Safe Drive Control including STO

Graph illustrating STO Function

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,]. 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.


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.


[1]    “Variable Frequency Drives – Industrial Wiki – odesie by Tech Transfer”, Myodesie​.com, 2017. [Online]. Available: https://​www​.myo​desie​.com/​w​i​k​i​/​i​n​d​e​x​/​r​e​t​u​r​n​E​n​t​r​y​/​i​d​/​3​040. [Accessed: 19- Jun- 2017]. 

[2] “Safe Torque Off (STO) – 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​-​t​o​r​q​u​e​-​o​f​f​.​a​spx. [Accessed: 19- Jun- 2017].

[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.


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!

Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. ( in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity.

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