Why is the earth ground important?

The PE is important for the safety of the person operating the equipment and safe operation of the devices. All grounds should have the same reference potential level between all the components in the system. This also increases performance by reducing the noise level in the system.

How would you check that the system components are correctly connected to the PE?

Check voltage ground connectivity with a digital voltmeter, after ensuring that all the connections are secure and well established with proper wiring according to the product’s schematics.

Are you allowed to connect or disconnect the drive while the power source is on?


How much time should you wait after shutting off the drive's power before touching or disconnecting parts of the system?

Minimum: 1 minute
Recommended: several minutes
The internal capacitance is discharged during this time.

What are the two main safety standards that Elmo drives comply with?

CE and UL

Two types of failures are known to you? What are the risks of each one?

Improper grounding: Can result in excessive voltage drop, drive failure or personal injury. Improper power supplied: Shorting terminals or improper isolation. Grounding the reference neutral ? of an AC input could result in drive failure and injury.

What steps are taken to minimize the chances of motor-drive failure?

Proper grounding, cable shielding, proper connections.

MOSFET for high voltage (AC drives), IGBT for low voltages (DC drives) – true or false?


What is the in-rush current? How does Elmo solve the in-rush current problem?

When an amplifier is connected to the power supply and the power is activated, initial high level in-rush current can occur. This problem is solved by using negative thermal voltage transistors (NTCs).

What PWM switching method is used in Elmo drives?

Advanced unipolar with fast technology.

What kinds of current and voltage sensing are known to you?

Resistor sensing on low side power bridges, hall effect sensors on motor phases.

Why is voltage sensing required?

For drive protection (under- and overvoltage) and current GS control loop.

What type of electrical protections are known to you that are implemented in Elmo drives?

Short protection: Phase-phase, phase-ground, phase-VP+ Under/overvoltage, temperature protection.

Why do we need a shunt regulator? Describe the situation that will cause it to operate

To dissipate energy that is absorbed into the system, without harming the stage up to a certain rated level. This can occur during rapid deceleration of a high internal load.

Describe the principle of torque generation.

Passing current through the coil of a starter, creating a magnetic field perpendicular to the rotor’s field.

What is commutation? How is commutation performed in a DC brush motor?

The ability to drive and move a motor based on an electrical input current. Two phases of the brush motor have voltage applied to the brushes creating a magnetic field to repeatedly turn the rotor 90°.

What is the relationship between the motor rotation speed and the BEMF voltage?

BEMF (Back Electromotive Force) is the energy voltage developed on the system during rotation as a result of the inherent motor characteristic properties. This is always less than or equal to the rotation speed on a torque-speed curve.

What are the advantages and disadvantages of DC brush motors?

Advantage: They are simple motors where no commutation is required.
Disadvantage: They require maintenance and are not very efficient.

What is the difference between six-step and trapezoidal commutation? What method is used in Elmo’s digital drives?

Six-step and trapezoidal commutation are the same methods with different names. Elmo uses sine commutation.

What is the maximum torque ripple in trapezoidal current control in a sinusoidal wound motor?

About 13%–14%

What is the maximum torque ripple at a sinusoidal current control of a sinusoidal wound motor? and of a trapezoidal wound motor?

Very minimal, less than 1% of a sinusoidal motor.
13%–14% of a trapezoidal motor.

What are the main advantages of an AC brushless (sinusoidal) motor over a DC brush motor?

An AC brushless motor is more accurate (mainly regarding velocity) and requires almost no maintenance. However, the drive electronics are more expensive and more complex.

What types of digital sensors are known to you?

Digital Halls, incremental encoder, a combination of digital halls and an incremental encoder, and absolute serial sensors.

For Elmo drives, how do you know which types of feedback they support when looking at the product catalog number?

Reference it according to the product matrix. “R” is for Resolver, and so on. It is usually the last product code of the catalog number.

What are digital hall sensors used for?

They assist in commutation of the motor for electrical cycle reference. The hall sensors give the location of the rotor to a tolerance of ±30 electrical degrees.

