CETA in force!

CETA comes into force today, 21-​Sep-​2017

If you are unfa­mil­i­ar with CETA, the Comprehensive Economic and Trade Agreement, this ground­break­ing trade agree­ment between Canada and the European Union will be a game-​changer for Canada. Until today, the actu­al date for imple­ment­a­tion of the agree­ment has been a mov­ing tar­get. There were at least two pre­vi­ous dates announced by the Canadian gov­ern­ment, but each time the dates passed without the agree­ment com­ing into force due to issues that needed to be resolved.

So what does this mean for Canadians? As of today, 98% of Canadian products can now enter into the EU tariff-​free. Within two years, 99% of products will be tariff-​free. The agree­ment embod­ies much of what the EU sys­tem is based upon: Four pil­lars of free­dom are entrenched in the agree­ment.

The Four Pillars include the free­dom of move­ment of people, goods, ser­vices and cap­it­al. This philo­sophy has brought sig­ni­fic­ant prosper­ity and free­dom to European cit­izens. Within the“Schengen Area”, EU cit­izens can move freely across nation­al bor­ders without passing through cus­toms, in a very sim­il­ar way to Canadians mov­ing from Province to Province. EU cit­izens can work in any Schengen coun­try without the need for work­ing visas or cit­izen­ship in the new coun­try they have chosen. Similar freedoms exist for goods, ser­vices and money.

Under CETA, sim­il­ar freedoms are avail­able to Canadians, although with some restric­tions since CETA does not mean that Canada is now an EU Member State. Goods can flow from Canada to the EU, and from the EU to Canada without tar­iff restric­tions, except in some lim­ited cases. Businesses who want to set up oper­a­tions in the EU can do this with lim­ited restric­tions, and Canadian pro­fes­sion­al work­ers can move to the EU to staff these new oper­a­tions without the need for restrict­ive work visas. Investment in EU oper­a­tions has gained pro­tec­tions through EU law so that these invest­ments are bet­ter pro­tec­ted. Canadian ser­vice busi­nesses can now provide their ser­vice products to EU cus­tom­ers with little restric­tion. Canadian busi­ness now has free access to a mar­ket­place of 500 mil­lion new cus­tom­ers, nearly 14 times lar­ger than the Canadian mar­ket. The EU mar­ket is worth nearly €2.4 tril­lion in exports alone. This is an oppor­tun­ity Canadians can’t afford to miss.

With the instabil­ity being cre­ated by the cur­rent US admin­is­tra­tion and the bully tac­tics that are being used to force the rene­go­ti­ation of NAFTA, Canadian busi­ness should take the oppor­tun­ity presen­ted to us today to turn our eyes to the EU, a uni­on of coun­tries who are open and friendly to Canadians. People who want to work with us, who want our products and ser­vices. Many Canadians sup­port scrap­ping NAFTA if key pro­vi­sions can’t be met.

For more inform­a­tion on CETA and what it will mean for Canadians, please see Doing Business in Europe — CETA: Canada and the European Union Usher In a New Era of Trade.

21-​September-​2017 is a day to cel­eb­rate. The future looks bright!

Machinery Safety Labels: 3 Top Tools for Effective Warnings

This entry is part 4 of 4 in the series Hierarchy of Controls

Machinery Safety Labels

The third level of the Hierarchy of Controls is Information for Use. Safety Labels are a key part of the Information for Use provided by machine build­ers to users and are often the only inform­a­tion that many users get to see. This makes the design and place­ment of the safety labels crit­ic­al to their effect­ive­ness. There is as much risk in the under-​use of safety labels as there is in the over-​use of safety labels. Often, machine build­ers and users simply select gen­er­ic labels that are eas­ily avail­able from cata­logues, miss­ing the oppor­tun­ity to design labels that are spe­cif­ic to the machine and the haz­ards present.

