Machine Guidance Sensors – An Overview (Part 1 of 2)

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

Created on Friday, 03 April 2009 03:34

Choosing and purchasing the right Machine Guidance system is an extremely important process (as well as expensive!) and you want to get it right. Part of the decision process when choosing your new Machine Guidance system is based on how well you appreciate the product and its capabilities as defined in the product specification. But be prepared for the minefield of jargon and data that the customer is expected to. I have been in this industry for over 20 years designing and using sensors, yet I still encounter confusing specifications. My intention is to clear up some of these issues and hopefully offer a clearer insight into the world of sensors and Machine Guidance. This article is intended to be unbiased as this is about understanding and not selling.

Why do we need sensors?

I will present my data based on an excavator example, this is the system with which I am most familiar and tends to use the greatest number of sensors.  The diagram below shows a typical excavator identifying the components that need to be monitored, please note that I have included the GPS as a sensor. Do not be confused by what you are told, the GPS is just another sensing element used by the system to assist in accurately position the machine tool to a known reference. The GPS gives position and heading, the other sensors give equipment position data.  The application software uses the data output from these sensors to accurately position the tool (e.g. bucket) within a 3D Digital Terrain Model. The information is displayed to the operator via a computer screen. I have no intention of discussing the GPS as it is not my field of expertise, suffice to say there are plenty of others better qualified to do so in future editions of Machine Control Online.

The equipment sensors monitor the position of the equipment to which they are attached. All of this data is used by the main application to calculate the tool position relative to a reference point on the machine – usually the slew or rotation centre. Any change in the sensor output data caused by machine movement will change the calculated tool position and displayed data.

Summary: Without the sensors we don’t know where we are.

How do we measure the equipment position…?

Each of the machine components can have its position measured by a range of different sensor types.

Extension sensor
An extension sensor monitors length or extension of a moving piece. This type of sensor will measure cylinder extension from which the relative positions of the machine equipment can be calculated. Extension monitoring can be achieved by using a reeling drum type sensor or a linear position sensor. Extension sensors can be used for boom, arm and bucket monitoring.

The three basic types of extension sensor are:
• Reeling drum sensor – monitors the payout of cable between the reeling drum base (fixed) and the cable end (attached to the moving piece). Voltage or a current output. Can provide relatively accurate information in relation to the cable extension, but is limited in its application due to the extending cable needing a clear path through which to move, has been used by some systems to monitor the cylinder extension, however, the cable is the weak link with this sensor and is easily damaged during normal excavation operations

• Linear position sensor – a range of different types available using different sensing technologies. They can be externally mounted e.g. the “cylinder type” or mounted inside the hydraulic cylinder itself – known as the “In cylinder linear position sensor”, usually a very accurate sensor.  Both of these have applications to monitor cylinder extension.

• Cylinder type sensor - are not usually suitable for the construction environment and tend to be employed in robotics applications. The “In cylinder linear position sensor” is best suited for factory fit as the cylinder needs to be removed from the machine, dismantled etc.

Note: These types of sensors are in the category of “direct drive” sensors. They are directly mechanically linked to the moving piece and for all intents and purposes do not suffer the response issues that “non-direct drive” sensors experience.

Summary: Extension sensors are vulnerable, expensive, difficult to install.

Flowmeter
A sensor that monitors the flowrate of hydraulic oil into the cylinder of the piece of equipment it is monitoring. The flowrate is proportionate to the cylinder from which position can be calculated. Flowmeters are usually very big and heavy pieces of kit as they need to cope with the extreme high pressures and flowrates associated with construction equipment. They are not absolute i.e. don’t know where they are, and need to have a known reference point from which to start measuring – they are usually referenced with the cylinder fully retracted or extended. One problem is that they will drift as the equipment is exercised due to losses in the system and need referencing on a regular basis. Their advantage is that they are installed away from the business end as the hydraulic pipework to which they interface is usually found on the on the main machine chassis, this means that it is an extremely rugged solution. Usually used in deepwater dredging applications for monitoring the bucket angle.

Please be aware that a flowmeter installation requires careful consideration and is not for the faint hearted. Ideally the installation should include additional items such as hydraulic filters and a flow by pass circuit for when the flowmeter is not in use. The by pass circuit should be engaged when the machine guidance system is not in use. Normally, machine guidance requires careful machine operation, however under other applications excavators will be worked very hard and dare I say it, not too carefully? This can cause huge pressures in the hydraulic system – believe me, you do not want the flowmeter to endure such pressures as damage can occur to flowmeter and machine.

Summary: Flowmeters are ideal for monitoring equipment that will be subject to environmental extremes.

Rotation / Slew sensors
Rotation or slew sensors 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, which provides heading relative to the earth’s magnetic field. The large ferrous object to which it is attached seriously affects a popular option, but its accuracy. Movement of the machine equipment, machinery electrics (generators) and passing vehicles will affect the sensor output. They are easy to install, but flawed due to the above reasons.

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. It is however a very easy installation.

Summary: only use this sensor if absolutely necessary (a 3D system does not require this device as heading / rotation is derived from the GPS data) and then choose one to suit the application e.g. if accuracy is not an issue go for digital compass, if accuracy is an issue and you are not working indoors go for a GPS compass. Important: If operating in 2D with a dual slope reference laser then it is essential to have a rotation sensor.

[End of Part One]
I think we have reached a natural ending for our overview. I hope this has proven a helpful taster into what is a complex, confusing, but never boring subject. The importance of the sensors in Machine Guidance cannot be emphasized enough and it is critical that the end user has the information he needs to make  a considered decision. Next time we will discover the “work horse” of machine guidance – the angle sensor -  and what you need to look for when assessing its performance. If you have any questions or comments regarding this article please feel free to contact me via MCO.

...to be continued

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