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

Written by Nigel Adams - Engineering Director - Prolec LTD - Poole, UK

Created on Saturday, 02 May 2009 23:13

Angle Sensor
Angle sensors come in many guises, depending upon technology and machine interface. They are usually mounted on the side of the moving piece and give an angle output relative to the angle of the equipment. From this information it is then easy to derive the relative positions of the equipment. Angle sensors can monitor the boom, arm, bucket, tilting bucket, pitch and roll of the machine; they are the most versatile and widely used sensor option.

The two main types of angle sensors to be aware of are:
Direct drive  – this has a mechanical link physically moving the measuring device. Gives an instant response and will not suffer the effects of over swing and oscillation of a pendulum driven device. This variant of angle sensor does require much greater installation time and care over the positioning of the sensor and its drive link, it is usually the more expensive option.

Non direct-drive – no mechanical link. The sensing element within this type angle sensor may be something as simple as a potentiometer or as high tech as the latest accelerometer technology, however they are relying on one reference and driving medium – gravity. It is gravity, which is moving the pendulum of the sensor be it a mechanically constructed device within the sensor housing or that created within the MEMS (Micro Electro Mechanical System) of the accelerometer device.  Unlike the direct drive sensor this variant is relatively easy to install and not too much care over the positioning of the sensor is required (depending upon the manufacturer).

The most popular technologies used for these sensors follow:

Note on abbreviations:
A Absolute:  Gives an immediate output that is correct and therefore does not require additional referencing.
NA Non – Absolute: Requires referencing before its output can be deemed correct.
DD Direct drive
NDD Non direct-drive

1) Liquid electrolyte A, NDD
This has a conductive fluid in a capsule; the fluid moves its position in the capsule depending upon angle. Can be highly accurate, but usually has a very limited operating range. Can suffer from inaccuracies when used in a high vibration environment or temperature extremes. Mostly used for tilt sensing applications where only a small angle measuring range is required.

2) Potentiometer A, DD, NDD
Pendulum driving the shaft of a resistive potentiometer - Not very accurate, requires some form of damping (usually oil) to stop excessive over swing and oscillation. The limited operating range is not a full 360º-output range. This represents older technology and not as popular.

3) Encoder A, NA
Pendulum driving the shaft of an encoder device (a digital wheel which gives a coded output depending upon its position) - Can be very accurate, requires some form of damping (usually magnetic) to stop excessive over swing and oscillation. Full 360º range. The encoder device is not usually affected by temperature (other than effects on any mechanically moving components or magnetic damping which may hamper response).

Note: The non-absolute output version requires movement through a reference point.

4) Accelerometer A, NDD
Using the latest technologies to bring you a truly amazing device, which is small, lightweight and can be extremely accurate. Usually fabricated out of silicon this will give an absolute output if calibrated correctly over a full 360º range. As they are “solid state” devices they lend themselves more readily to signal processing to characterise an accurate sensor output and the design engineer can tailor the response to minimise the effects of electrical noise and mechanical vibration. Do not confuse with dynamic accelerometers used for measuring ummm acceleration – dynamic types cannot measure the static gravity referenced signal that we are using for measuring the angle. However, angle accelerometers will suffer noise from the dynamics of the machine to which they are mounted, but these effects will be reduced if the designer has employed suitable filtering and noise rejection techniques.

Summary: Angle sensors are an extremely good compromise in having a device that delivers accuracy, ease of installation (certainly the non direct drive variants), and cost.

Outputs
Sensor outputs come in wide range of types and clearly are (or should be) designed to match the machine guidance system to which they are connected.

Output types fall into two categories – analogue and digital. Analogue output devices are now regarded as old fashioned, but can still be found in use. Digital outputs are more common and with the advent of lo power microprocessors and high speed communication protocols mean that the outputs allow for a range of additional data other than sensor information to be monitored by the machine guidance system.

