eliminated by selecting a motion pattern that does not contain sudden changes of acceleration. If we chose a mathematical function for our speed whose second derivative is continuous, then we reduce system vibration quite a lot. One ma
Read more

Importing OpenSCAD designs into Onshape

-
Image
After using Onshape CAD software for a while I am really fond of it. So one natural thing to do, specially once they have released the Instructor's kit is to start using it in the classroom too. But one thing I was not so sure how to do was to use old designs I made using OpenSCAD. It does not mean I am completely quitting using OpenSCAD but suddenly I can see I can do assemblies in Onshape that will look quite nice for showing and documentation purposes if only I could easily import from OpenSCAD. I am glad to report that the route OpenSCAD --> FreeCAD --> Onshape worked brilliantly. Even more when a few tricks were applied. While some people suggest to import CSG files from OpenSCAD to FreeCAD, I have had more success by directly importing SCAD files into FreeCAD. One trick that worked well for me was to get rid of $fn references, so now circles become real circles (or cylinders) in FreeCAD. If a part keeps some $fn references then your cylinders or circles wi
Read more