Free falling hammer

Question

Hello,
First I am not sure where to put this question, it is a difficult one.
I have a free falling hammer that falls vertically and crushes specimen being tested. The weight of the hammer is about 250 kg and it's speed 10 m/s. The deformation of the specimen is about 0.15 m.
I need to measure the force and the position of the hammer. Force measurement is an easy thing, problem starts when I try to measure hammer's position.
Linear encoder is precise enough, resolution 40 nm, total accuracy 1 micro m.I need accuracy about 0.01mm.
Problem- I cannot measure hammer's position from it's start but I start to measure just before collision. So the hammer hits the liner's encoder scale which starts to vibrate.
The encoders manufacturer specifies that encoder's redhead ridehight variation must be within 0.05mm.The scale's vibration makes measurement impossible.
My idea is to install a shock absorber to decrase scale's acceleration. Shock abrorber that has 0.3m stroke will decrease the acceleration to 333m/s^2. Seems acceptable.
The scale must be install on some kind of linear bearings or linear guideway. The tolerance of this must be smaller than 0.05mm what is difficult to buy.
I want to ask you for help, some ideas how to solve this problem.
Best regards,
przemmo
Attachments:
free falling hammer.png ‏9 KB

Answer 1

Hi Przemmo,
Just to run with the optical idea a little further:
Install a precise scale on the hammer, and a large zoom objective on the camera. A high speed camera on a very large zoom level can capture as the scale (markings) on the falling hammer move, similar to a quadrature encoder. The inspected area (the scale) could be 10mm, in which case you need a 200px vertical resolution to measure 0,05 mm. This is available. Make the markings little triangles, use visual inspection, and you can measure 10m/s at 0,05mm accuracy at 1000 FPS. Add a unique markings (ie a serial number for each triangle) and you can measure the 0,05mm precise analog position at any FPS you need.
All you need is a very acurate scale on the hammer, and a very stable fixture for the camera.
Best Regards,
ST
Best Regards,
T Simon
National Instruments
Applications Engineer
Certified LabVIEW Developer - Certified TestStand Architect