Product Safety and Liability Limitation

If your com­pany man­u­fac­tures machinery that has poten­tial haz­ards asso­ci­ated with its trans­port­a­tion, install­a­tion, use, main­ten­ance, decom­mis­sion­ing and/​or dis­pos­al, you likely have a very strong need to cre­ate effect­ive product safety labels. This task must be done right: product safety labels play an integ­ral role in your company’s product safety and liab­il­ity pre­ven­tion efforts. And that means that people’s lives and your company’s fin­an­cial well-​being are on the line. On that note, it’s import­ant to keep in mind these two factors when it comes to effect­ive safety labels:

  1. If prop­erly designed, they can dra­mat­ic­ally reduce acci­dents. This not only improves a product’s over­all safety record but adds to a company’s bot­tom line by redu­cing product liab­il­ity lit­ig­a­tion and insur­ance costs.
  2. If poorly designed, needed safety com­mu­nic­a­tion does not take place and this can lead to acci­dents that cause injur­ies. With these acci­dents, com­pan­ies face high costs set­tling or fight­ing law­suits because their products lacked “adequate warn­ings.”

With the rise in product liab­il­ity lit­ig­a­tion based on “fail­ure to warn” over the past sev­er­al dec­ades, product safety labels have become a lead­ing focal point in law­suits faced by cap­it­al equip­ment man­u­fac­tur­ers. Let’s look at three best?practice tools for product safety label design. These tools can provide insight to help you cre­ate or improve your safety label strategy in order to bet­ter pro­tect your product users from harm and your com­pany from litigation-​related losses.

TOOL #1: SAFETY LABEL STANDARDS

As a man­u­fac­turer, you know that your leg­al oblig­a­tion is to meet or exceed the most recent ver­sions of stand­ards related to your product at the time it’s sold into the mar­ket­place. Warning label stand­ards are the first place to turn to when it comes to defin­ing your product safety labels. Up until 1991, there was no over­arch­ing, multi-​industry stand­ard in the U.S., or in the rest of the world, which gave defin­it­ive guid­ance on the prop­er format­ting and con­tent for on-​product warn­ings. In the U.S., that changed nation­ally with the pub­lic­a­tion of the ANSI Z535.4 Standard for Product Safety Signs and Labels in 1991, and inter­na­tion­ally with the pub­lic­a­tion of ISO 3864 – 2 Design Principles for Product Safety Labels in 2004.

As of 2017, Canada does not have a warn­ing label stand­ard. Since Canada imports machinery from the U.S. and the EU, it is quite com­mon to see either ANSI Z535 style labels or ISO 3864 style labels on products. Under Canadian law, neither is more cor­rect. However, Québec has spe­cif­ic require­ments for French lan­guage trans­la­tions, and many CSA stand­ards pre­scribe spe­cif­ic haz­ard warn­ing labels that do not con­form to either ANSI or ISO styles.

Following the design prin­ciples in ANSI Z535.4 or ISO 3864 – 2 will give you a start­ing place for both the con­tent and format choices you have to make for your products’ safety labels, bear­ing in mind the lan­guage require­ments of your jur­is­dic­tion. Note that both of these stand­ards are revised reg­u­larly, every five years or so, and it’s import­ant to be aware of the nuances that would make one format more appro­pri­ate for your product than anoth­er.

Safety label standard ANSI Z535.4 Product Safety Signs and Labels
The ANSI Z535.4 product safety label stand­ard
Safety label standard ISO 3864-2 Graphical symbols - Safety colours and safety signs - Part 2: Design principles for product safety labels.
The ISO 3864 – 2 product safety label stand­ard

TOOL #2: RISK ASSESSMENT

From an engin­eer­ing per­spect­ive, your job is to identi­fy poten­tial haz­ards and then determ­ine if they need to be designed out, guarded, or warned about. From a leg­al per­spect­ive, your job is to define what haz­ards are “reas­on­ably fore­see­able” and “reas­on­able” ways to mit­ig­ate risks asso­ci­ated with haz­ards that can­not be designed out. This is where risk assess­ment comes into play.

In today’s world, a product is expec­ted to be designed with safety in mind. The risk assess­ment pro­cess helps you to accom­plish this task. At its most basic level, risk assess­ment involves con­sid­er­ing the prob­ab­il­ity and sever­ity of out­comes that can res­ult from poten­tially haz­ard­ous situ­ations. After identi­fy­ing the poten­tial haz­ards related to your product at every point in its life­cycle, you then con­sider vari­ous strategies to either elim­in­ate or reduce the risk of people inter­act­ing with these haz­ards.

