domingo, 30 de março de 2014

March 24th

On this week I’ve been doing a several things. As in every project, it’s essential to have the documentation of all the work! This week I started to draw all the circuits of my hardware and to project the PCB Board in Eagle, as I said that I would be doing before.

The board is meant to be able to gather all the data from all the sensors, which are a force load cell, a force sensor resistor (FSR) and an infrared distance sensor, amplify the signal and convert it from tension to current. This last requirement is very important because the sensors will be far away from the Arduino. If the signal goes from the sensors to the Arduino in tension the signal may vary due to the resistive wire losses and as well, in current the signal has a greater electrical noise immunity.

The first board that I made, as you can see on the figure, doesn’t have yet this V/I converter so I’ll be making a few changes on the next week to add that.

Figure 1 - Acquisiton board made in Eagle Software.

Also the board has only three slots for one of the kind of sensors that I mentioned, however I’ll be using two of each kind so I’ll have to add more connectors too.
As for the connectors, I am using molex connectors but they are not the best considering their size and utility, so I’ll be switching those for RJ45 connectors for the supply and the signals, that able to have 8 wires in just one cable, and JST-H connectors for the sensors, that are much smaller than molex.

At the same time I’ve been creating a new package in ROS with a script in C++ that reads the data from the hardware and displays it as a kind of progress bars on ROS-RViz. 

Figure 2 - Displaying data on RViz, green - load cell, blue - FSR and red - IR sensor.


terça-feira, 18 de março de 2014

March 17th

On this week I will be working on Eagle which is a software that allows to project PCB boards, because later I'll have to make some of these boards for the signals acquisition.


quinta-feira, 13 de março de 2014

March 10th

On this week I did some drawings of my prototypes ideas and built a test prototype so I can be prepared to test and get data when the sensors arrive.

Figure 1 - Isometric view of the test case for the sensors.

Also I made a list to order all the equipment that I'll need the most for my project. For example, force load cells, force sensor resistors (FSR), IR Distance sensors and acquisition boards and adapters.


quinta-feira, 6 de março de 2014

March 3rd

On this week I did some research on how could I measure the forces on a pedal. There are two systems, one using Force Sensing Resistors (FSR) and other using Force Load Cells.

The FSR are flat sensors that have variable resistances between their 2 pins depending upon the force applied. When the force applied increases the resistance goes down. These sensors are easy to use and also relatively inexpensive, however it has some disadvantages such as drift and poor accuracy, errors up to 25%.
Depending on how long and how hard it's pressed, the FSR value will come back to approximately 95% of its initial value almost instantly, and than drift the final 5% over a few seconds. In conclusion these sensors aren't the best choice when it's required accuracy and repeatability.

Figure 1 - Force Sensing Resistor.

The Force Load Cells are transducers that convert force into an electrical signal. The applied force deforms a strain gauge and that strain gauge measures the deformation as an electrical signal. The output signal is typically very small so it's required to amplify it by using an instrumentation amplifier. The load cells have higher dimensions than FSR, however they have high accuracy and repeatability, which, in my opinion, are very important aspects when building a system like mine.

Figure 2 - Force Load Cell.

Also, I wanted a sensor that would be able to measure the distance from the pedal to the car floor. After some research I found an interesting IR Distance sensor for very short distances (4-80cm).

Figure 3 - IR Distance Sensor.


sábado, 1 de março de 2014

Feb 24th

On this week I had a workshop about C++ and Robot Operating System (ROS) at the LAR, where I learned some important things of C++ language and some basics of ROS.

To develop my software I'll be using C++ and ROS so this really was a major added value.