Post by thread:[PICLIST] [EE] [PIC] Speed measurement (variable

Hello to all, actually I have to do a speed measurement of a moving metal part with the use of two variable reluctance sensors (magnetic). Did anybody knows a good design for the comparator schematic or any hints in using these sensors?? Thanks Thomas -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Sat, 18 May 2002, Thomas Lehmann wrote: >Hello to all, > >actually I have to do a speed measurement of a moving metal part with the >use of two >variable reluctance sensors (magnetic). >Did anybody knows a good design for the comparator schematic or any hints in >using these sensors?? Basically there are two situations: a) A magnet is moving across the sensor You use a straight comparator with the coil connected differentially to its inputs. You establish the default output level by manipulating the offset of the comparator. The output will be a short pulse. b) No magnet is moving or in the stator part You put a certain amount of dc current through the sense coil to generate a field, using a constant current generator (in the simplest case a resistor) and sense voltage as above with an AC coupled comparator. The output is a short pulse as above. c) You need to do static sensing. This involves setting up an ac bridge. Ask ... Peter -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

The sensor consist of a thin magnet cylinder with the coil wounded on it, so that at the front end of the sensor there is a static magnetic field. When a ferrous part moves 90 degree to the front surface of the sensor, a voltage pulse is generated. My actual problem is, that the sensitivity depends strongly on the distance between the sensor and the moving metal part. I have wired the sensor on a dc-coupled comparator. The trigger point is setable via pot. Actually I get some "ghost"-pulses. Don4t know the reason why. I4ll try it with a ac-coupled comparator. Do you know an application circuit? Thomas -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Sat, 18 May 2002, Thomas Lehmann wrote: >The sensor consist of a thin magnet cylinder with the coil wounded on it, so >that at the front end of the sensor >there is a static magnetic field. >When a ferrous part moves 90 degree to the front surface of the sensor, a >voltage pulse is generated. >My actual problem is, that the sensitivity depends strongly on the distance >between the sensor and the moving metal part. >I have wired the sensor on a dc-coupled comparator. The trigger point is >setable via pot. Actually I get some "ghost"-pulses. >Don4t know the reason why. I4ll try it with a ac-coupled comparator. >Do you know an application circuit? The signal will be proportional to the inverse of the sqared gap probably. This is to be expected, since it's a magnetic field. The coil rings probably helped by mechanical play and/or circuit capacitance. Damp it with a parallel resistor chosen such that Rdamp ~= 10 times the resistance of the coil (for starters). This will lower the signal. You can avoid this by using the coil in current mode. The trigger setpoint should be automatic, you should only tweak it a little to avoid amplifier noise. There are ICs that do all this for you automatically. There are a number of basic schematics in National databooks (older) under LM339 and such. Also look at 'magnetic tape head amplifiers' (the digital kind). Peter -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Thomas Lehmann wrote: > > Hello to all, > > actually I have to do a speed measurement of a moving metal part with the > use of two > variable reluctance sensors (magnetic). > Did anybody knows a good design for the comparator schematic or any hints in > using these sensors?? Hi Thomas, you will get heaps of problems with the sensors behaving differently (different slope deltas, wave shapes, voltages etc) as the metal moves past at differing speeds. I would seriously suggest using optical (beam break) sensors if you want good speed measuring. -Roman -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Hello Roman, the metall part is moving in a tube with approximately 10 - 40 m/sec (compressed air). The movement is non-periodic. Optical measurement is not possible because of dust and oil fume in the environment. Any additional idea? Thomas -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

{Quote hidden}> -----Original Message----- > From: Thomas Lehmann [SMTP:.....t.lehmann.electronicdesignKILLspam@spam@T-ONLINE.DE] > Sent: Saturday, May 18, 2002 8:11 PM > To: PICLISTKILLspamMITVMA.MIT.EDU > Subject: [EE] [PIC] Speed measurement (variable reluctance sensor)? > > The sensor consist of a thin magnet cylinder with the coil wounded on it, > so > that at the front end of the sensor > there is a static magnetic field. > When a ferrous part moves 90 degree to the front surface of the sensor, a > voltage pulse is generated. > My actual problem is, that the sensitivity depends strongly on the > distance > between the sensor and the moving metal part. > I have wired the sensor on a dc-coupled comparator. The trigger point is > setable via pot. Actually I get some "ghost"-pulses. > Don4t know the reason why. I4ll try it with a ac-coupled comparator. > Do you know an application circuit? > > Thomas > This is a very common problem in engine/chassis management systems that tend to use variable reluctance sensors for just about everything (flywheel position, ABS sensors). Automotive systems use signal conditioning IC's specificaly designed for these sensors. The main problem is that the magnitude of the pulse varies with both speed and distance to the moving part. Using a DC coupled comparator with a fixed decision threshold means your pulse timing will only be accurate at one (calibrated) speed. Everywhere else the pulse will be retarded or advanced. To prevent this, you can AC couple the sensor, and look for a zero crossing on the appropriate slope. This will always be in the same place. You could roll your own circuit, or use something like tge Nat-Semi LM1815 IC ( http://www.national.com/pf/LM/LM1815.html ). Regards Mike -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

Hi, > the metall part is moving in a tube with approximately 10 - 40 m/sec > (compressed air). The movement is non-periodic. > Optical measurement is not possible because of dust and oil fume in the > environment. > Any additional idea? > > Thomas Sounds to be a perfect applications for hall effect sensors. Take a look at http://www.allegromicro.com and you should find what you want. Best regards, Alexandre Guimaraes -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body