The best prac­tice risk assess­ment stand­ards that exist today (i.e. ANSI Z10, ANSI B11, CSA Z432, CSA Z1002, ISO 12100, ISO 31000, ISO 31010) give you a pro­cess to use to quanti­fy and reduce risks. Using these stand­ards as the basis for a form­al­ized risk assess­ment pro­cess will not only help you to devel­op bet­ter safety labels and a safer product, but it will also provide you with doc­u­ment­a­tion that will help you to show the world that you are a safety-​conscious com­pany who uses the latest standards-​based tech­no­logy to reduce risks. This will be highly import­ant should you be involved in product liab­il­ity lit­ig­a­tion down the road.

From an engin­eer­ing per­spect­ive, your job is to identi­fy poten­tial haz­ards and then determ­ine if they need to be designed out, guarded, or warned about. From a leg­al per­spect­ive, your job is to define what haz­ards are “reas­on­ably fore­see­able” and “reas­on­able” ways to mit­ig­ate risks asso­ci­ated with haz­ards that can­not be designed out. This is where risk assess­ment comes into play.

MIL-STD 882 risk assessment form
A typ­ic­al risk assess­ment scor­ing mat­rix (based on MIL STD 882 as defined in ANSI B11/​ISO 12100 Safety of Machinery – Risk Assessment Annex D)

TOOL #3: PICTOGRAPHIC  SAFETY LABELS FOR GLOBAL MARKETS

A large num­ber of machinery man­u­fac­tur­ers sell their products around the globe and when this is the case, com­pli­ance with glob­al stand­ards is a require­ment. The ANSI Z535.4 and ISO 3864 – 2 product safety label stand­ards, and the EU machinery dir­ect­ive place an emphas­is on using well-​designed sym­bols on machinery safety labels so inform­a­tion can be con­veyed across lan­guage bar­ri­ers.

The EU Machinery Directive 2006/​42/​EC requires that all inform­a­tion for use be provided in the offi­cial lan­guages of the coun­try of use. Information for use includes haz­ard warn­ing signs and labels that bear mes­sages in text. Adding sym­bols also increases your labels’ notice­ab­il­ity. The use of sym­bols to con­vey safety is becom­ing com­mon­place world­wide and not tak­ing advant­age of this new visu­al lan­guage risks mak­ing your product’s safety labels obsol­ete and non-​compliant with loc­al, region­al and inter­na­tion­al codes. In ISO 3864 – 2’s latest, 2016 update, a major change in ISO label formats was made: a new “word­less” format that con­veys risk sever­ity was added to the stand­ard. This new label format uses what ISO calls a “haz­ard sever­ity pan­el” but no sig­nal word. It com­mu­nic­ates the level of risk through colour-​coding of the haz­ard sever­ity pan­el. This format option elim­in­ates words – mak­ing trans­la­tions unne­ces­sary.

It should be noted that some­times sym­bols alone can­not con­vey com­plex safety mes­sages. In these cases, text is often still used. When ship­ping to non-​English speak­ing coun­tries, the trend today is to trans­late the text into the lan­guage of the coun­try in which the machine is sold. Digital print tech­no­logy makes this solu­tion much more cost effect­ive and effi­cient than in the past.

Safety label by Clarion Safety Systems on a machine
A typ­ic­al Clarion machine safety label that uses an inter­na­tion­ally format­ted graph­ic­al sym­bol and a format that meets both ANSI Z535.4 and ISO 3864 – 2 design prin­ciples (Design ©Clarion Safety Systems. All rights reserved.)

Concluding Thoughts

The safety labels that appear on your products are one of its most vis­ible com­pon­ents. If they don’t meet cur­rent stand­ards, if they aren’t designed as the res­ult of a risk assess­ment, and if they don’t incor­por­ate well-​designed graph­ic­al sym­bols, your com­pany risks lit­ig­a­tion and non-​conformance with mar­ket require­ments. Most import­antly, you may be put­ting those who inter­act with your machinery at risk of harm. Making sure your product safety labels are up-​to-​date is an import­ant task for every engin­eer respons­ible for a machine’s design.

