H3 Inertial Measurement Unit

Product Specification User’s Guide

Document Number:  PSD-1001

Document Revision:   D

Document Owner:

Design Engineering

TABLE OF CONTENTS

1.0       Purpose. 1

1.1      The High Performance Inertial Measurement Unit 1

2.0       Communications. 2

2.1      Commands. 2

2.2      Sample Format 2

2.3      Measurement 4

3.0       Mechanical. 5

3.1      Dimensions. 5

3.2      Coordinate System.. 6

4.0       Hardware. 7

4.1      Connections. 7

4.2      Pin Function Description. 7

4.3      RS422 Connection Description. 8

4.4      Digital Sensor Encode Pulse Output 9

5.0      Part Numbers. 10

5.1      USB Data Acquisition (DAQ) Module Options. 10

5.2      Specifications. 11

6.0       Terms, Conditions and Warranty. 12

1.0         Purpose

1.1           The High Performance Inertial Measurement Unit

This user’s guide documents the features and use of the H3 Inertial Measurement Unit Series of products.  The H3 Inertial Measurement Unit provides serial digital outputs of triaxial acceleration, angular rate and magnetic field data. Custom algorithms provide high performance, temperature compensated data in real time via the RS-422 protocol at sample rates up to 800 Hz.  Two performance options are available the HN for normal applications or the HP which dramatically reduces the accelerometer bias offset and noise. The H3 Inertial Measurement Unit also supports 2 external analog inputs and 2 external digital inputs that allow other sensors, digital components or switches to be integrated from your application. The H3 is available in a custom package measuring 2.000 in. × 1.110 in. × 0.645 in. height.  The H3 IMU is provided with a 15 pin Bi-Lobe connector.  Table 2 details the pin-out of the connector configuration. (see Section 5.1 for part numbering specifications).

Figure 1 - H3-IMU Functional Block Diagram

2.0         Communications

2.1           Commands

The H3 Inertial Measurement Unit does not currently offer a command structure or API that allows modification of device characteristics at runtime.

2.2           Sample Format

Data samples are formatted as shown in Table 1.  Each data channel (i.e. gyro, accelerometer, magnetometer) is represented by a signed (2’s complement) 2-byte short (16-bit) integer that must be converted to its corresponding engineering unit before use (see Section 2.3).  An individual data packet is collectively referred to as a sample.

Table 1 - Sample byte order/format

Graphically, the sample has the format shown in Figure 2:

All samples begin with four (4) synchronization bytes, where each byte is encoded with 0xFF hex.  Synchronization bytes aid in the identification of the beginning of samples as they arrive from the device.  There are two cases in which synchronization is necessary: 1) initial synchronization of data once the device is powered and 2) re-synchronization if data is lost or errors are encountered.  The complete structure of a sample is as follows (Note: all byte offsets are zero (0) based):

1. Synchronization bytes: bytes 0-3 with each byte encoded as 0xFF hex.
2. Message size: byte 4, size, in bytes, of entire data message, including complete header.
3. Device ID: byte 5.
4. Message ID: byte 6, type of message.  Currently, only data messages are transmitted by the device with MID 0x14 hex (20 decimal).
5. Pulse Marker / External Digital Inputs: byte 7, bit3 of byte 7 is the pulse marker. If this bit is high a pulse output should be correlated to the frame otherwise the bit is 0. Bits 0-2 map to external digital inputs 1-3 respectively. Bits 4-7 are not used.
6. Byte 8 is reserved for future use.
7. Product ID: byte 9, product identification number.
8. Frame Counter: bytes 10- 12, The frame counter is a 23 bit unsigned integer with a maximum count of 16,777,216.
9. Serial Number: bytes 13 & 14 serial number of each H3-IMU.
10. Payload: byte 15, the payload size can be calculated as follows:

payload_size = message_size – 13(header) – 1(Checksum byte)

11. Checksum byte: 8-bit checksum byte.
    1. Sum sample contents (header + payload).  DO NOT include the checksum byte.
    2. The summed value should equal the checksum if the message is valid.
    3. Note:  If greater precision (larger than 8-bit) addition is used to calculate the checksum, the checksum will be the remainder of a divide by 256.

2.3           Measurement

Accelerometer, gyro and magnetometer data is temperature compensated on the H3 Inertial Measurement Unit.  The payload element of the data packet contains accelerometer, gyro and magnetometer samples, which must be converted to values that represent usable data (e.g. rotational rate, G-force, gauss).  The data is transmitted as signed (2’s complement) 16-bit integers.  The following function must be used for conversion of sample values:

Equation 1:    result = raw_payload_value x digital_sensitivity

where result is the converted value in the appropriate units (e.g. deg/sec), raw_payload_value is the raw component-specific value from the payload (e.g. accelerometer X), and Digital_Sensitivity is the sensitivity expressed in engineering unit per bits.  Digital sensitivity values are listed in the Specification Table 5 on page 11 (NOTE:  You must use the value specific to the dynamic range of the device you have purchased).  For example, if you have purchased a ±300 deg/s, ±2 G H3-IMU, the corresponding equations for the X component would be:

value_x = raw_payload_value_x_gyro x 1.3733 x 10^{-2  deg /}s /bit

_{value_x = raw_payload_value_xaccel x 9.1553 x 10-5 G/bit}

where raw_payload_value_x is taken from the sample payload corresponding to the x-components of the gyro and accelerometer, respectively.  The resulting values have units of degrees/sec and G’s, respectively.

