Honeywell HMR4007 manual Magnet Considerations, Magnet apparent length 900301 01-03 Rev. a

HMR4007

SENSOR PRODUCTS

The second function, "CurrentPosition", is used to query the current position of the magnet. The following 16-bit integer values are returned:

0-1024	normal position response where 1024 represents full scale response.
-1, -2	communications failure. The sensor failed to respond to the PC quickly enough. This is
	usually caused by improper wiring, improper power settings, a timeout value too small for
	the PC speed, or the DLL not being initialized before attempting to get the position.
-3	position error. The sensor communicated correctly, but the sensor card was not able to
	calculate a valid position. A position error is usually caused by the magnet being off the end
	of the board or not present.

The functions "SetAxialMode" and "SetRadialMode" are used to configure the sensor to use magnets in either the axial mode or radial (see the introduction for a description of axial and radial modes). The sensor is shipped with the axial mode selected. Once set into an operating mode, that mode is retained in nonvolatile RAM.

The functions "EnableDAC" and "DisableDAC" are used to enable and disable the digital to analog (DAC) output on port J2. The sensor is shipped with the DAC enabled. Once set into an operating mode, that mode is retained in nonvolatile RAM.

HMR4007.exe: This is a simple Visual Basic 5.0 executable that demonstrates the use of the DLL and shows a simple graphical display of the current position. Source code for the program is available and can be modified as desired. The position window will turn yellow in the event of a communications failure, or turn red in the event of a position error.

MAGNET CONSIDERATIONS

The choice of magnet to be used with the HMR4007 is critical to its accuracy. Because a magnet's field extends beyond the boundaries of the material, a magnet has two dimensions to consider: its physical (material) length and its apparent (magnetic) length. The heart of the HMR4007 is Honeywell's HMR1501 magnetoresistive (MR) sensor that responds to the angle between the chips' sensitive axis and the magnetic field. The HMR1501 has a maximum response when this angle is +/-45 degrees. We can, therefore, define the magnet's apparent length as the distance between the +45 and -45 degree points at the gap distance.

		HMR1501		As the gap between the magnet and the sensor array increases, the
Bx			= f(4)	apparent length of the magnet increases as well. A general rule of
		response		thumb for a cylindrical bar magnet is that the apparent length will
		B
	4			increase by the amount of the gap. A twenty-five millimeter long
				magnet for example will appear to be thirty-five millimeters long at a
				gap of 10 millimeters. Since magnetic fields are highly nonlinear and
				subject to the exact geometry and properties of the material, this is only
				a general guide.

		N			The HMR4007 uses three sensors in its array at all times in order to
			physical	apparent	calculate position. It automatically selects the sensors closest to the
		S	length	length	magnet for this purpose. The apparent magnet length must, therefore
					be greater than he spacing between three sensors, and this distance is
					23.2mm for the HMR4007.
					The disadvantage of increasing the gap between the sensor and
					magnet in order to increase its apparent length is that a magnet's field
					strength decreases exponentially with gap. In order to maintain the MR
					sensor's response the magnetic field at the sensor must be greater
					than 35 gauss. Unless extremely strong rare earth magnets are
gap					economically feasible or a large gap is required because of restrictions
					on where the magnet and array can be mounted, it is usually better to
					keep the gap to a minimum and use a longer magnet.
	Figure 3. Magnet apparent length
900301 01-03 Rev. A
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