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EMDCSS

The GDS Electro-Mechanical Dynamic Cyclic Simple Shear System Hardware Handbook

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Introduction

This handbook provides an outline of the design and implementation of the Electro-Mechanical Dynamic Cyclic Simple Shear (EMDCSS) system.  For additional details on system software please refer to the separate GDSLAB manual.

Suggested System Layout

Overview of Components

The EMDCSS system consists of the following major subsystems:

• Axial and Horizontal Electro-Mechanical Actuator Units

• Axial and horizontal motor drive units

• GDS Advanced Digital Control System V2 (AdvDCS V2)

• Long and short-range vertical displacement measurements

• Long and short-range horizontal displacement measurements

• Vertical force measurement

• Primary and secondary horizontal force measurement

• Optional high accuracy shear LVDT upgrade

• Optional pore pressure measurement

Dynamic control unit

The dynamic control unit inside the AdvDCS V2 control box interfaces to the PC via a USB connection. It provides closed-loop feedback for both the vertical and horizontal axis, eight channels of analogue to digital conversion, digital inputs and outputs and a quadrature counter. It also features a compliance estimation unit that adapts the control responsiveness on the softness of the sample.

The dynamic control unit runs with a control loop frequency of 1 kHz (1000 control loops per second). The compliance estimation unit runs at 5 kHz.

Setting up the System

Setting up the Computer System

A high-quality, high-speed PC is needed for overall system functionality. We recommend an i5 processor or better (or AMD equivalent) at least 8GB RAM and running a fully updated version of Windows 10 or later.

The AdvDCS unit is connected to the computer using a USB cable. Acquired data is stored locally on the AdvDCS and passed to the controlling computer by a separate communications process.

Please follow the instructions referred in the section “Calamari Drivers” in the helpsheet below to install the software.

Setting up the Hardware Components

The hardware set up comprises of an LVDT Signal Conditioning board and a Digital Control System unit (AdvDCS V2). 

The LVDT Signal Conditioning Unit is used to convert signals from the LVDT transducers used on the EMDCSS, from a moderated signal to read as a voltage. Therefore the transducers get plugged straight into this unit and then a connecting DIN-to-LEMO cable is connected to the corresponding channels on the AdvDCS box. 

The EMDCSS has two connections to the DCS control box.

• A digital CAN cable with an encased RJ45 connector on the termination connecting to the EMDCSS and a normal RJ45 connected on the termination connecting to the AdvDCS box.

• An analogue 9 pin to 9 pin connector for the axial quadrature signal.

The remaining transducers from the EMDCSS are plugged into their corresponding channels on the AdvDCS unit. The Diagram below represents all the system connections.

Setting up the Transducer Arrangement

Under normal setup, there are two displacement transducers for the vertical axis and one displacement transducer for the horizontal axis. The vertical axis has a P & G type transducer for long-range coarse control and an LVDT type transducer for its short-range fine control.  These have a range of 125mm and ±2.5mm respectively.

An encoder reading is also available for the vertical axis, but because of the error induced by the deformation of the components on the vertical axis this reading should not be used for test purposes.

The Horizontal axis has a short range LVDT transducer with a range of ±10mm as well as a transducer built into the encoder of the actuator.

As an optional upgrade a high accuracy shear LVDT is available. This transducer will be attached to the sample pedestal, with a spring-loaded armature resting on the side of the top cap (please refer to the helpsheet ‘Simple Shear Sample Preparation’ attached in the ‘setting up a sample’ section).

The long-range axial displacement transducer is plugged into the yellow channel on the AdvDCS box. The axial and horizontal LVDTs are connected to the matching serial number channels on LVDT conditioning box. Then the DIN-to-LEMO cables are connected to the green and grey channels (channel #5 and channel #6) on the AdvDCS box for the axial and horizontal LVDTs, respectively.

The axial load cell is plugged into the black channel (channel #0) and the primary and secondary shear load cells are plugged into the brown and red channels respectively (channel #1 and channel #2). 

Make everything tidy

Check all cables are neatly arranged.  Plumb any required fittings into the pedestal and topcap.  Put the required pedestal in place. 

Safety Precautions

When working around the EMDCSS unit, be aware that it has moving parts, even though the majority are encased. 