Although the sensor data is temperature compensated, a customer’s application may require the use of temperature information, therefore a temperature value obtained from each gyro is provided. The temperature data provided in the payload requires a different conversion process. The data is transmitted as signed (2’s complement) 16-bit integers.  The following function must be used for conversion of temperature sample values:

[Equation 2: result_deg_C = (raw_payload_value_x_temperature x digital_sensitivity) + 25]

where result is the converted value in degrees Celsius, raw_payload_value is the raw component-specific value from the payload in bits and the digital sensitivity is the temperature sensitivity expressed in degrees C per bit (digital sensitivities are listed on page 11 in the Specification table).

In the cases where a custom dynamic range has been ordered, the digital sensitivity can be found by the following equation:

Equation 3: digital_sensitivity = dynamic_range x 4.57764 x 10^-5

where digital sensitivity is expressed in engineering units per bit and dynamic range is the unipolar range for the specific sensor axis (e.g. ±0075 º/s then 75 º/s should be used for the dynamic range in Equation 3).

3.0         Mechanical

3.1           Dimensions

The H3-IMU is available in a custom package measuring 2.000 in. length × 0.1.110 in. width × 0.645 in. height.  Holes are located near the center of each side allowing #3-48 machine screws to be utilized to mount the IMU to a PCB or chassis. The use of two 1/16 in. alignment pins is also supported on each side of the lower housing. Figure 3 depicts the physical dimensions of the part and its features.

Figure 3 - Physical Dimensions

3.2           Coordinate System

The coordinate system for the Inertial Measurement Unit follows the right hand rule convention.  The sign convention for the accelerometers is configured to produce a positive signal when the IMU is accelerated in the opposite direction of the axis arrow. As an example, the Inertial Measurement Unit pictured in Figure 4 below (given the X and Y axis are parallel to the earth’s surface) will produce 0 gs for the X and Y axes and a positive 1 g for the Z-axis.  As a further example, if the Inertial Measurement Unit were moved backwards (left side of the page) the X-axis accelerometer would produce a positive output.  A counterclockwise rotation of the IMU about any of the depicted axis will produce a positive angular rate output for the corresponding axis.

Figure 4 - H3-IMU coordinate system, side view

4.0         Hardware

4.1           Connections

The H3 IMU’s output connector is a dual row metal shell 15-pin Nano connector produced by Omnetics that meets all MIL-DTL-32139 specifications.  The mating connector is a dual row Bi-Lobe Nano connector with a Omnetic part number of 28000-015.

4.2           Pin Function Description

The pin functions for the Inertial Measurement Unit and mating connector are listed in Table 2 below:

Figure 5 – Omnetics Bi-Lobe Nano interface connector A29100-015

Table 2 – H3 IMU Connector Interface Pin Functions.

4.3           RS422 Connection Description

The H3 IMU RS422 connection is factory configured to 921600 Baud.  The RS422 connection is configured as an 8-bit UART with one start bit, eight data bits, and one stop bit.  Data is sent from the H3 IMU via the YZ differential driver pair and should be terminated with a 120 ohm resistor.  The H3 IMU is not currently configured to receive data.

Figure 6– RS422 Full-duplex direct connection diagram

4.4           Digital Sensor Encode Pulse Output

The H3 IMU’s pin 5 output contains a digital pulse train that indicates timing for when each sensor signal is encoded. The rising edge of the pulse is when the encode begins while the falling edge indicates the completion of the encode for the associated signal. The time between encodes is 5 µs. Figure 7 below details the pulse waveform for pin 5.

Figure 7 - Digital Sensor Encode Output Waveform

Table 3 - H3 IMU Sensor Signal Encode Orders

5.0  Electrical Specifications and Options

5.1           Part Numbers

Table 4 - Standard Part Numbers

1. Custom Acceleration Ranges of ± 30g , ± 50g, ± 100g and ± 200g Also Available
2. Custom Angular Rate Ranges Available up to ± 5400 °/s

5.2           USB Data Acquisition (DAQ) Module Options

The USB DAQ is available to purchase with your IMU to facilitate simple data collection using a PC. The module converts the IMU RS422 output to USB signals and in the case of the USB-H-8.5UR model utilizes an internal charge pump to boost the USB 5 volt power up to 8.3 volts to power the IMU. Model number USB-H-8.5XR allows the use of an external power supply and has maximum voltage of 8.5 volts. Each USB DAQ model number in Table 5 below is compatible with the H3 IMU and is available for order.

Table 5 – USB DAQ Module Options

5.3           Specifications

Table 6 - Specifications

1.) Other bandwidth configurations are available upon request.

2.) Other configurations are available on a special order basis.  Contact sales for more information.

6.0         Terms, Conditions and Warranty

DEFINITION : As used herein: “Seller” means MEMSense, 2693D Commerce Road, Rapid City, SD 57702. “Buyer” means the party purchasing Product(s) from the Seller. “Product” means all articles, materials, work or services offered by the Seller and described in the accompanying quotation, acknowledgement, invoice, or other Seller form. “Order” means any purchase Order or contract issued by the Buyer for Products provided by the Seller.

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