For more inform­a­tion on effect­ive product safety labelling and resources that you can put to use today, vis­it www​.clari​on​safety​.com. Clarion also offers com­pli­ment­ary safety label assess­ments, where we use our exper­i­ence with the latest stand­ards and best prac­tices to assess your labels and ensure that they’re up-​to-​date in meet­ing today’s require­ments.

Ed. note: Additional Canadian mater­i­al con­trib­uted by Doug Nix.

Digiprove sealCopyright secured by Digiprove © 2017
Acknowledgements: Derek Eversdyke, Clarion Safety Systems, LLC
Some Rights Reserved

Safe Drive Control including Safe Torque Off (STO)

This entry is part 12 of 13 in the series Emergency Stop

Ed. Note: This art­icle was revised 25-​Jul-​17 to include inform­a­tion on safe stand­still.

Safe Drive Control

Variable Frequency Drive for conveyor speed control
Variable Frequency Drive for con­vey­or speed con­trol [1]
Motor drives are every­where. From DC vari­able speed drives and index­ing drives, through AC Variable Frequency drives, servo drives and step­per motor drives, the cap­ab­il­it­ies and the flex­ib­il­ity of these elec­tron­ic sys­tems has giv­en machine design­ers unpre­ced­en­ted cap­ab­il­it­ies when com­pared to basic relay or contactor-​based motor starters. We now have the cap­ab­il­ity to con­trol mech­an­isms using motors in ways that would have been hard to ima­gine at the begin­ning of the indus­tri­al revolu­tion.

Since we are con­trolling machinery, safety is always a con­cern. In the 1990’s when I star­ted design­ing machinery with motor drives, deal­ing with safety con­cerns usu­ally meant adding a suit­ably rated con­tact­or upstream of the drive so that you could inter­rupt power to the drive in case some­thing went wrong. With early servo drives, inter­rupt­ing the sup­ply power often meant los­ing pos­i­tion data or worse, so con­tact­ors were placed between the drive and the motor. This occa­sion­ally caused the drive stage of the servo con­trol­ler to blow up if the switch-​off happened with the motor run­ning and under high load. Motor drive man­u­fac­tur­ers respon­ded by provid­ing con­tact­ors and oth­er com­pon­ents built into their drives, cre­at­ing a fea­ture called Safe Torque Off (STO).

STO describes a state where “The drive is reli­ably torque-​free” [2]. The func­tions dis­cussed in this art­icle are described in detail in IEC 61800 – 5-​2 [3]. The func­tions are also lis­ted in [10, Table 5.2]. Note that only Safe Torque Off and Safe Stop 1 can be used for emer­gency stop func­tions. Safe Torque Off, Safe Stop 1 and Safe Stop 2 can be used for safety-​related stop func­tions ini­ti­ated by a safe­guard­ing device.

If you have been a read­er of this blog for a while, you may recall that I have dis­cussed stop cat­egor­ies before. This art­icle expands on those con­cepts in rela­tion to motor drives and their stop­ping func­tions spe­cific­ally. I’ve also talked about Emergency Stop extens­ively. You might be inter­ested in read­ing more about the e-​stop func­tion in the post “Emergency Stop – What’s so con­fus­ing about that?”

Safe Torque Off (STO)

According to Siemens, “The STO func­tion is the most com­mon and basic drive-​integrated safety func­tion. It ensures that no torque-​generating energy can con­tin­ue to act upon a motor and pre­vents unin­ten­tion­al start­ing.” Risk assess­ment of the machinery can identi­fy the need for an STO func­tion. The devices used for this applic­a­tion are described in IEC 60204 – 1 in clause 5.4 [4]. The design fea­tures for pre­ven­tion of unex­pec­ted start­ing are covered in more detail in EN 1037 [5] or ISO 14118 [6]. If you are inter­ested in these stand­ards, ISO 14118 is in the pro­cess of being revised. A new ver­sion should be avail­able with­in 12 – 18 months.

