New Technology - GPS Compass Sensor

Written by Nigel Adams – Engineering Director, Prolec Ltd. Poole, UK

Created on Monday, 02 November 2009 14:19

New Technology - GPS Compass Sensor

In my first article for Machine Control Online I explained in some detail the various sensor options available for excavator machine guidance solutions.

One area that was covered was that of Rotation / Slew sensors and to recap:

A quick word on rotation or slew sensors, these are usually used in 2D machine guidance systems to monitor the rotation (also known as slew) of the excavator cab or turret. They are available as:

a)    A direct drive type, which consists of an encoder device mounted to wheel, which is spring, loaded against the cab slew base. As the cab rotates the wheel turns and the encoder outputs corresponding data. Whilst being very accurate they are clumsy and difficult to install on all but large excavators.

b)    A digital compass that provides heading relative to the earth’s magnetic field. A popular option, but its accuracy can be affected by the large ferrous object to which it is attached. Movement of the machine equipment, machinery electrics (generators) and passing vehicles have the potential to affect the sensor output depending upon its installation location. They are popular and easy to install, but may be affected in performance due to the above reasons, however these negative effects on performance can be compensated for during calibration.

c)    A gyro sensor, using solid state gyro technology rotational movement can be measured, but a gyro drifts and becomes inaccurate unless you are willing to pay the +$10K for a device (which is usually fitted on underwater ROVs).

d)    A GPS heading sensor will give accurate heading information, but will not work indoors and may be affected in its performance due to any large nearby obstructions (tall buildings, densely built up areas etc). It is however a very easy installation.

Summary:
If operating in 2D with a dual slope reference laser then it is essential to have a rotation sensor. If high accuracy is less of an issue then go for the digital compass solution, if high levels of accuracy are an issue and you are not working indoors or in densely built up areas then go for a GPS compass.

With 3D applications only use the Rotation / Slew sensor if absolutely necessary - a twin RTK 3D system does not require this device as heading / rotation is derived from the GPS data. Most 3D applications require the high accuracy solution.

Prolec have had a great deal of experience in installing direct drive technology (a) and have on rare occasions used gyro sensing (c). However we have always been on the lookout for incorporating new technology as a means of finding an improved solution to this sensing problem and this led us down the path of using independent GPS heading solutions (d). We identified the products available from Hemisphere GPS as being the most suitable for our applications and implemented both their Vector V100 and VS100 products within our 2D and 3D product ranges.

Using this technology makes the installation a lot easier than fitting a direct drive solution which can add more than half a day to the installation time and also allowed an easy upgrade path to 3D – remember, if you have an accurate, reliable heading source, then you only need to add a single GPS position sensor to upgrade the system to a full real time 3D solution and has clear, obvious economic benefits. Whilst the technical benefits were very clear and measurable, the economic benefits weren’t as great as they could have been, this being because we were providing a 3rd party solution within our product portfolio – something needed to be done.

The solution appeared in the form of the Hemisphere GPS LV100 OEM board. This product maintained the high precision we had come to expect from Hemisphere GPS, but as it was available as a single pcb it was a more compact and economical purchase and allowed Prolec to package it in a manner that complemented the existing Prolec product range.

Hemisphere describe the GPS LV100 as follows:
Based on Hemisphere GPS' patented Crescent Vector™ technology, the LV100 integrates two GPS antennas, an NMEA 2000 communication processor, a single axis gyro, a tilt sensor and a power supply into a single system.  The dual antennas allow for ease of integration into your system and provides for precise heading and GPS sub meter position accuracy even while sitting stationary.  The gyro and tilt sensor improve system performance and provide backup heading information if the GPS-based heading is ever lost.  The Crescent technology provides more accurate code phase measurement and improved multipath mitigation resulting in excellent accuracy and stability.

Prolec is now in a position to announce the release of their GPS Compass Sensor and have now started shipping this sensor with its Digmaster systems. As with all sensors there are pros and cons, and yes there are some limitations in using this technology, but no more so than with any other GPS or GNSS solution. The main benefits it gives are in its accuracy and ease of use, but other things such as a gyro compensation to enable the sensor to continue to give heading information during temporary loss of GPS are of a great bonus too.










The sensor also includes Prolec’s well proven angle sensing technology providing a dual axis Pitch and Roll output as well the GPS heading data. The sensor integrates with the Prolec CAN bus keeping the installation common and simple.

The installation of the sensor is easy and simple, and the sensor is located using a purpose built mounting plate. The sensor is held firmly in place to the mounting plate by 3 very strong rare earth magnets. This allows the sensor to be easily removed from the machine if required. The sensor itself can be mounted at the top of a mast on the machine counterweight or on the machine roof.

Specification for the sensor:

* 2D GPS heading accuracy better than 0.75 degree rms
* Integrated gyro and tilt sensor deliver fast start-up times and provide heading updates during temporary loss of GPS
* Fast heading and positioning output rates up to 20 Hz
* Pitch and Roll output of 0.2? accuracy
* CAN2.0B communication
* Operating Voltage: 10V to 30V
* Operating Temperature: -20?C to +60?C
* Environment: Sealed to IP67

For more information visit:
www.prolec.co.uk
www.hemispheregps.com