Trip Switch Accepts Digital or Analog Inputs

Steve Oxenberg, Honeywell Inc. Fort Washington, PA.

With so many industrial digital fieldbus protocols in use today, the development of a digital PV alarm trip switch was not unexpected. Especially, considering the fact that in some plants as many as 60% of field transmitters are connected to some sort of safety interlock or shutdown system.

A PV alarm trip switch is a configurable HIGH or LOW relay contact trip that may be used to provide simple ON/OFF control or shutdown plant equipment when a hazardous process situation is approached. A typical safety shutdown must be independent of the control system and override it when necessary to avoid a process upset or plant incident. This is not to be confused with control redundancy, in which a backup controller takes over if the primary controller fails.

Use of traditional analog transmitters in shutdown applications allow other conditions to affect the validity of the signal going into the trip device and thereby affecting the trip action. Among that which can influence the signal are coupled noise impulses, loose wires or failed transmitters. Consequently, over the years, manufacturers have provided configurable delays and hysteresis within shutdown devices to compensate for those uncertainties with hopes of avoiding false shutdowns. Unfortunately those same precautions cause delays in the response to a valid shutdown.

With digital transmitters, most, if not all, of the uncertainty can be eliminated, shutdown response time can be improved and the likelihood of a false shutdown can be avoided. Also available from digital transmitters is a means of differentiating a maintenance problem from a process problem. However, utilizing digital transmitters effectively can be a challenge.

A new device manufactured by Vektron Corporation appears to have advanced trip switch technology to the next level with digital, multivariable and analog capabilities in one compact module. Vektron Corporation, whose specialty is customized industrial interfaces calls it MTS, Multivariable Trip Switch .

MTS is a zero-error multivariable digital PV alarm trip switch. What makes this particular device unique is that it accepts inputs from either a digital transmitter or a traditional 4-20ma analog transmitter and can also provide a precision analog repeat output. Users can now mix digital and analog instrumentation while retaining the key benefits of each.

Figure 1 - Trip Switch Block Diagram

The MTS has application possibilities with integrated or stand alone solutions, and with single or multivariable transmitters. The MTS is based on the same technology as an earlier product Vektron produced, the Multi-Variable Analog interface (MVA), and has many of the same features. Compatible digital instrumentation can directly utilize the transmitter's accurate PV, status and configuration database. At the same time, analog instrumentation can utilize the module's 80% accuracy improvement, multivariable PV access, independent transmitter status and more.

The MTS's digital input is based on the Honeywell Smart Transmitter Digitally Enhanced (DE) protocol and can also be configured to trip on a standard 4-20ma. analog PV signal. This eliminates the need to procure and stock different alarm modules. While the MTS is not as sophisticated as a PLC interlock solution, it is a cut above ordinary analog trip switches, placing the MTS in a class by itself.

The MTS accepts a digital signal from the transmitter and provides a HIGH or LOW PV alarm trip. The PV trip point is adjustable from zero to 99.9 percent. Its trip relays have enough to capacity to directly handle small applications with 5 Amp. @ 250 Vac contact ratings.

In addition, the MTS has an independent "smart status" relay output whose state is linked to the digital transmitter's status. The availability of this separate transmitter status relay eliminates the need for additional HIGH/HIGH or LOW/LOW alarms which are typically required only to determine if a wire has broken or a transmitter has failed. For digital inputs, the MTS also converts the digital PV value into a high precision analog output voltage signal.

FASTER and SAFER
Shutdown systems can be configured to respond faster and avoid process upsets or false shutdowns because of the greater validity associated with digital PV transmission and transmitter status. Including the communication transport time, the trip switch's digital input response is two times faster than typical analog switches.

Figure 2 - Digital Safety System

The independent transmitter status relay output is useful in differentiating a maintenance problem from a process problem. False shutdowns may be avoided by incorporating this status differentiation into the shutdown strategy. Vektron also indicated that the MTS design has already taken into consideration many of the emerging requirements of IEC 1508 for safety related equipment.

MULTIVARIABLE
Its digital input is able to accept multivariable transmitters and is user configurable to trip on any one of the four process variables (PV) or even one of the secondary variables (SV).

Since each of the digital PVs carries its own status and is separately processed, depending on transmitter type, the failure of one PV will not necessarily effect other PVs. This enables a more reliable trip action based on the transmitter's predetermination of PV validity.

Figure 3 - Expanding with Digital Communications

Digital communications enable accurate, secure measurements and device diagnostics that protect system security and integrity. Enabling the transmitter to communicate digitally means more measured variables can travel through the same wire pair. With the multivariable SMV3000 Pressure Transmitter from Honeywell, a user can gain access to either DP, AP, temperature or flow process variables (PV) or even the sensor temperature secondary variable (SV). This may also reduce the overall solution cost by not having to install additional transmitters or new field wiring to make those same measurements.

Honeywell now has at least three multivariable transmitters on the market; SMV3000 Pressure Transmitter, SGC3000 Gas Chromatograph, and MagneW Plus Flowmeter. Vektron indicated that the MTS also works with all 3rd party DE field instruments like the multivariable smart level transmitter from Drexelbrook Engineering Company . As explained, the MTS was designed as a generic DE interface enabling it to be compatible with any DE transmitter regardless of who, when or where it was manufactured. This assures both past and future compatibility.

MULTIDROP
Because of its high input impedance, several MTS modules may be multidropped on a single transmitter with essentially no penalty. This has the advantage of delivering an independent PV alarm trip, transmitter status and precision analog signal anywhere and at virtually any distance.

