MDT Transducer Fundamentals

Magnetostrictive Displacement Transducers (MDT) are absolute position transducers designed for use in rugged industrial environments. They are non-contact, wear-free, highly reliable, and offer accurate and repeatable linear position measurement. In the motion control industry, magnetostrictive displacement transducers are typically inserted into hydraulic cylinders for measurement of the cylinders position.

For MDT Start/Stop or PWM feedback with a typical calibration value of 9 µs/in, the maximum resolution supported by the RMC70 is 0.0005 in with 1 recirculation. The maximum resolution supported by the RMC150 is 0.001 in with 1 recirculation.

MDT feedback is supported by the following modules:

• RMC70: MA axis module
  Each axis on the RMC70 MA axis module can be individually configured for MDT or SSI inputs.

• RMC150: MDT module

 

Each RMC70 or RMC150 MDT axis can be configured for a Start/Stop transducer or a Pulse Width Modulated transducer.

Start/Stop

To make a measurement with a Start/Stop transducer, the RMC sends an interrogation pulse to the transducer. The transducer responds by returning 2 pulses—a Start pulse and a Stop pulse. The RMCs internal counters begin to count when the first pulse, Start, is received and stop counting when the second pulse, Stop, is received. The time between the start pulse and the stop pulse is proportional to the transducer position.

Start/Stop Pulse Transducer

Blanking Period (For Neuter Outputs)

After the RMC receives the start pulse, it waits a brief amount of time before looking for the stop pulse. This time, called the MDT blanking period, gives time for noise to settle out.

The RMC150 has an MDT Blanking Period parameter, with options of 5µsec or 21µsec. The default value is 5µsec. Some older transducers, such as Temposonics I and II with neuter outputs, require the longer blanking period of 21µsec, due to noisy signals.

The RMC70 does not have an MDT Blanking Period parameter and always uses a 5µsec blanking period. Therefore, Temposonics I and II transducers with neuter outputs should not be used with the RMC70, but can be used with the RMC150.

PWM

To make a measurement with a Pulse Width Modulated transducer, the RMC sends an interrogation pulse to the transducer. The transducer responds with a return signal. The return signal is high while the transducer is determining its position. The RMC counts during the time that the return signal is high. The time that the return signal is high is proportional to the transducer position.

Pulse Width Modulated Transducer

 

The value obtained from the PWM or Start/Stop counter is put in the Raw Counts register for that axis. The Raw Counts are converted to Counts and then into an Actual Position in user-defined units.

Recirculations

Delta does not recommend using recirculations on MDT transducers. Recirculations have historically been used to gain more accuracy in the measurement. However, with faster counters on the controller, recirculations have become less important. In addition, the highest accuracies are only available for MDT transudcers with SSI ouput, which don't have any recirculation options.

Transducer Lengths

An MDT transducer takes a certain amount of time to determine its position. The time it takes increases with position. Therefore, the length of the transducer dictates the minimum Loop Time you can use.

The approximate formula for determining the maximum transducer length for a given loop time is as follows:

Length(in.) = (LoopTime(ms) - 0.22)/0.0095

Using the formula, the following are maximum lengths for a given loop time. This assumes no recirculations.

Loop Time	Maximum MDT Transducer Length
500µs	29.4 inches
1000µs	82.1 inches
2000µs	187.3 inches
4000µs	397.9 inches

 

See Also

MA Module (RMC70) | MDT (M) Module (RMC150)

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