Analogue outputs
These will be either voltage or current.  They are (usually) not restricted by resolution issues (see later), but have limitations in their application. Voltage outputs that are single ended e.g. 0 to 5V output are more tolerant to interference than those with a differential milliviolt output (such as that found with a strain gauge type sensing device). Voltage outputs will suffer from electrical noise and will have accuracy affected by the resistance of the cable to the main system – restricted to short cable runs and low bandwidth systems. Current output (usually 4-20mA) are far more tolerant of electrical noise and do not suffer from loss of accuracy and is a recommended analogue solution for long cable runs.

Summary: Low tech and you are unlikely to find these within your machine guidance system, but at least you now know their limitations.

Digital Output
This is the output in a digital (on/off) form that is not affected by any output level changes due to interference (within reason) – unlike their analogue counterparts. Digital outputs can be transmitted over long cable runs with no effect to the sensor data they are transmitting.

Frequency Output
The signal frequency is proportionate to the sensor data.  Whilst being tolerant to electrical interference the frequency can wander giving misleading data and the bandwidth i.e. quantity of data is limited by the output frequency which is usually quite low.

Pulse Width Modulation Output
The frequency of the signal remain the same, it is the mark space ratio that changes in relation to the sensor data. Does not suffer as much from “frequency wandering” as it is the ratio between the mark and space being measured however the bandwidth i.e. quantity of data is limited by the output frequency which is usually quite low.

Serial Data Output
Data is encoded into a serial format and transmitted to a receiver (within the machine guidance computer) which decodes the format to retrieve the sensor data. Serial data has undergone many standards, but the most commonly used now for Machine Guidance is CAN Bus. This encodes the serial data and gives it unique identifier. This allows the sensor to be recognised on the CAN Bus so all moving equipment can have its own designated sensor and conditions such as data loss, sensor failure, Bus failure etc. can be recognised. It is an extremely flexible method of passing data and allows other sensor specific data to be passed as well such as sensor health, temperature etc. to be transmitted. Serial data can be corrupted by heavy electrical interference, but this will be recognised by the receiver and report such conditions to the user. The method is therefore highly reliable. The CAN Bus has a very high data rate and for machine guidance applications does not suffer from any bandwidth limitations.

Note : CAN Bus communication between different Machine Guidance manufacturers is not possible. It is not possible to mix and match sensors etc. from different manufacturers because we all like to have our systems working independently to our own protocols giving us control over our own development and system operation. There are standards that exist to allow compatibility between manufacturers, but they are not widely employed in Machine Guidance, and even those that do use them, tend to use their own interpretation of the standard.

More recent machine guidance systems are using telemetry (radio comms) to communicate sensor data to the main computing unit. This innovative method is extremely welcome, but is still in its infancy and does suffer the drawback of not being able to work if submerged and requiring recharging of the sensor batteries. However it is an exciting new development in machine guidance sensors and can only improve with time.

Summary: Best to go for a modern output format such as CAN Bus, it highly  reliable, can be transmitted over long distances and has been designed for the automotive and industrial environment, the data will be correct, if interference corrupts the data it can be recognised and error conditions raised.

Understanding the spec sheet ….

Operating range
What is the angular operating range of the sensor? Mechanical operating range refers to the amount the sensor can be physically rotated, electrical operating range refers to the sensing range of the device. If you need to monitor something moving through 200º then a device with an operating range of 100º will not do.

Accuracy
What is the sensor accuracy, look or ask for the accuracy quoted in degrees e.g. 0.2º as it’s then easy to do a quick calculation. The horizontal and vertical measuring accuracy of your system will not be any better than the following reference table :

Sensor accuracy    measuring accuracy / ft radius

0.1º            0.0017ft            
0.2º            0.0035ft
0.3º            0.0052ft
0.4º            0.007ft
0.5º            0.0087ft

Therefore a machine with a 30ft reach and a 0.3º accurate sensor will not be better than 0.0052 x 30ft = 0.156ft accuracy, ignore anyone who tells you it is better, its just not possible and is governed by the laws of basic trigonometry as proven many years ago by the Greeks. Note that the spec may give accuracy of ± Xº, therefore you’re accuracy in the previous example will be within the tolerance range of ±0.156ft

Note: This example hasn’t taken into account the additional errors from monitoring the machine pitch and roll and any positioning system (GNSS) which has its own inherent errors.