Figure 4 - Multidrop Trips and Multivariable Access

Other manufacturers, like ProSoft Technologies Inc. , have also developed MTS compatible DE multivariable interfaces for platforms such as the Allen-Bradley PLC-5 and SLC-5 programmable logic controller series.

The MTS can be used either to enhance an existing installation or, because of the ability to provide loop power, in a stand alone installation.

For existing installations, the MTS operates in a high input impedance listener mode and can be connected without disconnecting or disrupting the existing wiring. Any currently installed single variable transmitter, such as the Honeywell ST3000 Pressure Transmitter, can be used to access its secondary variable, temperature and transmitter status.

Figure 5 - Single Variable Transmitter SV Access

STAND ALONE OPERATION
Vektron has also provided complete stand-alone capability in the MTS with the ability to provide the transmitter loop power and 250 ohm loop resistor. The MTS also utilizes a self-resetting electronic fuse that completely protects the field wiring from short-circuits.

Since the MTS also accepts any analog 4-20ma. input, its use is not limited to DE compatible devices. As a matter of fact, Vektron incorporated a few smart features in the module's analog design such as indicating bad transmitter status should the analog signal fall outside the nominal 4-20ma. range. Also incorporated is the ability to hold the last known good PV input value should there be an input wire break. Although performance of these features is not expected to match that of a digital input, it is definitely an enhancement over traditional PV alarm switches and should prove useful in safety applications.

Figure 6 - Stand Alone Digital Transmitters

IMPROVING ANALOG ACCURACY
Also provided is a means of interfacing Honeywell Smartline® digital transmitters with analog instrumentation and expansion outside the digital world by incorporating a high precision analog repeat output in the same module. This analog output actually improves the overall analog measurement accuracy by 80%.

The improved performance is the result of incorporating a precision voltage repeat output instead of a traditional current output. This enables installation without the traditional loop resistor and its associated error. Since virtually all instrumentation actually requires a voltage input, this has the effect of an 80% accuracy improvement. That means that by introducing the MTS into an existing analog loop, better analog performance is obtained from the same transmitter!

Figure 7 - Typical Analog Accuracy

Assuming a typical analog transmitter accuracy of ±0.1%, the loop resistor contributes an additional ±0.1% inaccuracy for a worst case error of ±0.2%. With the MTS installed, the transmitter's digital accuracy is typically ±0.075%. But since the MTS output is a precision ±0.045% voltage source, the error introduced by the loop resistor is eliminated. The worst case error is now only ±0.12%, an improvement of 80%.

Figure 8 - Enhancing Analog Accuracy with MTS

Analog instrumentation such as other safety shutdown systems, backup or critical loop safety controllers and recorders can obtain their analog input signal from the MTS. The additional benefits gained through utilization of the digital field communications are bi-directional communications, improved noise immunity and PV value validity.

OSHA 1910 VALIDATION
The MTS also has other refined features like user configurable forced "output mode" action. This permits the user to select whether or not a transmitter's forced output value causes the MTS's "smart status" to trigger a bad status or pass through as normal. When disabled, this feature may be used to validate PV alarm trip shutdown operation. When enabled, it acts as a watchdog, preventing the user from inadvertently leaving a transmitter in a forced state.

For routine validation and maintenance, the MTS provides a TEST terminal that trips the PV alarm and status relays and forces the analog output to 3.00 volts, 50%. The MTS's life cycle costs are minimal since each module is factory characterized and does not require any periodic calibration.

ENHANCED PERSONNEL SAFETY
The analog repeat output fail-safe direction is user configurable to go to burnout high, burnout low or even more useful, hold the last known good (LKG) PV input value. By utilizing the LKG and the "smart status" relay output, the user now has a simple means of reducing the likelihood of process upsets, false shutdowns and may allow more time to take corrective action by holding the analog output at its last good value. Unnecessary personnel excursions out to the field in a potentially hazardous environment may be avoided with the additional visibility of transmitter status. These are sophisticated features found mainly in large distributed control systems. With features like this, it doesn't take much to recover the MTS investment.

Figure 9 - Leveraging Maintenance Features

HOW IT WORKS
The entire MTS is configured through simple switches and does not require any special tools or software. Installation and checkout is simplified with five colored LED indicators. The MTS is a self-contained diagnostic tool with LEDs indicating power, communication activity, trip status and transmitter status. It even alerts the user if its analog repeat output is shorted.

Figure 10 - MTS Functional Diagram

The MTS provides two security features found in Honeywell DCSs; Bad PV protection and Bad Database protection. This means that only validated PVs are passed through and status is only considered good when the transmitter configuration matches that of the MTS. If a problem exists, the MTS's "smart status" LED provides a visual indication of specifically what is wrong.

Table 1 - Comparing Input Features

For current users of DE compatible instrumentation, the MTS enables every transmitter to communicate digitally regardless of other signal destinations. Having accomplished that, all transmitters can take advantage of centralized configuration, database security and digital control accuracy.

The MTS appears to provide a multitude of application options utilizing digital transmitters with products from other manufacturers. This situation is not unlike those we will face when digital Fieldbus transmitters arrive. As Vektron explained; "The MTS's flexibility is attributed to a close working relationship with Honeywell."

With Vektron, Honeywell appears to have taken an excellent step towards applying technology to achieve new and unique solutions, while at the same time fulfilling its promise of providing continued support for the DE protocol and meeting the needs of open interfaces. Vektron has also indicated that the MTS is the second of a series of products it plans to offer. Honeywell users will be glad to know that more flexibility is available for use with the first industrial digital fieldbus protocol.