Shunt or magnetic sensors? A comparison of current measurement methods
In contrast to voltage and temperature measurement high-precision current measurement are especially difficult in the area of several hundred Amps. Current sensors in hybrid and electric vehicles usually implement one of two technologies which are completely different in their physical properties: Magnetic current sensors (hereinafter referred to as MCS) such as GMR, Hall Sensors and current transformers, or current-sensing resistors (shunts). Table (T1) shows the advantages and disadvantages of these physical measuring principles. For this, the sum of all advantages as a whole is more important than any single advantages one product might offer.
The following is a detailed list of differences between these physical metering methods:
1. Construction form and mounting space
Keeping the mounting space as small as possible is an important factor for alternative drives, while the shunt and PCB's customizable designs offer high added value. The shunt may be built in various ways as a part of a bus bar.
2. Dynamic range
The dynamic range necessary for these applications that may be easily covered by the IVT is between some 10 mAmps and 1.5 kAmps. A comparable MCS would have to be built in a double sensor construction to achieve the range, which would negatively influence weight, mounting space and precision.
3. Voltage and current measurement precision
A comparison with the MCS is not possible for voltage metering as those systems are current sensors exclusively. Where required, customers would have to built-in a fully separate, isolated and therefore expensive voltage sensor.
The IVT has an integrated, highly precise voltage measurement at 0.1% for 40 mV. Concerning current measurement, the IVT-A's measurement precision is about one order of magnitude higher than that of the respective MCS. In this area, the module reaches a precision of 0.1%rdg 20 mAmps (current range +/-300 Amps) and 0.5 % at 100mAmps respectively (current range 1500 Amps) for the complete required temperature range between -40 and +85°C.
Detailed metered values:
Current range 300 Amps
Table 2: Measurement errors for current at room ambient temperature
Current range 1500 Amps
Table 3: Temperature dependence for current and voltage measurement
4. Sampling rate
The maximum sampling rate is 3.8 kHz. Compared with some MCS, this seems rather slow at first glance. To achieve a similar noise behavior using a MCS, however, a filter is necessary to filter signal noise at high frequencies of 100 kHz down to 100 Hz, for instance. This means that the IVT-A also has a clear advantage at the same noise intensity.
5. Redundant safety systems for over-current sensing
The IVT-A includes a redundant and independent over-current sensor. Its hardware limit is set to a minimum threshold value. In addition this value can be increased in steps by software programming. An optimum hysteresis may be adjusted using firmware.
Any voltage and current channels may be equipped with anti aliasing filters and it is also filtered by an internal integration/low pass filter.
7. Galvanic isolation
The IVT-A uses complete galvanic isolation for both signal and supply paths. This isolation is voltage proof up to 2.5 kV.
8. Output signal
Currently, the IVT may be delivered with an SPI-interface, a CAN-interface is in development. The IVT provides customers with the requested data in calibrated form directly as a digital signal via a standard interface. The interface protocol may be adapted to the user.