What is "bias" in an inertial measurement unit (IMU), and why is it the most critical specification?

What is "bias" in an inertial measurement unit (IMU), and why is it the most critical specification?

 

When selecting an inertial measurement unit (IMU), almost every engineer looks first at "bias stability." Yet, 90% of people overlook a crucial detail: the bias values listed in product datasheets are almost always specified at a room temperature of 25°C.

An inertial measurement unit (IMU) that performs perfectly in a laboratory can easily fail once deployed in the field. The root cause of such issues often points to an overlooked metric: "bias over the full temperature range."

What is "bias" in an inertial measurement unit (IMU)?

 

Bias, in simple terms, refers to the error value in an IMU's output when it is completely stationary. For a gyroscope, bias is measured in °/h (degrees per hour), representing the amount of angular error that accumulates over the course of an hour.

This is why bias is the most critical performance metric for an IMU: it directly determines how long the system can maintain its accuracy in the absence of external corrections.

Room-Temperature Bias ≠ Full-Temperature Bias

The bias of all MEMS inertial sensors changes as a function of temperature. This is an inherent characteristic of MEMS technology, and no manufacturer can avoid it.

 

What is "room-temperature bias"?

In 99% of cases, the "bias stability" specified in product manuals refers to a value measured in a laboratory environment at a constant temperature of 25°C, following a sufficient warm-up period. This figure is typically very impressive, that’s why many manufacturers choose to display it prominently in their promotional materials.

 

What is "full-temperature bias"?

Full-temperature bias refers to the maximum bias value of an IMU across its entire operating temperature range (typically -40°C to +85°C). This is the performance metric that truly matters for your actual industrial applications.

There is a significant difference between room-temperature bias and full-temperature-range bias:

•  IMUs without full-temperature calibration: Full-temperature-range bias is 10–50 times the room-temperature bias.

•  IMUs with standard full-temperature calibration: Full-temperature-range bias is 3–10 times the room-temperature bias.

•  IMUs with high-precision full-temperature calibration: Full-temperature-range bias can be kept within 1–2 times the room-temperature bias.

 

Why do most manufacturers avoid explicitly specifying full-temperature bias?

•  It doesn’t look good: Disclosing these figures might scare off many potential customers.

•  High calibration costs: Every IMU requires point-by-point calibration within a thermal chamber, which is both time-consuming and labor-intensive.

•  Lack of customer awareness: The vast majority of engineers only look at room-temperature bias and are not familiar with full-temperature bias specifications.

 

How can you verify the true capability of full-temperature calibration?

•  Demand that the manufacturer provide actual full-temperature data. If they cannot supply specific specifications for zero-bias across the full temperature range—or if they only offer a vague range—it is safe to assume their full-temperature calibration capabilities are inadequate. Furthermore, do not believe claims that "zero-bias at full temperature is the same as at room temperature"; such assertions defy fundamental physical principles.

•  Examine the temperature range and the number of calibration points. To cut costs, many manufacturers calibrate at only three points—typically -40°C, 25°C, and +85°C—and then rely on linear fitting. In practice, this approach yields very poor full-temperature compensation.

In contrast, our calibration covers the full range from -40°C to +85°C, with calibration and error compensation performed at every 1°C interval.

•  Request empirical data measured under high and low-temperature conditions. Examples include a zero-bias drift curve recorded during 24 hours of continuous operation in an environmental test chamber, or a zero-bias response curve during rapid temperature transitions.

Room-temperature bias is merely an "idealized figure," whereas full-temperature bias reflects "true performance." An IMU that performs perfectly in a laboratory setting—if it has not undergone rigorous full-temperature calibration—may, when deployed in the field, underperform compared to a unit with slightly inferior nominal specifications but superior performance across the full temperature range.

•   Temperature Range: Covers the entire operating temperature range of the product.

•   Temperature Points: Calibration is performed at 1°C intervals; every unit undergoes individual testing at each of these temperature points.

•   Compensation Algorithm: Utilizes a proprietary compensation algorithm (with independent intellectual property rights) to achieve higher accuracy.

•   Individual Calibration: Every IMU unit is independently calibrated across the full temperature range, with a unique calibration report generated for each.