The STO func­tion oper­ates as shown in Fig.1. The blue line rep­res­ents the drive speed/​velocity, V, on the y-​axis, with time, t, on the x-​axis.

Graph showing motor drive output over time when the STO function is activated.
Figure 1 – Safe Torque Off func­tion [1]
At the begin­ning of the stop­ping pro­cess (orange arrow and dot­ted line), the drive gate pulses are imme­di­ately shut off, remov­ing torque from the motor (i.e., zero torque). The speed of the driv­en equip­ment will drop at a rate determ­ined by the sys­tem fric­tion and iner­tia until stand­still is achieved. The zero torque con­di­tion is then main­tained until the safety func­tion per­mits restart­ing (area out­lined with yellow/​black zebra stripe). Note that drive stand­still may occur if the fric­tion and iner­tia of the sys­tem per­mit, but it is pos­sible that the driv­en equip­ment may coast for some time. You may be able to move the driv­en equip­ment by hand or grav­ity with drive in STO.STO is an uncon­trolled stop [4, 3.56]:

STO is an uncon­trolled stop [4, 3.56]:

uncon­trolled stop
stop­ping of machine motion by remov­ing elec­tric­al power to the machine actu­at­ors
NOTE This defin­i­tion does not imply any oth­er state of oth­er (for example, non-​electrical) stop­ping devices, for example, mech­an­ic­al or hydraul­ic brakes that are out­side the scope of this stand­ard.

The defin­i­tion above is import­ant. Uncontrolled stops are the most com­mon form of stop­ping used in machines of all types and is required as a basic func­tion for all machines. It can be achieved in a num­ber of ways, includ­ing the use of a dis­con­nect­ing device, emer­gency stop sys­tems, and gate inter­lock­ing sys­tems that remove power from machine actu­at­ors.

The concept of an uncon­trolled stop is embod­ied in stop cat­egory 0 [4, 9.2.2]:

stop cat­egory 0 — stop­ping by imme­di­ate remov­al of power to the machine actu­at­ors (i.e., and uncon­trolled stop, see 3.56)

Stop cat­egory 0 is only appro­pri­ate where the machinery has little iner­tia, or where mech­an­ic­al fric­tion is high enough that the stop­ping time is short. It may also be used in cases where the machinery has very high iner­tia, but only for nor­mal stop­ping when coast­ing time is not a factor, not for safety stop­ping func­tions where the time to a no-​motion state is crit­ic­al.

There are a few oth­er stop­ping modes that are often con­fused with STO:

  • Safe Stop 1
  • Safe Stop 2
  • Safe Operating Stop
  • Safe Standstill

Let’s explore the dif­fer­ences.

Safe Stop 1 (SS1)

If a defined stop­ping time is needed, a con­trolled stop­ping func­tion will be required fol­lowed by entry into STO. This stop­ping func­tion is called “Safe Stop 1” (SS1).

SS1 is dir­ectly related to Stop Category 1 [4, 9.2.2]. As described in [4], Stop Category 1 func­tions as fol­lows:

stop cat­egory 1 — a con­trolled stop (see 3.11) with power avail­able to the machine actu­at­ors to achieve the stop and then remov­al of power when the stop is achieved;

A “con­trolled stop” is defined in [4, 3.11]:

con­trolled stop
stop­ping of machine motion with elec­tric­al power to the machine actu­at­or main­tained dur­ing the stop­ping pro­cess

Once the con­trolled stop is com­pleted, i.e., machine motion has stopped, the drive may then be placed into STO (or cat­egory 0 stop). The stop­ping pro­cess is shown in Fig. 2 [7].

