Monday, April 4, 2016

How to select a load cell

Single Point Load Cell (Also known as Side Mount Load Cells, Parallelogram Sensors or Belt Buckle Sensors): This Single Point Load Cell is used in small jewelry scales and kitchen scales. It’s mounted by bolting down the end of the load cell where the wires are attached, and applying force on the other end in the direction of the arrow. Where the force is applied is not critical, as this load cell measures a shearing effect on the beam, not the bending of the beam.

Pancake Load Cells, also known as low profile Load Cells: The nature of its structure is less sensitive to extraneous load and moments (especially Torque) compared to other inline sensors such as the S Beam Load Cell, diaphragm or column type. For lower capacities it’s designed with bending beams.

Piezo Film Sensors

Applications: Higher frequency applications strain gauge.
Pros: flexible, lightweight, and tough. In its simplest mode the film behaves like a dynamic strain gauge except that it requires no external power source and generates signals greater than those from conventional foil strain gages after amplification.
Cons: Operation down to fractions of Hz can be achieved using either conventional charge amplifiers or, since signal levels are relatively high, simple high impedance FET buffer circuits. Very slow events, 0.0001 Hz, for example, are not normally possible to detect with piezoelectric film.
Costs: Under 200$

Saturday, April 2, 2016

Controlling DC motor by means of direction of rotation and torque/power insertion

For running a DC motor in both direction (in our case CRS Robotics gripper) you need to have a bridge to be able to switch the polarity of the source. For this purpose I have used  EVAL6206PD board which not only provides a protected bridge but also it has PWM current control for both of its output independently.

The two bridges are designated A and B and their output pins designated as OUT1A, OUT2A, OUT1B, and OUT2B. These outputs are controlled independently by logic inputs IN1A, IN2A, IN1B, and IN2B respectively.

A logic high or low on any of these inputs will drive its corresponding output to the positive supply rail or to ground. Both of the A outputs will be forced to an off (high impedance) state if the ENA pin is taken logic low, as will the B outputs if ENB is taken low. Each bridge also has an analog control signal, VREFA and VREFB, which control the current.