Signal Chain Basics #113: Precision current measurement optimizes motor control

In this article learn about options for measuring current in motor control applications, the strengths and challenges of these options, and how using precision current measurement can optimize the motor control solution.

Basic motor topologies

First, let’s look at the different motor control topologies.

There is the half-bridge with single high- and single low-side drivers where the motor is attached to the connection point of these two drivers (Figure 1 ). This configuration is typically used with a brushed DC motor, and offers three operating modes: run, coast, and break.

Figure 1

Simplified circuit diagram for a half-bridge motor topology.

Next is the H-bridge, which resembles an H (Figure 2 ), which is used typically with a brushed DC motor. In addition to run, coast, and break operating modes, it can also reverse a motor.

Figure 2

Simplified circuit diagram for a H-bridge motor topology.

Last is the 3-phase topology (Figure 3 ), which is commonly used on brushless DC (BLDC) motors. BLDC motors are electrically commutated and offer all four operating modes.

Figure 3

Simplified circuit diagram for a 3-phase motor topology.

Why measure current for motor control?

There are two primary reasons for measuring current in motor control circuitry:

• Fault protection
• Input for motor control algorithm

Overcurrent protection circuitry is used for fault protection to identify: stall conditions, bad connections, or the motor’s health. By detecting when an overcurrent condition occurs, overcurrent protection allows the system to take action to prevent potential damage. This protection can range from simple overcurrent detection to complex current monitoring.

Current measurement provides both torque and speed information that can be used by algorithms that control the motor. Current measurement is directly proportional to the motor torque. Speed information can be calculated by understanding how the control algorithm affects the current level.

Current monitoring options

There are three options for measuring current in both half-bridge and H-bridge topologies (Figure 4 ).

Figure 4

Current sense resistor placement options for a half-bridge motor topology.

High-side sensing has the advantage of a stable common-mode voltage and it enables fault detection. However, depending on the motor, the common-mode voltage could be very high, limiting the choice of devices for this implementation. Additionally, the driver current, which is actually what is being measured, does not necessarily equal motor phase current.

Low-side sensing also measures the driver current, not the motor-phase current. It introduces a ground variation of the motor relative to system ground. Due to the sense element’s location; fault detection is limited. It does offer the advantage of having more options for implementation as the common-mode voltage is essentially ground, enabling use of low-voltage amplifiers.

In-line sensing offers true motor phase current measurement for optimizing the quality of the information being provided to the motor control algorithm. The major challenge is that the common-mode voltage is a pulse-width modulated (PWM) signal, which disrupts the output signal unless good PWM rejection circuitry is enabled. This causes more strenuous requirements for the current sense amplifier; for example, it must have both very good DC and AC common-mode rejection ratio (CMRR).

As with half-bridge or H-bridge topologies, we have the same three current measurement location options (Figure 5 ).

Figure 5

Current sense resistor placement options for 3-phase motor topology.

The strengths and challenges for high- and low-side sensing are the same as earlier. With the understanding that if the low-side is done on each leg (Figure 5 ), the current measured by any one of the amplifiers represents only one-third of the total current. Now it must be recombined by the management controller to understand the actual motor current.

Measuring inline leaves no guess work on the phase current, as that is what is being measured at all times. However, as noted earlier, the common-mode voltage seen by the current sense amplifier is a high-voltage PWM that must be rejected. The frequency of the signal seen by the current sense amplifier has two contributors:

1. Differential signal (useful information) is relatively narrow-band and small amplitude
2. Common-mode PWM signal (not useful) is wide-band and large amplitude

An ideal inline current sense amplifier only processes the differential signal; while rejecting the common-mode signal. This high-voltage combined with high Δ V/ Δ T poses a steep challenge that limits the availability of suitable current sense amplifiers. This can limit the adoption of this topology to those applications requiring precise phase current measurement, such as electronic power-steering systems.

Summary

The decision on where to measure current for motor control depends on the application requirements. High-side is a simple approach that enables easy fault identification, but does not make it easy to determine the actual motor phase current. Low-side is a proven approach that is perceived to have a lower system cost, but has difficulty scaling to higher precision and lower drift. It creates a non-linear output signal that must be post-processed to extract the proportional base current information. In-line provides a faster response, higher precision, and the ability to have less drift over a wide temperature range. It reproduces a continuous proportional signal of the phase current, which does not need further processing. If accurate representation of the phase current is critical to the systems operation, then in-line motor sensing is the best option.

Join us next time, when we discuss the effects of DC/DC switching regulator noise and address power supply ripple when designing circuits for precision SAR ADC applications.

References

More from TI about current sensing.

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