Graph showing the reduction of drive speed over time following the beginning of a controlled stopping process.
Figure 2 – Safe Stop 1

The stop­ping pro­cess starts where the orange arrow and dot­ted line are shown. As com­pared to Fig. 1 where the decel­er­a­tion curve is gentle and expo­nen­tial, the act­ive stop­ping peri­od in Fig. 2 is a lin­ear curve from oper­at­ing speed to zero speed. At the blue dot­ted line, the drive enters and stays in STO. The yellow/​black zebra striped area of the curve out­lines the com­plete stop­ping func­tion. This stop­ping meth­od is typ­ic­al of many types of machinery, par­tic­u­larly those with servo driv­en mech­an­isms.

Safe Stop 2 (SS2)

In some cases, the risk assess­ment may show that remov­ing power com­pletely from a mech­an­ism will increase the risk. An example might be a ver­tic­al axis where the motor drive is used to main­tain the pos­i­tion of the tool­ing. Removing power from the drive with the tool raised would res­ult in the tool­ing crash­ing to the bot­tom of the axis in an uncon­trolled way. Definitely NOT the desired way to achieve any kind of stop!

There are a num­ber of ways to pre­vent this kind of occur­rence, but I’m going to lim­it the dis­cus­sion here to the Safe Stop 2 func­tion.

Let’s start with the defin­i­tion [4, 3.11]:

con­trolled stop
stop­ping of machine motion with elec­tric­al power to the machine actu­at­or main­tained dur­ing the stop­ping pro­cess

Wait! This is exactly the same as a stop cat­egory 1, so what is the dif­fer­ence? For that we need to look to [4, 9.2.2]:

stop cat­egory 2 — a con­trolled stop with power left avail­able to the machine actu­at­ors.

The first thing to know about stop cat­egory 2 is that this cat­egory can­not be used for emer­gency stop [4, 9.2.5.4.2]. If you have tool­ing where stop cat­egory 2 is the most appro­pri­ate stop under nor­mal con­di­tions, you will have to add an anoth­er means to pre­vent the axis from fall­ing dur­ing the emer­gency stop. This could be a spring-​set brake that is held released by the emer­gency stop sys­tem and is applied when the e-​stop sys­tem removes power from the tool­ing. There are many ways to achieve auto­mat­ic load-​holding besides brakes, but remem­ber, whatever you choose it must be effect­ive in power loss con­di­tions.

As shown in Fig. 3, the oper­a­tion of Safe Stop 2 dif­fers from Safe Stop 1 in that, instead of enter­ing into STO when motion stops, the sys­tem enters Safe Operating Stop (SOS) [8], not STO. SOS is a stop cat­egory 2 func­tion. Full torque remains avail­able from the motor to hold the tool­ing in pos­i­tion. Safe stand­still is mon­itored by the drive or oth­er means.

Graph showing speed reduction to zero, followed by entry into stop category 2.
Figure 3 — Safe Stop 2

Depending on the ISO 13849 – 1 PLr, or the IEC 62061 SILr needed for the applic­a­tion, the drive may not have high enough reli­ab­il­ity on its own. In this case, a second chan­nel may be required to ensure that safe stand­still mon­it­or­ing is adequately reli­able. This can be achieved by adding anoth­er means of stand­still detec­tion, like a second encoder, or a stand­still mon­it­or­ing device. An example cir­cuit dia­gram show­ing this type of mon­it­or­ing can be found in Fig. 4 [10, Fig. 8.37], show­ing a safety PLC and drive used to provide an “inch­ing” or “jog” func­tion.

Circuit diagram for a safe inching mode using a motor drive. Taken from Fig 8.37 in BGIA Report 2/2008e
Figure 4 — Safely lim­ited speed for inch­ing mode – PLd, Cat. 3 [10]
In Fig. 4, the encoders are labelled G1 and G2. Both encoders are con­nec­ted to the safety PLC to provide two-​channel feed­back required for Category 3 archi­tec­ture. G1 is also con­nec­ted to the motor drive for pos­i­tion and velo­city feed­back as needed for the applic­a­tion. Note that this par­tic­u­lar drive also has a con­tact­or upstream, Q1, to provide one chan­nel of the two required for Category 3. The second chan­nel would be provided by the pulse block­ing input on the drive. For more on how this cir­cuit func­tions and how the func­tion­al safety ana­lys­is is com­pleted, see [10].

