Inertial Measurement Unit

Highly accurate measurements of pitch, roll, and yaw rate using three rate gyros, as well as x, y, z acceleration via three accelerometers.

The RACELOGIC IMU can be used as a standalone sensor or as an inertial navigation system (INS) connected to a VBOX GPS data logger.

When used in conjunction with VBOX 3i, data from the IMU can be seamlessly integrated with GPS to produce pitch and roll angle accurate to 0.06° (RMS) as well as smoother velocity data. This ensures GPS data even when satellite reception is interrupted.

Key Features


High Accuracy

Using synchronous 16-bit sampling for each of the internal sensors provides a high degree of accuracy, with yaw rate resolution of 0.00085°/s and acceleration resolution down to 0.04 mg. When used in conjunction with a VBOX 3i pitch/roll is accurate to 0.06°(RMS) and the yaw angle to 0.5°(RMS).

Waterproof

The IMU is constructed with a splash proof casing, which is rated to a limited ingress IP of 65 and IP67 when unused connectors are fitted with Lemo blanking plugs. This makes it ideal for use on boats or in harsh environments, as well as automotive testing.

GNSS Integration

When used in conjunction with VBOX 3i as an inertial navigation system (INS), data from the IMU can be seamlessly integrated with GNSS to produce accurate position, velocity and body angles, even when satellite signal reception is interrupted or degraded.

Internal Temperature Compensation

The IMU is fully calibrated for temperature effects, scale factor, bias and misalignment errors. It has improved calibration and stability.

CAN Bus Interface

When used as a standalone sensor the IMU04 can be easily connected and configured via the CAN or serial interface.

IMU Integration

Survey-grade high-speed GPS is the most accurate way to measure velocity - as long as view to the sky is clear.

Problems arise when buildings or tall trees obstruct the testing ground. If the GPS signal is interrupted, dropouts cause spikes in data, which is not ideal when you are relying on a clean velocity signal.

In order to keep high GPS accuracy even where sky visibility is less than perfect, Racelogic couples data from an IMU and the VBOX 3i GPS data logger.

By blending GPS with data from an Inertial Measurement Unit, housing three gyros and three accelerometers, smoother, more reliable data is produced. The solution can deal with GPS dropouts, maintaining high accuracy. IMU integration realises the high accuracies that VBOX 3i can achieve even when external conditions are compromised, meaning that data has now become more reliable and easier to interpret.

How does IMU Integration improve the GPS data channels?

Velocity
IMU integration decreases the noise for a more accurate reading and maintains a precise velocity measurement throughout the GPS drop outs.
Heading
The heading reading here is very consistent, providing more accurate results than the GPS only data, which exhibits some noise, even in low speeds.
Position
When mapping vehicle position along a heavily tree lined road, the GPS signal incurs some dropouts and reflections, producing noisy position readings. However, IMU integration corrects the position measurements, creating smoother, more accurate data.
Acceleration
IMU Integration data provides a smoother, more accurate representation of longitudinal acceleration (measured in G-force) during an ABS brake stop.

Brake Testing using IMU Integration

Conducting brake tests on tall vehicles with long suspension travel can result in a speed overshoot of the velocity data, due to the measurements being taken at the high roof position of the GPS antenna. As the brakes are initially applied, there is a higher rate of change in velocity at the roof than there is at the vehicle's centre of gravity (COG).

However, the integration of an IMU04 with a suitably upgraded VBOX 3i can be used to counteract this 'lever-arm' effect by placing the IMU at the COG, which measures the vehicle pitch as it brakes. This data, when combined with that from GPS, provides a compensation for the overshoot and allows for consistent brake stop testing.

Counteracting the lever-arm effect will also aid test engineers when conducting high-dynamic manoeuvres other than brake stops. In slip angle measurements the speed overshoot can occur if the antenna is moving through a greater arc of travel than that of the vehicle's centre of gravity as it corners. Procedures such as lane change manoeuvres can therefore benefit from IMU integration and lever-arm compensation.

Point A has travelled further than point B.

This graph shows how point A has travelled further than point B.

Blue trace = GPS Speed; Red trace = IMU-corrected data

In this example of a high-dynamic brake stop, the blue trace (GPS Speed) overshoots at the initial point of brake application, and then exhibits a damped oscillation as the deceleration continues. The IMU-corrected data (red trace) accurately records the brake stop from the vehicles centre of gravity.

