Product Focus Mobile Elevating Work Platforms and the ANSI A92.20 Standard

Thursday 7 February 2019

SUMMARY

In this article, Curtiss-Wright Industrial Group’s David Taylor looks at the new ANSI A92.20 standard for access equipment and its implications for the fitment of additional or alternative sensors to Mobile Elevating Work Platforms (MEWPs), including scissor- and boom-lifts.

BACKGROUND

The new ANSI A92.20 standard for access equipment is in draft and will likely become effective late-2018/early-2019. The standard, which covers design aspects of access equipment, will replace two existing standards – A92.5 and A92.6 for boom- and scissor-lifts respectively.

In addition to providing design and safety guidance, A92.20 defines the term Mobile Elevating Work Platform (MEWP) to replace Aerial Work Platform (AWP), and then further defines by type. This document is primarily aimed at Type 3 MEWPs, which allow travel with the platform in an elevated position. Additionally, A92.20 defines Group A, including scissor-lifts, and Group B, boom-lifts.

Much of the content of ANSI A92.20 harmonizes with the European equivalent standard – EN 280:2013+A1:2015 – which itself refers to EN ISO 13849-1:2008 for guidance on the implementation of safety functions. With regards to safety function implementation, the new ANSI standard also directly references EN ISO 13849-1.

From a fitment of additional or alternative sensors perspective, there are two elements of ANSI A92.20 which should be referenced – 4.3.2 Chassis Inclination and 4.4.1.2 Load-sensing System.

CHASSIS INCLINATION

ANSI A92.20 requires every MEWP to be fitted with a means of detecting that the chassis is not tilted beyond safe working limits and, if it is detected, then further operation of the MEWP must be prevented. A similar requirement existed in both A92.5 and A92.6, but this only required a warning to be issued, rather than prevention of operation.

Because machine operation now has to be prevented, it is likely that higher-performing tilt sensors will be required. For example, traditional types have been subject to the effects of vibration, which has been negated by damping the signal from sensor to control unit. While this method has provided a solution, it can mean significant delays in the measurement of actual tilt angle.

Sensors that can measure both tilt angle and vibration can filter out unwanted effects at source and therefore lead to a quicker response to actual tilt angle, regardless of whether vibration is present. This performance can be provided by Inertial Measure Unit (IMU) based sensors.

Ansi-vts-(1).JPGCurtiss-Wright’s VTS2021 dual-axis, vibration-tolerant tilt sensor uses the latest IMU technology and fast-acting software algorithms to filter out disturbances caused by vibration and vehicle motion. This provides output stability without the measurement delays usually associated with heavily-damped, traditional sensing methods.

The VTS2021 feeds output data over CANbus using the SAE J1939 protocol, with each measurement axis having two sensing elements, which are constantly compared to ensure correct operation. If an error is detected, its condition is communicated to the host electronics to ensure a safe operating situation is assumed. Additionally, each output signal is calibrated to account for thermal drift, ensuring accuracy over the operating temperature range.

LOAD SENSING

This requirement relates to MEWP operation and the amount of physical load (or weight) on the platform. Essentially, the maximum permissible platform load will vary and depend on platform position. Sensors that detect both position and load are therefore required.

Platform position measurement techniques will vary between scissor- and boom-lifts. If a scissor-lift is considered, then platform elevation can be measured in two ways – a rotary sensor fitted to a pivot point of the scissor mechanism, or a tilt sensor fitted to a scissor arm.

Ansi-reach_lift.JPGIn the case of a pivot point of the scissor mechanism, two-part, no-contact sensors may be preferable as the absence of any operating shaft means such a piece cannot be broken. Two-part sensors generally comprise of a magnet and a sensor body, with the relationship between the magnet’s rotation and a Hall-effect device in the body being used to determine an angular output. Measurement of platform position on a boom-lift is more complex, as the platform moves in multiple axes. It will therefore be necessary to fit multiple sensors and consider the output from each to determine true platform position.

Load measurement techniques may also differ between MEWP types. Because a scissor-lift only provides vertical movement of the platform, a pressure sensor fitted in the hydraulic lift system is an effective way to determine weight on the platform. The more complex motion related to platform position on a boom-lift means a load cell fitted to the platform itself is usually required.

Curtiss-Wright Industrial’s NRH27x series of ‘no-contact’ rotary position sensors offer an optimal combination of performance, safety and cost for MEWP OEMs designing vehicles and control systems.