Safe Operating Stop (SOS)

During a safe oper­at­ing stop (SOS), the motor is brought to a spe­cif­ic pos­i­tion and held there by the drive. Full torque is avail­able to keep the tool­ing in pos­i­tion. The stop is mon­itored safely by the drive. The func­tion is shown in Figure 4 [9].

A graph showing a drive maintaining position following a stop
Figure 5 — Safe Operating Stop

In Fig. 5, the y-​axis, s, rep­res­ents the pos­i­tion of the tool­ing, NOT the velo­city, while the x-​axis rep­res­ents time, t. The start of the pos­i­tion hold­ing func­tion is shown by the orange arrow and dashed line. The peri­od fol­low­ing the green dashed line is the SOS peri­od.

SOS can­not be used for the emer­gency stop func­tion. Under cer­tain con­di­tions it may be used when guard inter­locks are opened, i.e., the guard door on a CNC lathe is opened so that the oper­at­or can place a new work­piece.

There a quite a few addi­tion­al “safe” drive func­tions. For more on these func­tions and how to imple­ment them, see [2] and applic­a­tion data from your favour­ite drive man­u­fac­turer. Reference is also provided in [9, Table 5.2].

Safe Standstill

Safe stand­still is a con­di­tion where motion has stopped and is being mon­itored by a safety-​rated device whose out­put sig­nals are used to con­trol the release of guard lock­ing devices. Safe stand­still is not the same as zero-​speed because zero-​speed can be achieved without the use of safety rated con­trol com­pon­ents and design, while safe stand­still requires both suit­able com­pon­ents and design.

There are a num­ber of ways to achieve safe stand­still. Here are three com­mon approaches [12]:

  1. Rotation sensors
    Sensors includ­ing prox­im­ity sensors, resolv­ers, and encoders can be used to mon­it­or the motion of the drive com­pon­ents. A safe stand­still mon­it­or­ing device is used to when stand­still has occurred.  When a machine has an unstable rest pos­i­tion, a prox­im­ity sensor should be used to ensure the machine is in a safe con­di­tion before the guard lock­ing devices are released.
  2. Back EMF mon­it­or­ing
    Back elec­tro­mot­ive force or Back EMF is the voltage cre­ated in an elec­tric motor due to the rota­tion of the arma­ture in the mag­net­ic field in the motor. This voltage opposes the applied voltage and is approx­im­ately pro­por­tion­al to the rota­tion­al speed of the motor. Back EMF remains after the sup­ply voltage has been removed, allow­ing mon­it­or­ing devices to indir­ectly meas­ure motor speed and stand­still.
  3. Failsafe timer
    Failsafe timers are time delay relays designed for use in safety func­tions. Failsafe timers can be used when the stop­ping per­form­ance of the machinery is con­sist­ent and known.
    Following remov­al of power from the drive motor, the time delay starts. At the end of the time delay, the relay releases the guard lock­ing devices.
    Regular time delay relays can­not be used for this pur­pose, only fail-​safe relays designed to be used in safety func­tions can be used, along with suit­able safety sys­tems design tech­niques like ISO 13849 or IEC 62061.

Conclusions

As you can see, there are sig­ni­fic­ant dif­fer­ences between STO, SS1, SS2, SOS and Safe Standstill. While these func­tions may be used togeth­er to achieve a par­tic­u­lar safety func­tion, some are func­tions of the imple­ment­a­tion of the motor drive, e.g., STO, a func­tion of the design of the motor drive itself, e.g., STO, SS1, SS2, and SOS, or the design of con­trols extern­al to the motor drive, e.g., safe stand­still. The sim­il­ar­it­ies between these vari­ous func­tions can make it easy to con­fuse them. Care needs to be taken to ensure that the cor­rect tech­nic­al approach is used when real­ising the safety func­tion required by the risk assess­ment.

References

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[3]      Adjustable speed elec­tric­al power drive sys­tems – Part 5 – 2: Safety require­ments – Functional. IEC Standard 61800 – 5-​2. 2nd Ed. 2016.