There is an emergency stop button on the front side of the machine, which when pressed stops the machine immediately. A system error message is then displayed in the GDSLab device status tab.

Initial Power Up

The 120/240 Volts A/C, 50/60Hz, connector is plugged into the applicable socket on the side of the EMDCSS.

Before powering up the system, make sure that the emergency stop button is pressed. Power on the computer, then power on the DCS.

Initial Software Set-up and Transducer Check

Run GDSLAB. When GDSLAB first runs, you should select the relevant ini file for the test station from the list of available ini files which will have been provided by GDS in the GDSLab CD or USB stick. Please refer to GDSLAB manual for detailed instructions. After selecting the ini file, a window will pop up showing the connection of DCS box to the PC.

Select ‘OK’ to confirm the connection of AdvDCS box and proceed to the ‘Object Display’ – Within the Object Display, you must be very careful to ensure that the following parameters are correctly and accurately set.

• Load cells (axial and shear) range and sensitivity

• Pore pressure range and sensitivity

• Axial displacement range and sensitivity

• Shear displacement range and sensitivity

Please refer to helpsheet 216 ‘ADVDCSv2 Device Interface’ (attached above) for checking the calibration details of transducer.

Once these have been set up you can proceed to perform ‘sanity checks’ on the transducers. Left click on the digital image of DCS box to open the transducer window. Live readings of the transducers can be seen in this window.

Load Cell Check

Displacement Transducer Check

To check the displacement transducer, the safest way to do is to use the motor control as the motors communicate with the acquisition system. To do so, release the emergency stop button which will energize the motors. Since the abort button was pressed earlier, there will be errors in the memory and the transducer readings will appear red in the GDSLab object display. Click on the read button twice in the GDSLab object display to remove errors in the memory and the transducer reading should appear blue. The machine is now ready to be moved.

To perform sanity checks on the axial displacement transducers, in the GDSLAB object display, click on the control parameter tab to target axial displacement (encoder). Set target displacements and see if the top cap platen is moving correctly. Make small movements at first to conform the distance moved is the same as the target value (eg move 2mm and measure with a ruler). Now drop the LVDT armature and set it at the middle marked position with the transducer body (zero reading point, see pic to the left below). Tighten the screw on the armature block (see pic to the right below) to hold the armature in place. Target 2mm axial displacement (encoder) in the GDSLab and check if all the axial displacement transducers move by 2mm.

For the horizontal transducer sanity checks, target a small displacement (e.g. 2mm) on the horizontal displacement (encoder) and check that both horizontal transducers move by the same value. Conform the distance moved is the same as the target value (measure with a ruler).

Top Cap and Pedestal Alignment Check

The short range horizontal LVDT should be as close to zero as possible when the top cap and pedestal are exactly in line. This is important so that when the machine is loading the sample, the sample is loaded normal to the axis and not at an oblique angle. 

To do so, put the top cap and pedestal (without the porous discs) in the EMDCSS and tighten the bolts. From the GDSLab object display, use the CP command to bring the top cap down to the maximum positive position using the Axial Encoder option. Also, target zero position on the horizontal LVDT. Check that the top cap and pedestal are now align with each other.

When setting the transducer in place, it is near impossible to get it exactly to 0, therefore a slight offset is applied. A soft zero offset can be applied in the transducer window. This offset will remain fixed unless the user redefines it. It is also important to check this offset every so often to ensure that this offset hasn’t wavered, and redefine it if necessary.

Before shipping the EMDCSS machine to you, the horizontal LVDT is set close to 0 when the top cap and pedestal are aligned. If the offset value is high, then please follow the instructions in the ‘LVDT Transducer Setup’ section to bring the offset close to 0.

In addition to this, the vertical short range displacement transducer needs to be set up correctly, in terms of specimen height etc.  For example, when using a specimen height of 20mm, when the sampled is docked the transducer needs to be near the top of its range.  The vertical LVDT transducer has a range of +/- 2.5 mm, therefore the transducer should read approximately -1.5.  This is in order for the transducer to be able to read the full displacement of the sample, without going over its reading threshold. 

