Current transducers go digital
07 October 2016
A couple of years ago, LEM’s Sales & Marketing observed the first signs of a technological turnaround: some of the leading OEMs in the servo-drives and robot industry were thinking of abandoning analogue interfaces on their systems.
This change was driven by the evolution of controllers: the “brains of the machines”. The controllers function fully digitally internally and generally only had A/D’s (analogue-to-digital converters) on their interfaces. This is no longer the case; the A/D’s are disappearing in new controllers and customers are now asking for current transducers with a digital output so that they can easily connect to their new micro-controllers. Another advantage of a digital interface is that it is less sensitive to electro-magnetic interference.
After a market survey conducted in 2013 by LEM’s marketing & product management teams, the R&D department started developing a Hall-based open-loop ASIC. The analogue to digital (A/D) conversion is performed by an on-board sigma-delta modulator, giving a 1-bit serial stream output, with a sigma-delta modulator at the output.
The first prototype transducers were delivered to “alpha customers” at the end of 2015 where the initial feedback was positive
LEM proposed a range of digital output versions of the successful HO and HLSR open-loop Hall effect current transducers. These new components are for nominal current measurements of 10, 32, 50, 80, 100, 120, 150, 200 and 250 ARMS in three different mechanical designs (PCB and panel mounting) and provide up to 12-bit resolution with 20kHz bandwidth. The single-bit output minimises the connections required, enabling highly compact transducers, and the digital output allows the user to choose the filter used on the bitstream to optimise between resolution and response time, according to the application. Digital outputs are also intrinsically immune to noise in hostile environments.
The digital interface
The digital filter is implemented by the user. The advantage is that the number of connections to the transducer is minimised; each user can decide the filter(s) best suited to the application and the output format can be selected to match the system requirements.
Performances and filter choices
Any conversion from an analogue to a digital signal involves quantisation, and the error between the digital signal and the exact value of the analogue signal it represents is equivalent to the addition of noise. The output from a sigma-delta modulator is more than simply a bitstream with a certain density of 1’s and 0’s; the sequence is randomised in a way that pushes the quantisation noise out of band to frequencies higher than those of interest for current measurement. The user processes the bitstream in a digital filter which rejects the high frequency noise. As with any filter, compromises are made to optimise system performance: a narrow bandwidth gives lowest noise (or highest resolution) at the expense of response time, and vice versa.
Bits are processed one at a time in the digital filter. Due to the modulator oversampling ratio (OSR), the digital filter output can process every OSR bits with no loss of information within the band of interest.
The latency of filter depends on its very nature: output is delayed by 2 x OSR x CLK period for a sinc2 whereas 3 x OSR x CLK period are needed to get the exact output after a step response with the very common sinc3 filter. The bit rate at the output of LEM’s new sensors is 10Mb/s. The combination of OSR, filter choice and bit rate leads to the response time, the bandwidth and the effective resolution of each of the signal paths connected to the bitstream.
The resolution of the complete system including the analogue part of the transducer, the sigma-delta modulator and the digital filter is limited either by the quantisation noise inherent to the system or by the analogue noise from the Hall cells and amplifiers. For fast response times (for example with an OSR of 16 and a sinc2 filter) the resolution is defined by the system and will be the same with any transducer. If the filter is changed to sinc3 and the OSR is increased the effective resolution is improved but will be limited to 11 – 13 bits (depending on the sensor sensitivity) by analogue noise. The term “effective” resolution is used because for system convenience the filter may output a word with a length of 16 bits or 2 x 8 bits. However only the most significant bits corresponding to the effective resolution contain useful information, the less significant bits contain noise.
Usually the digital filter output is sampled at a frequency equal to the bit rate divided by the OSR; this is referred to as decimation.
To transmit the bitstream, LEM offers the choice between two physical interfaces. In both cases the bit rate is 10Mb/s:
With the first, clock and data are provided as single-ended CMOS levels (Uc and GND). This is suitable for transmission over short distances, up to some tens of centimeters, after which EMC issues may become important. The maximum allowed capacitive load is 30pF.
The second interface is suitable for transmission over longer distances. In this case the clock and data are combined as a Manchester coded signal. This is output on transducer pin 3 and its complement on pin 4. The differential signal thus generated is compatible with the RS422 standard. By keeping the two signal tracks physically close, EMC effects, both transmitted and received, can be kept to a low level.
This technological leap is not “just a new family of transducers” for LEM and the industry.
The outlook for digital output transducers is very encouraging: customer feedback shows that a significant part of the market will shift to digital interfaces, starting with the high-end servo drives. It is expected that more industry segments to follow this trend.
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