Current transformer as inexpensive, non-invasive timing trigger

Article By : Jeff Radtke and William Ferris

Build an inductively coupled trigger with respectable time response using a current sense transformer.

We needed to detect motion in a commercial, stepper motor driven system and subsequently initiate motion in a servomotor based system we developed.  Because both systems simulate human respiratory motion, a 10 ms response time would suffice.  Control of the servomotor system was possible using an optically coupled input to a Parker EtherCAT motor drive.  Following initial motion detection, it was acceptable to manually reset the motion detection device.

Several options were explored.  A voltage level trigger was considered, but because the stepper motor system was borrowed, it was deemed unacceptable to splice into either motor or drive cables.  Software triggering was unavailable in the stepper motor system.

Electronic sensing of motor drive was preferred over optical or magnetic motion detection because of repeatability concerns and a desire to avoid mechanical complexity. A non-invasive trigger sense based on inductive coupling was chosen after the identification of an inexpensive current sense transformer with a split-core to facilitate non-invasive implementation.  The Nidec C-CT-6, featuring a 5 kHz high frequency corner and 1:3000 turns ratio, appeared to be up to the task.

Circuitry to detect stepper motor current and change the state of an LED in the EtherCAT drive input is shown in Figure 1.

Figure 1 Inductively coupled trigger circuit employs a standard current sense transformer (T1), large burden resistor (R2), input comparator (U1), flip-flop (U2), and output comparator (U3).  Pulse generator and load resistors (R1 and R11) replaced motor hardware for test purposes.

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The C-CT-6 current transformer typically is used with a 10 ohm burden resistor to provide an accurate representation of primary current.  Because we didn’t care about amplitude accuracy, a large (33 K) burden resistor (R2) with 3.1 V zener diode voltage limiter was chosen to improve the time response and prevent damage to the comparator U1. Full-wave rectification (D1-4) of the current transformer output was implemented to simplify interface connection by removing polarity effects.

Comparator U1 compares the rectifier output to a threshold voltage set by RV1.  Positive feedback of comparator U1 is provided via the offset adjustment, pin 5.  A positive-going U1 output triggers a D-type flip-flop (U2A).  Pushbutton switch SW1 provides manual reset of the flip-flop between trigger events.  To help determine when this switch needs to be pushed, the LED (D6) indicates status of the flip-flop.  An output comparator, U3, acts as a low side switch to control the opto-coupling LED in the EtherCAT  motor system.  Common-mode chokes (FL1, FL2) on the trigger input and output reduce RF interference and prevent false triggering in the electrically noisy environment.  These chokes consisted of five turns of RG-174 on ferrite split cores.

To simulate drive system connections and measure trigger time response, the stepper motor and drive were replaced by a 56 ohm load (R1) and a B&K 3011B pulse generator. This provided a 200 mA primary current in the current transformer (T1). The optoisolated input to the EtherCAT drive was replaced by a 2 K load (R11).  Trigger repeatability using the motor drives was also confirmed.

Oscilloscope observations of the simulated system [Figure 2] show the voltage across R1 is a 12 V positive going step; this was used to trigger the oscilloscope.

Figure 2 These oscilloscope traces indicate rectifier output noise and slope, as well as trigger response time during our test.  Channel 1 is the voltage across R1, Channel 2 is the voltage at rectifier output, Channel 3 is the input comparator threshold, and Channel 4 is the output of U3.

Inputs to the comparator, U1, are also shown to indicate when the rectifier output exceeds the threshold voltage.  It is believed that the current transformer output rise time is limited by the secondary coil capacitance. (Lee) Threshold voltage was chosen to be as small as possible, but well above the noise floor to minimize false triggering.  The rectifier output voltage exceeds the 0.25 volt threshold 13 microseconds after the leading edge of the input pulse. Comparator response time adds another 1.2 microseconds.

The 15 microsecond response time of this trigger exceeded our requirements as well as expectations based on the bandwidth specification of the current transformer we used.  The choice of a large burden resistor and diode voltage limiter on the current transformer output improved time response at the expense of amplitude accuracy.

As shown in Figure 3, it is possible to use an inductively coupled trigger in more demanding applications.

Figure 3 Characteristics of a hand wound (10:1) ferrite core current transformer for high frequency triggering.  Channel 1 is the voltage across 56 ohms, sourced through the primary coil.  Channel 2 is the current transformer output voltage across a 33 K burden resistor.

The time response of the current transformer is greatly improved by reducing the secondary coil to a few turns, thereby decreasing secondary capacitance.  A high permeability ferrite split core, such as an inexpensive common mode choke core, further increases high frequency response. To reduce insertion loss at the expense of secondary pulse amplitude, the air gap in a split core can be adjusted with shims.  Core material may be chosen for magnetic properties to meet a range of bandwidth requirements.

References

  1. Lee, “Making sense of current sense transformers,” 6 Feb. 2019. [Online]. Available:  http://www.epsma.org/page85.html. [Accessed Sept. 2020]

 

This article was originally published on EDN.

 

Jeff Radtke holds an MS degree in Nuclear Engineering and Engineering Physics from the University of Wisconsin-Madison.  He has been employed in nuclear instrument development for educational, industrial and medical applications since 1986.  

William Ferris holds an MS degree in Medical Physics from the University of Wisconsin-Madison. He is currently a dissertator and research assistant in the same department. His thesis is concentrated on patient motion management for radiation therapy.

 

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