[4]     Safety of machinery — Electrical equip­ment of machines — Part 1: General require­ments. IEC Standard 60204 – 1. 2006.

[5]     Safety of machinery — Prevention of unex­pec­ted start-​up. EN Standard 1037+A1. 2008.

[6]     Safety of machinery — Prevention of unex­pec­ted start-​up. ISO Standard 14118. 2000.

[7]     “Safe Stop 1 (SS1) – Safety Integrated – Siemens”, Industry​.siemens​.com, 2017. [Online]. Available: http://​www​.industry​.siemens​.com/​t​o​p​i​c​s​/​g​l​o​b​a​l​/​e​n​/​s​a​f​e​t​y​-​i​n​t​e​g​r​a​t​e​d​/​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​p​r​o​d​u​c​t​-​p​o​r​t​f​o​l​i​o​/​d​r​i​v​e​-​t​e​c​h​n​o​l​o​g​y​/​s​a​f​e​t​y​-​f​u​n​c​t​i​o​n​s​/​P​a​g​e​s​/​s​a​f​e​-​s​t​o​p​1​.​a​spx. [Accessed: 19- Jun- 2017].

[8]     “Safe Stop 2 (SS2) – Safety Integrated – Siemens”, Industry​.siemens​.com, 2017. [Online]. Available: http://​www​.industry​.siemens​.com/​t​o​p​i​c​s​/​g​l​o​b​a​l​/​e​n​/​s​a​f​e​t​y​-​i​n​t​e​g​r​a​t​e​d​/​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​p​r​o​d​u​c​t​-​p​o​r​t​f​o​l​i​o​/​d​r​i​v​e​-​t​e​c​h​n​o​l​o​g​y​/​s​a​f​e​t​y​-​f​u​n​c​t​i​o​n​s​/​P​a​g​e​s​/​s​a​f​e​-​s​t​o​p​2​.​a​spx. [Accessed: 19- Jun- 2017].

[9]     “Safe Operating Stop (SOS) – Safety Integrated – Siemens”, Industry​.siemens​.com, 2017. [Online]. Available: http://​www​.industry​.siemens​.com/​t​o​p​i​c​s​/​g​l​o​b​a​l​/​e​n​/​s​a​f​e​t​y​-​i​n​t​e​g​r​a​t​e​d​/​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​p​r​o​d​u​c​t​-​p​o​r​t​f​o​l​i​o​/​d​r​i​v​e​-​t​e​c​h​n​o​l​o​g​y​/​s​a​f​e​t​y​-​f​u​n​c​t​i​o​n​s​/​P​a​g​e​s​/​s​a​f​e​-​o​p​e​r​a​t​i​n​g​-​s​t​o​p​.​a​spx. [Accessed: 19- Jun- 2017].

[10]     M. Hauke, M. Schaefer, R. Apfeld, T. Boemer, M. Huelke, T. Borowski, K. Büllesbach, M. Dorra, H. Foermer-​Schaefer, W. Grigulewitsch, K. Heimann, B. Köhler, M. Krauß, W. Kühlem, O. Lohmaier, K. Meffert, J. Pilger, G. Reuß, U. Schuster, T. Seifen and H. Zilligen, “Functional safety of machine con­trols – Application of EN ISO 13849 – Report 2/​2008e”, BGIA – Institute for Occupational Safety and Health of the German Social Accident Insurance, Sankt Augustin, 2017.

[11]     “Glossary”, Schmersalusa​.com, 2017. [Online]. Available: http://​www​.schmersa​lusa​.com/​c​m​s​1​7​/​o​p​e​n​c​m​s​/​h​t​m​l​/​e​n​/​s​e​r​v​i​c​e​/​g​l​o​s​s​a​r​y​.​h​t​m​l#S. [Accessed: 25- Jul- 2017].

[12]     Schmersal Tech Briefs: Safe Speed & Standstill Monitoring. Schmersal USA, 2014.

Acknowledgements

Special thanks go out to two of my reg­u­lar read­ers for sug­gest­ing this post: Matt Ernst and con­trols­girl, who com­ments fre­quently. Thanks for the ideas and the ques­tions that sparked this post!