sensor for continous motion

hi everyone,
my design is a continuous motion device (lifting up and down) with certain range of motion (degrees). i need a sensor so that the moving base will change the direction of lifting when it reach the range limit. any suggestion for the sensor to be used?

Use Gyros.....very simple.....3-axis are available for around $10

ENJOY!

Gyros are good but they are a lil difficult to use...If u r new to sensors then u should first use simplest sensor a button...If u wanna use non-contact sensor then go for proximity sensors...they are very easy to use

yes, i am new to sensor. the range of motion of my device is within 0° to 130° , with adjustable angle range of ±5° , meaning that if we set 30°, the moving base will move from 0° to 30°, then change the direction of lifting. so i need a sensor that enable me to adjust its initial location according to the favour angle range .
the size of the moving base is basically similar with the size of the lower arm..any suggestion?

engkeat,

It seems like you want to instrument a mechanically-intensive device. Could you post some mechanical drawings or sketches? Could you tell us what the device (or the subsystem that you're working on) will be doing?

- Nick

the device is used for performing passive arm exercise for those who undergo arm physio, the mechanical design is basically consist of a stationary base and a moving base . the angle range of the exercise increase slowly according to the pain tolerance of the user. so, i need a sensor to detect the positon of the moving base..
the upload image is retrieved from wikipedia which is a bit similar with part of my design,
my rough idea is to fix it at the pivot. the favour angle range is set by adjusting the two movable "needle sensor" initially, my problem is what type of sensor can i used to detect if the moving base have reached the angle that i set previously

According to ur Mech Drawing...Gyros wont do the job....u`ll need proximity sensors for that purpose...And u have to install the sensors on a disc so that the limits can be adjusted by changing the sensor position (degrees).. and disc can be attached at the bottom of ur system...Proximity sensors are of various types...use those which can detect metal if ur guage moving parts are metal...

For measuring angles, you could use a rotary encoder, unless I've misunderstood your design. It has a turning thing, and it will tell you how far it's turned.

Another approach would be to use a potentiometer as a voltage divider. That would probably be cheaper, but less precise. Probably precise enough, though.

I'll second sketerpot:
- Rotary encoder (RE). Optical RE - more rugged, longer life, more expensive. Mechanical/contacting RE - shorter life, usually cheaper.
- Potentiometer. Continuous output. Limited angle, or limited number of turns. Relatively short life (usually around 10k cycles). Cheap.
- Rotary Variable Differential Transformer (RVDT).

precise angle is crucial for my device, i am thinking of using a push button switch, when the moving base touch the switch, a signal will send to the PIC and yet change the motor direction

engkeat said:

precise angle is crucial for my device, i am thinking of using a push button switch, when the moving base touch the switch, a signal will send to the PIC and yet change the motor direction

Click to expand...

Oh, so you need to detect only certain limiting positions. How accurately do you need to detect the angle? On what radius? If the desired accuracy is not too high, a number of proximity sensor types can do this job (mechanical switch - as you've mentioned, Hall effect, beam interruption, eddy current, capacitive).

If you want to detect only the maximum and minimum limit of your working arc then limit switch is the simplest way to acheive. If you want to keep a log of each position then you need rotary encoder. It is best to buy one but if not possible you cab build yourself a simple one. In any graphics software draw a cricle with 360 equidistant marks/dots around its edge. Print it on tranparency and cut it out of that. Mount it on a servo motor in a way that its edge is just between transmitter and sensor of an optical detector. Each signal of optical detector will represent a movement of 1 degree. you can put a simple set/reset logic counter to count the movement steps. A very easy logic circuit can give you a control on motion & direction of servo motor.

It seems mess up of practical work but it will be a good practice to learn a lot of inside a initial control system.

goodluck

A POT can also simply be an absolute rotary encoder.