Resolution
Not to be confused with accuracy. It is sometimes included in a spec. sheet to make the sensor look better than it actually is. Resolution is the level to which a signal can be measured (resolved). An analogue sensor output could be measured to 0.0000001º and beyond (until you reach the limits of physics) however its accuracy may only be 0.5º due to the mechanical and fabrication constraints. Don’t be fooled.

Summary: Always look for the accuracy and ignore the resolution.

Electrical characteristics
Is the operating voltage of the sensor correct for the machine? Is the sensor protected against electrical transients, power surges? Does it have reverse polarity and load dump protection? Has it been tested for EMC (Electro Magnetic Compatibility – this is protection against strong electrical and magnetic interference which is quite common in the environment in which they are used). All of these issues should not be an issue as the majority of sensors will be part of an overall machine guidance system, but it doesn’t hurt to ask.

Bandwidth
Some specification sheets quote additional data such as bandwidth. This may refer to the update rate of the serial output or the response of the sensor. If it refers to the update rate of the serial data then the higher the value the more data will be transmitted – this is mostly a good thing. If it refers to the sensor response then the higher the value the more quickly a sensor will respond. This, however, is not necessarily a good thing as increased bandwidth response means an increase in noise levels and potential rogue sensor readings. Best to ignore this data and use the system check (referred to in the conclusions) to see if the system performs, as you want it in real time without any adverse effects.

Temperature
Make sure that the storage and operating temperatures suit your working environment. Most devices will operate beyond their operating temperature; however under these conditions they cannot be guaranteed to operate within their given specification.

Environmental Protection
The sensor is going to be used in a harsh environment. It is attached to a mechanical device that will suffer vibration, shock and knocks. It will be used outside in all weathers and temperature extremes. It will be covered in mud, slime, dirt, oil and be submerged; you need to make sure that the sensor can withstand all of this. Confirm that it has been tested against any of the above. When asking about its waterproof qualities ask for the IP rating. This is a rating which defines protection against dust and water and consists of a 2 digit number with each digit giving a protection rating the chart below explains :

Table explaining IP numbering system:
e.g. IP67 = dust tight and protection from immersion in water (this does not mean permanent immersion and is only good up to 1m depth)

If a sensor is being quoted as IP68 ask to what depth and duration, just saying IP68 doesn’t tell you if it is good for 20ft or 200ft. Make sure the sensor meets your needs.

In Conclusion:
Ultimately when sourcing suitable sensor / machine guidance solutions the best method of evaluation after all the appropriate questions have been asked is to try it. Ask to trial a system and watch for the following things:

1    Does the screen respond in real time, i.e. when I move the machine does the display (or data) follow the action instantly or is it delayed or jerky?

2    Likewise is the screen data too jerky / noisy suggesting that the sensors are too responsive and it will be very difficult to use. If so can these effects be reduced without any detrimental effect to accuracy and overall system response?

3    Do some measurement tests with a number of fixed reference points in the ground, make sure that the sensors are working within spec as indicated in the accuracy section of this article. If the measurement results aren’t too favourable then machine guidance system and GNSS positioning system independently to evaluate where any errors may be present.

4    Exercise the machine through its entire working envelope to make sure there aren’t any dead spots in the sensors.

5    And finally if in doubt ask, the vendor should be capable of answering any of the points I have highlighted to reassure you that you are being looked after by someone who knows what they are doing.

Now, before any of you engineers out there having read this article decide to reach for the keyboard to tell me I don’t know what I am talking about and am a disgrace to my profession please take note – this isn’t for you – its for the guy in the field, the one who has to use these systems and is tired of inconsistent engineering and sales psycho babble and just wants to get on with the job using a system that he can trust. So put yourself in his shoes and ask yourself if we provide enough information to the end user in a format that he can understand and be confident with?