The contact-less registration of positions, driving movements etc. are a required method for procedure automation, so that re-registration can take place in the actual situation of the system. Often precision in the µm range are needed. The systems need to be robust, with little required maintenance and (of course) inexpensive. Several years ago a method was established, where ultrasonic delay time on a wire was used to determine a position.
A wire is strung over the entire length of the positioning equipment. A strong permanent magnet is situated at a certain position. A current pulse is sent by wire at regular intervals (i.e. 1k/sec) with several ampere and a length of several microseconds. This high current pulse results in a magnetic field around the wire, which changes rapidly. The position where the magnetic field of the permanent magnet is situated, an energy is generated that winds around the wire and generates an ultrasonic wave, which extends into both directions of the wire (at app. 2,700 m/sec) with ultrasonic speed.
In the ?dead" side the ultrasonic impulse is absorbed, whereas the "active" side detects the impulse via ultrasonic receiver, which also converts it into a digital receiver impulse. The time between the start of the pulse and receiving of the receiver impulse is measured via TDC so that the location of the permanent magnet can be determined. In cases where the permanent magnet moves with the positioned parts (i.e. a milling cutter) the equipment can be positioned precisely.
This procedure permits positioning precision of 2-5 µm, which equals an ultrasonic delay time of 800 ps ? 2 ns, which is exactly the range that a TDC has to offer. With a high resolution, which is located slightly above the requirements, TDCs are ideal for time difference measurements. In addition, these chips are extremely small in comparison to past solutions, are ten times as precise while using only 1/10 of the power. All of these features contribute to the increase in distribution of TDCs.
Magnetostrictive Positioning Measuring Principle
Magnetostrictive TDC Application Notes
Position Sensing with Magnetostrictive Devices Improving Precision using TDCs
Non-contact, linear position sensing is necessary for many industrial applications. Magnetostrictive transducers in addition offer a very high precision of less than 0.001 inch. They are well established in applications like plastic injection molding machines, hydraulic and pneumatic cylinders or woodworking machinery. A basic challenge for the electronics is high-precision time measurement and can be easily solved usind TDCs (Time-to-Digital Converter)
Ultrasonic Magnet Position
The Measuring Principle
The measuring element consists of a magnetostrictive waveguide. Magnetostrictive materials are elastically deformed when a magnetic field is present. This effect is used in the following manner: The magnetostrictive waveguide is built as tube with a copper rod inside. The start of measurement is initiated by a short current pulse. This produces a circular magnetic field around the waveguide. The position of the movable part is marked by a magnet whose magnetic field is perpendicular to the circular field of the current pulse. The interaction between the two magnetic fields produces a strain pulse, which travels at sonic speed along the waveguide. A sensor placed at the end of the waveguide converts the sonic pulse into an electrical signal. The travel time is directly proportional to the position of the magnet. The sonic speed in the waveguide is approximately 2830m/s, which corresponds to approx.
9µs/inch or 0.35µs/mm. To achieve a resolution of 0.0002 inches (5µm), the precision in time measurement must be 1.8ns! This corresponds to a reference clock in the GHz range.
Time Measurement with TDCs
TDC-GP1 is a good solution for the time measurement task in magnetostrictive applications. This sin-gle-chip time-to-digital converter has 2 channels with a single shot resolution of 250ps and therefore easily fullfills the needs of high precision positioning. Furthermore, the 30bit dynamic range of TDC-GP1 allows measurements up to 200ms with the same resolution. With it‘s 4-fold multihit capability up to 4 magnets can be handled at once. TDC-GP1 is available in a small TQFP44 package allowing compact board design.
Several different measurement modes are available with TDC-GP1. For magnetostrictive applications the one called ‚measurement range 2‘ is the right choice. In this mode, the TDC uses a predivider to extend the