How many hall code states are there in one electrical cycle?

Six states

What are the hall codes’ illegal states?

There are two illegal states in Elmo drives. Those states are ‘111’ and ‘000’

Why do we need 120 Ohm input resistance in the A/B internal differential OP AMP input?

Mainly for noise immunity.

Does the drive perform auto phasing when the Feedback is digital halls + incremental encoder?

No, only phase correction. Namely offset correction of the absolute hall’s zero with respect to the motor’s electrical zero.

What is ampacity? What is the connection between the ampacity and the wire AWG?

The amount of current carrying ability of the wire with respect to its diameter. For example: If you have a thinner cable you can’t transfer so much current. If you have a larger cable diameter you can transfer more current. When the AWG is high the ampacity is low, and when the AWG is low the ampacity is high.

True or false? – For the PE (Protective Earth) the AWG should be as high as possible.


What is recommended regarding the motor – drive power connection cables?

They should be as wide as possible (low AWG), not twisted but shielded. Ground the outside of the motor.

In which drives must we usually use an isolated transformer? Why do we need the isolated transformer?

Low DC voltage drives (less than 200 VDC). The isolated transformer provides isolation between the logic and the power stages of the drive.

What is recommended regarding the main power supply and the drive cable connection?

Twisted and shielded cable. In case of using isolation the PE should be shorted to the power supply PR. In case of DC offline drives (when an isolated transformer is not essential, PE should not be shorted with the PR of the PS.

What would you do in a situation where the cables between the main PS and drive are very long?

Add capacitance; make the wires as wide as possible. This is in order to prevent high voltage drops across the cable edges.

What is the purpose of the auxiliary power supply? Where would you connect the PE to?

It is for the logic in the event of power loss to the driver. The PE is connected to ground via the PR.

What is the maximum current that the internal 5V encoder/halls supply can apply to the encoder/hall device?

200 mA

Why do we use differential encoder signal inputs?

To reduce the noise level and obtain a better signal integrity.

What is the maximum encoder frequency?

20 MHz.

What is the maximum differential analog input voltage that is allowed?

±10 VDC

What is the resolution of the analog input? How do we achieve that with 12 bit ADC?

14 bit. We oversample the 12 bit ADC to reach 14bit resolution.

What is the minimum required voltage to operate the optic isolated digital input? What is the maximum current allowed in this input device?

5 V or 24 V depending on the device.
20 mA

What is the minimum required voltage to operate the optic isolated digital output?

5 V

What types of unit modes (UM) are available in the SimplIQ technology?

UM=1 Current    UM=4 Position
UM=2 Velocity    UM=5 Auxiliary Position
UM=3 Step

What is the precondition before switching from one mode to the other?

Motor Off, MO=0

You have an input analog voltage of ±5 VDC, and you need to apply a current command of ±1 Amps. What would you assign to AS[1], AG[1], RM, TC, MO?

AS[1]=5      RM=1
AG[1]=0.2   TC=0

Speed can be changed "on the fly" during motor jogging in UM=2. True or False?


What is the SF used for? What is its maximum value? What are the units?

To smooth the velocity corners (as jerk). The maximum value is 100 msec.

What are the 5 modes of position referencing known to you?

PTP, PVT, PT, Jog, idle

Write the required commands to move the motor from its current position to position 1000 at a speed of 1000cnt/sec.

MO=0    PX=0    MO=1    PA=0    PV=1000     PA=1000   BG

What commands are required for the drive to accept P&D input from its auxiliary input?

MO=0, UM=4     YA[4]=0    EF[2]=<value for noise>    MO=1

What will be the mechanical count revolution for a resolver with 2 pole pairs (on the resolver) and 13 bit resolution?

2 x 213 = 16,384

Which challenge does the drive face when it has to deal with an analog sensor?

Matching phase offset properly, amplitude matching and lissajous creation of the analog input.

Does the resolver feedback require an auto phasing method?