Red (below) = Pitch measured by the IMU; Blue = GPS speed; Red = IMU integrated GPS speed

The traces between the green and red vertical lines are of a car going over a speed hump. Note how the GPS speed alters as the vehicle roof moves independently of the COG as it goes over the hump. The integrated speed logs the correct speed of the vehicle.

Mounting Options

1) On the roof

The easiest method of mounting the IMU is by placing it directly on the vehicle roof, co-located with the GPS antenna.

A specially designed IMU roof mount allows for an IMU04 to be securely fastened within the machined enclosure while it's magnetic base ensures that it stays safely in place.

The antenna can then be placed or screwed directly on top of the IMU Roof Mount (RLACS216 for magnetic antennas / RLACS230 for non-magnetic ground-plane antennas) allowing for the data sources to be measured at the same point.

By co-locating the antenna and IMU on the vehicle’s roof the distance between the two does not have to be measured, as you do in a standard setup, leaving only the required translation to – typically – the vehicle’s centre of gravity. This method is easier and isn’t prone to human measurement error. Co-locating the antenna with the IMU also greatly improves the performance of the Kalman filter.

When using a vehicle with a non-ferrous roof such as aluminium, carbon fibre or glass (sunroof), the magnetic based IMU Roof Mount will not hold and a suction mount is required along with the magnetic base.

With its three points of contact the Vacuum Tripod Mount (RLACS256) ensures a very stable fixture and reduces IMU vibration that might be transmitted by the vehicle's roof.

The adjustable lever arms allow for the levelling of the IMU on a non-flat roof whilst extending the mounting options on the vehicle. For example, you can fix the mount partially or fully on a coupé's glass roof when trying to achieve a greater antenna separation in a dual antenna setup.

If you are testing with a VBOX 3i Dual Antenna and are looking for a maximum antenna separation* the use of a Roof Mounting Pole (RLACS171) in combination with the Pole Mount with Ferrous Plate (RLACS257) will allow for a quick dual antenna alignment.

An additional pair of pole suction arms is supplied to stabilise the IMU on the roof.

*Maximum antenna separation improves the accuracy of slip angle measurement


2) Within the vehicle using a mounting arm

If roof mounting of the IMU is not possible the IMU can be fixed inside the vehicle. A flexible way to fix the IMU rigidly within the vehicle is by using the Racelogic mounting pole (RLACS212-v2). The three-part telescopic handle is fully adjustable to any length between 70 and 150 cm to which another 20 cm can be added by extending a third section using the compression lever.

Both ends are fixed to an 8 x 13 cm plate which sits on a joint to accommodate for uneven surfaces. The IMU can be screwed on the additional IMU pole mount bracket (RLACS294) which firmly sits around the pole. Pressed against the floor and the vehicle's ceiling, the mounting prop ensures that the IMU is fixed tightly.

3) Within the vehicle fixed to the vehicle body

It is also possible to fix the IMU firmly to the body of the vehicle. Make sure it is mounted in the direction of travel - as shown in the image below. It is also important to mount the sensor so that it is level with the ground.

For best results, mount the IMU and GPS antenna as close to each other as possible. For example: Bolt the IMU to the seat rails and place the GPS antenna on the roof directly above.

If you are using an IMU04 or IMU03, you must measure the relative position of the antenna in relation to the IMU to within +/- 5 cm. These distances must then be entered into the VBOX either via VBOX Tools > VBOX Setup or using a VBOX Manager.

Technical Specifications


Please not that inertial measurement sensors are highly sensitive mechanical systems. Their performance and life span can be impacted by severe vibration or heavy knocks, and we can only guarantee the gyroscope and accelerometer specifications for a maximum of 2 years from the date of purchase.

Gyroscopes (Angular rate sensors)

Dynamic range
±450°/s
Nonlinearity
0.01% FS
Effective Resolution
20 bits (0.00085 °/s)
Bandwidth
50 Hz
Noise density (Random walk)
0.01 °/s/√Hz (0.6 °/√hr)
Bias stability
±0.0028 °/s
Bias repeatability (1 year)
0.2 °/s

Accelerometers

Range
±20 g
Nonlinearity
0.01% FS
Effective Resolution
20 bits (0.00004 g)
Bandwidth
50 Hz
Noise density (Random walk)
90 µg/√Hz (0.055 m/s/√hr)
Bias stability
15 µg
Bias repeatability (1 year)
0.005 g

Environmental & Physical

Operating Temperature
-40 to +85°C
Maximum Ratings (Shock)
Powered (0.5 ms): 2000 g
Current
~150 mA
Voltage
8 – 30 V DC
Dimensions

IMU04 dimensions

feedback
feedback