Checking Machine Functionality

The EMDCSS machine should stop automatically when it reaches the upper or lower limit. This can be checked by targeting values beyond the upper travel limit (eg -60mm). A warning will be displayed on the Object Display status bar when this occurs.  If the machine does not stop automatically or makes any loud noises when reaching the limits, push the Emergency Stop button immediately and contact GDS for further instructions.  Carry out the same procedure for the bottom limit switch (+60mm). Then repeat the same procedure for the shear limit switches.

Double check the load-cell calibration. If you have a (small) proving ring or known working load cell, put that between the topcap and pedestal, and compare its reading with the reading shown in GDSLab for the system load cells. If you do not have such a convenient load measuring system then adopt some ad hoc means. For example, you could use the "rubber bung" dummy test specimen described below. You know roughly your own weight. Press your whole weight down onto the dummy test specimen and observe the deformation or ask a colleague to observe it for you.  You will now have a rough but very convenient indicator of load. 

Machine functionality checks also involve running a few tests (explained below) on a dummy sample. Please refer to the GDSLab handbook for a step by step guide on setting up a test stage (Chapter 3) and selecting the relevant test modules (Chapter 6) for carrying out the tests.

Running the First Test

Install a dummy test specimen.  This could be a rubber sample for instance.  Place the rubber sample in the cell.  Perform a constant rate of strain and constant rate of shear test (using the Advanced Shear test module).  Choose parameters appropriate to the dummy test specimen. Use a strain rate of 50mm per hour and termination conditions of 2mm for each axis.  Plot the results and check for noise levels.  On the screen, plot load cell against displacement for each axis.  From the plot, calculate the stiffness of the dummy test specimen in kN/mm.  Please refer to the article below for the importance of stiffness estimate in dynamic load controlled tests.

Running the First Dynamic Test

Select the Dynamic Shear module and choose axial displacement control. Select 0.1Hz at +/- 0.2mm axially as well as in the shear plane (if the dummy test specimen will stand it).  Choose 100 data points per cycle, 5 cycles on, 0 cycles off, terminate after 5 cycles.

Running Other Tests

Going back to the GDSLAB object display, you can check that the load control is working by setting a small change in target load. Observe the deformation of the dummy test specimen. From your ad-hoc calibration of the stiffness of the dummy test specimen, check that the observed deformations are about right.On the screen, note the resolution (i.e. variation) of control of axial force (load).  Normally, you should expect better than 1% of full range at loads greater than approximately 1/20th of full range.

If all is OK you can then start testing with real soil.

User Adjustments

So long as the system hardware remains unchanged there will be no user adjustments required on a day to day basis. If transducers are changed it is very important to update the calibration details for the data acquisition system. The main user software adjustment between test is the selection of stiffness estimate of the test specimen (explained above).

Setting Up a Sample

Test Procedure

The typical testing procedure to be carried out on an EMDCSS is consolidation stage, followed by a period of dynamic shearing and then a monoclinal shearing, to failure.  This will essentially give the user the required data.

Please refer to helpsheet 216 ‘ADVDCSv2 User Interface’ for selecting the waveform shape for dynamic cyclic test, and to enable the realtime graphs for transducers to be displayed during the test. Refer to the GDSLab handbook for setting up the test parameters and the test stages.

LVDT Transducer Setup

It may be necessary at some point during the life of the EMDCSS device to realign both the horizontal and vertical short range transducers. 

Although they are quite difficult to access it should be possible to adjust the position without removing the EMDCSS front cover. If you find it too difficult to access, the lower front cover can be removed. Align the topcap and pedestal using the Object Display. This is best achieved by getting them as closely lined up as possible, then with the flat edge of a metal ruler progressively use small increments to align them perfectly.

As shown in the figure below, the horizontal LVDT transducer wire is fed through a hole in the base of the device and the LVDT body clamped in place.  As mentioned previously, it is vital that when the topcap and pedestal are exactly in line, the reading of the transducer is as close to 0 mm as possible.  This is so that any displacement fore or back of the pedestal can be measured accurately.  This horizontal transducer has a range of +/- 10 mm.  When aligning it there is bound to be a slight reading either side of zero which can be set as a zero offset.

When locking the LVDT into place, also ensure that the black lead is pushed as far down, toward the base of the device, as possible to avoid interaction with the plate that sits over the top of it.