No in the case of single pole resolver, it is absolute. In other cases it does require auto phasing.

What is the meaning of emulation in our drives?

When an analog signal is channeled into the main feedback, it is emulated to the auxiliary feedback output. This can be converted to a digital signal.

How is it possible to output a digital encoder signal (from the main to the aux) with the Whistle drive?

You must use a line driver to buffer out the signal externally to the WHI

What is the difference between differential and single ended inputs?

Differential is between a ± valve (i.e. ±5 V) and single end is a + valve with respect to zero (i.e. +5 V to 0 V). Differential is better for noise immunity.

Why can the Tacho feedback not close the position servo loop?

It is a generator based on the voltage from the motor. It drifts around 0.

Is it possible to close a position loop with a potentiometer?


What are the impacts of incorrect wiring, grounding and shielding?

Possible drive failure, unsafe conditions, sparks or arching.

What is important to implement when wiring the motor and feedback cables?

Keep the wires as short as possible, with proper shielding and grounding. Also separate them as far as possible (motor and feedback, hot and cold)

Motor cables should be shielded and twisted. True or false?


Feedback cables should be twisted and shielded. True or false?


Power supply cables should be twisted and shielded. True or false?


When will you add additional capacitance to the power supply output? Where would you place it?

One scenario is for a switch mode power supply over a long distance placed near the power supply leads. Capacitance could be in some cases be a replacement for a “shunt” resistor. In some cases it is not sufficient and a shunt regulator is recommended.

What is important to maintain in the connection of several drive grounds?

Common node, star connection (not daisy chain)

When you have shielded a long feedback cable in the system, how would you connect the shield to the PE? Why?

Both ends to eliminate voltage drop to the PE system frame with one open hole end and the other through proper connection shielding.

What elements in the system would you connect to the central grounding point?

All of them, as that is the point of proper grounding. I.e., the power supply, drive and controller.

In which situation would you add ferrite core to the cables?

When there is noise in the lines to the motor phases. Do not wrap the ground wire.

In which situations would you add chocks?

If the motor’s inductance is too low or when the current ripple is too high.

Draw a simplified 3 cascade PIP control loop. What are the advantages of using this structure over a traditional PID control loop?

PIP is easier to tune, can use feed forward and gain scheduling is also possible.

What are the 5 main steps of the composer wizard tuning process?

Current gain tuning, commutation, digital inputs and outputs, velocity gain tuning and position tuning

What types of communications are available for the drive? What is the baud rate of each?

RS232 – up to 57,600 bps
CAN bus – up to 1 Mbps

What is auto phasing? When is it used?

To obtain the proper commutation angle each time the drive is switched on. This is done when no absolute sensor is available.

What is the method for manual velocity tuning?

Set KP=0 and KI=0. Set a range of movement (±1000 counts) and a velocity (±1000 counts/second), and observe the velocity command with respect to the actual velocity. Double KP until there is a 25% overshoot, decrease KP by 75% to start KI and obtain the rise time. Take KP/Rise time and start KI. Increase until the velocity command and the velocity track well.

What is a good indication for instability of the system in the step response?

Signal Oscillation, high overshoot and undershoot.

What would you do in the case of current saturation during the step response tuning of the drive?

Slightly lower the speed of travel. If the saturation is large, decrease it significantly.

What is KI of the velocity used for?

To stabilize the velocity during steady state motion (0 velocity error).

What is the position error (PE)?

The difference between the commanded position with respect to the position feedback, i.e., PCMD-Px.

What is the rise time of a step input?

The time it takes for the velocity to go from 10% of the desired velocity to 90% of the desired velocity.

What types of advanced filters are available in the Composer?

Encoder jitter filters (main and auxiliary), DI LPF, notch filter, low pass single and double pole, lead lag.

Describe the tuning process in 4-5 brief, bullet point steps

Set KP=0 and KI=0. Set a range of movement (±1000 counts) and a velocity (±1000 counts/second), and observe the velocity command with respect to velocity. Double KP until there is a 25% overshoot, decrease KP by 75% to start KI and obtain the rise time. Take KP/Rise time and start KI. Increase until the velocity command and the velocity track well.
Add position tuning: (0.5/Rise time x KP)

What are the advantages of distributed control?