Ansi-nrh27x.JPGIts latest offering from the range is the NRH27C, a no-contact rotary position sensor offering a CANbus J1939 output making it ideal for OEMs of on/off-highway vehicles destined for challenging environments, and as a cost-effective solution for medium volume applications where a wide-ranging options or degree of customization is required.

The NRH27C’s two, physically-independent, Hall-effect sensing signals are sent separately with the CAN message, allowing for error checking of positional data and meaning that high-performing, safety-critical applications can be addressed. An on-board diagnostic function means predefined error messages can be sent to define the present state of the sensor. The versatile, factory-programmable electronics can also be set to different Baud, Node ID and Frame rates.

For load-sensing applications where an alternative to position sensors is required, Curtiss-Wright Industrial also offers numerous models and configurations of pressure sensors. These include stainless-steel technology versions (operating from 0-4000 bar) that use thin-film polysilicon resistors applied to a stainless-steel diaphragm; and silicon versions (from 0.1-40 bar) using a silicon diaphragm into which pressure-dependent resistors have been diffused.

Piezo-resistive sensing technology is inherently accurate and both stainless-steel and silicon versions are integrated into robust stainless-steel housings with a choice of pressure port and electrical connection options, enabling them to be adapted into numerous configurations to suit specific needs.

Pressure modes include absolute, gauge, differential and sealed reference options, with measurement accuracies nominally 0.5% at room temperature, with an option to 0.1% in the high-precision range. ATEX/EX approved versions are also available for use in Zone 0 and 1 hazardous areas.

SAFETY FUNCTION PERFORMANCE REQUIREMENTS

Ansi-sky.JPGANSI A92.20 gives guidance to the safety function performance requirements necessary for Chassis Inclination and Load Sensing. The techniques used to determine the performance levels are based on assessment of the severity of risks resulting from failures of a safety function, which are defined within EN ISO 13849-1:2008.

Performance Level (PL) quantification is defined as a scale of a to e, with PL a being the least severe and PL e the most. Within EN ISO 13849-1:2008, there is then guidance relating to the hardware/software architectures needed to implement such performance levels; and the table below illustrates how ANSI A92.90 defines performance levels for the two previously described safety functions.

Safety Function Performance Level (per EN ISO 13849-1)
Chassis Inclination c
Load-sensing System d

 

A performance level is implemented by a combination of: architecture (physical and logical structure, e.g. hardware and software, and referred to as a Category), MTTFD (Mean Time to Dangerous Failure of the hardware and software making up the safety function) and DCavg (average Diagnostics Coverage within the safety function) per the table below.

Ansi-Graph.JPG

Simplifying the table – in the context of practically-achievable MTTFD and DCavg from widely-available and commercially-viable components – it is reasonable to assume an easier implementation of PL d can be achieved with a Category 3 architecture, whereas PL c requires Category 2. Essentially Category 2 is a single-channel architecture with some associated monitoring of function, with Category 3 being a dual-channel architecture with constant cross-checking of each. Selection of sensor type – single- or dual-channel – should therefore reflect the required Category.

THE SOLUTIONS

Curtiss-Wright Industrial Group designs and manufactures components and sub-systems for mobile elevating work platforms, variable lifts and access equipment, including vertical mast lifts; indoor scissor-lifts; outdoor rough-terrain scissor-lifts; articulating and telescopic booms.

Its product portfolio addresses the electrical system requirements of MEWPs, including joysticks, sensors and motor controllers. Many of its products are available off-the-shelf and have been designed to provide an optimal combination of performance and cost that the MEWP industry demands; while CANbus options of both CANopen and J1939 ensure simple and reliable interconnection. In addition to its comprehensive standard range, custom solutions can also be provided to meet customer’s exact requirements.

Curtiss-Wright’s capabilities also extend beyond simple component supply and includes custom Human-Machine Interface Consoles (HMIC) for all aspects of MEWP control.

Further information on Curtiss-Wright Industrial Group’s range of MEWP sensors can be found on this website.

ABOUT CURTISS-WRIGHT CORPORATION

Curtiss-Wright Corporation is a global innovative company that delivers highly engineered, critical function products and services to the commercial, industrial, defense and energy markets. Building on the heritage of Glenn Curtiss and the Wright brothers, Curtiss-Wright has a long tradition of providing reliable solutions through trusted customer relationships. The company employs approximately 8,600 people worldwide. For more information, visit www.curtisswright.com.

 


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