As with the horizontal LVDT, the vertical LVDT is threaded through a hole in the lower side of the device and up to the bracket (shown in the picture below). The stainless steel armature has a magnet, clearly distinguishable, in one end of the armature.  This end needs to be fed through the LDVT and be in position through the LVDT bracket.  The other end of the armature is placed within the stainless steel block on the base of the device.  It is important to leave some lee-way for future adjustment - therefore the user should only position the armature a portion of the way into the block before tightening it in place.

The armature should also be as straight as possible. The bracket holding the LDVT in place can be re-positioned by undoing the bracket screws attached to the device and manoeuvring the bracket so that the LVDT and rod line up perfectly.

The metal plate positioned below the bracket holding the LVDT in place is a resting plate for the armature when working with the sample (see picture below). By pulling the armature up and sliding the metal plate across, there is an indentation on the plate that the armature can rest in.  This is necessary for two reasons; for ease of access to the sample, as well as not to bend the armature while taking the pedestal/topcap in and out.

User Maintenance

The EMDCSS requires only occasional user maintenance and should perform for many years, assuming it is cared of correctly during the use.  Care must be taken especially during sample preparation, insertion and removal not to drop particulate matter onto the linear guides or to allow water to drain onto the linear guides.

 It is recommended that the user undertakes a check for friction annually and applies a small amount of oil to the linear guides if required at the same time.  There should be no other user maintenance required.

Annual Test for Friction

The following tests check that the level of friction from the linear guides in the EMDCSS is within the expected range.  These should be carried out with no sample present so the axial loadcell and horizontal loadcell 2 can measure only the friction in the system.

To check for static friction on both axes use the object display to set position to 0mm, 2mm, 0mm, -2mm, 0mm, 2mm, 0mm.  Do this for both vertical and shear axis.  Note the relevant loadcell (axial loadcell or shear loadcell 2) reading at each position.  The friction value is: 

• Static axial friction should be <±0.025kN worst case ≤±0.03kN

• Static shear friction should be <±0.025kN worst case ≤±0.03kN

To check dynamic friction setup an advanced shearbox stage to ramp from 0mm to +10mm in 1 minute, logging every 3 seconds. Then setup a second ramp coming back to 0mm in 1 minute, again logging every 3 seconds.  Dynamic friction is (max load– min load)/2 where max load is max load observed during positive ramp and min load is minimum load observed during negative ramp.

• Dynamic axial friction should be <±0.025kN worst case ≤±0.03kN

• Dynamic shear friction should be <±0.025kN worst case ≤±0.03kN

 If you have difficulty performing these tests of are concerned your EMDCSS does not pass the tests please contact GDS technical support by sending an email including your results files to support@gdsinstruments.com.

Annual Lubrication

A small amount of lubrication may be required to maintain the linear guides of the EMDCSS.  Please note that excess lubrication will likely not reduce the friction in the machine, but may instead lead to dust/particle accumulation and thus additional friction in the guides.

Note: a water dispersing oil such as WD40 is NOT suitable for lubricating this machine.  A good quality machine grease such as Castrol EP2 is recommended.

If following the friction checks the apparent friction on the axis is less than ±0.01kN you should not have to apply any lubrication.  If the friction exceeds ±0.01kN continue as described below.

To lubricate the horizontal guides set the EMDCSS to its reverse horizontal travel limit.  This can be easily done using the object display and targeting beyond the travel range, eg horizontal displacement =-20mm.  Next place a small drop of grease onto each of the exposed guides as far back into the machine as possible (reaching underneath the back rubber skirt at the front of the base platen).  A small syringe is ideal for this.  Lastly cycle the machine between the forward and rear travel limits 5 times to distribute the grease across the guides.

The vertical guides are more difficult to reach but can be exposed by setting the machine to its upper travel limit.  They can then be accessed behind the topcap position.  Again, place a small drop of grease onto the bearing as high up as possible, then cycle the vertical axis between upper and lower travel limits 5 times to distribute the grease.

Note: Take care when inserting hands/fingers into the EMDCSS.  It is recommended to push the STOP button whenever there is a risk of trapping hands/fingers being trapped in the machine.