Less wiring, management and monitoring of amplifier status. Overall reduced cost of ownership after everything is assembled. Less EMI.

What is the function of the 120 Ohm resistors?

To maintain the line impedance and noise immunity.

How long does it take to send a PDO message when the baud rate is 500,000 b/s?

About 5 kbps x 100 = 200 µs

What is the binary interpreter method?

The storage and execution of Elmo commands on PDO2.

What will be the COB-ID if I want to send a message to the drive by the binary interpreter to node ID 93?


What is the meaning of the message with COB-ID 0xAF?

Node ID 47 sends an emergency message.

How the does heartbeat mechanism carry out its function?

It pings/polls the CAN bus for the status of Node IDs

Which type of messages require an acknowledgement?

Confirmed messages, usually SDOs

What will be the boot up message for node ID 83?

0x753 – an error message

What is the function of the tdif function?

Time difference counter (counter from a set point with respect to the time declare)

How do the drives represent numbers with a reminder?

Floating will yield a decimal

If I defined three integers, for example, int A = 3; int B = 2; int C = A/B; what will be the representation of the integer C after the calculation?

1, it rounds down

What is the condition for changing between the different unit modes?

The motor must be off.

What are the possible sources of current commands in current mode?

TC (internal)
Analog (external)
PWM (external)

What are the possible sources of the velocity command in velocity mode?

Profiler, external analog command, follower on external encoder

Explain the usage and meaning of the SF (smooth factor)?

Smooth factor is the sinusoidal adjustment of the acceleration.

How would you set VH[2] in relation to HL[2]?

HL[2] is the limitation of VH[2] so VH[2] = HL[2]/2

Elmo drives can output 50% and 100% PWM velocity duty cycle command format through the auxiliary encoder? True or false

100% only since it enables the highest possible resolution.

What is the PM parameter? What is the difference between PM=0 and PM=1?

PM=1 Use the profiler (acc + dec + SF)
PM=0 Bypass the profiler – gives step response to the system.

What movement is faster – SF=50 or with SF=100?


What are the additional functional features of the Advanced version of an Elmo drive?

ECAM, 32 KB of memory.

Why do we need the Follower feature?

Synchronize motion with an external axis.

What is dual position mode? Why do we need it?

Using two encoders as an input to close the position and commutate the motor. For example, it is used when closing the position on  the linear encoder of a ballscrew stage and commutation for the motor on the end.

What is the position reference generator? What are the 5 modes of software referencing?

The position reference generator is a software command, external position reference and stop manager.
Point-to-point, jog, PT, PVT, idle

PTP May be initiated at any time. True or False?


What is Jog? At what mode of operation do we run it?

Moving at a constant velocity. Position unit mode.

What is pulse and direction? Explain

This involves a separate input through the Elmo auxiliary encoder input. Each pulse advances the position by one count and gives the direction. The width of the pulse determines its speed.

Give an example of a machine that can use ECAM.

A textile curtain machine rotating a drum while outputting positions to 50 separate motors in order to sew a certain pattern.

Why may user programs be needed?

For proper machine flow control and to react to external situations that may take place in the machine.

What can affect the execution speed of the user program?

CPU loads. Servo is the highest priority, communication is higher than the user program, and automatic routines can interrupt the program.

How can we measure the execution speed of a given code segment within a user program?

With tdif commands, TM or tick-tock

Can we execute a software interrupt routine (AUTO) when the program is not running?

No, we must use “while” infinite loop

Explain at least two possible structures for a user program.

Wait execute wait
Infinite loop waiting for a condition
Use autoroutines

Explain the usage and the syntax of the “until” command. Can you create an infinite “until” by mistake? If yes, give an example.

Until waits for a condition to be true.
“until (0)” will never end

Which types of variable are supported by the SimplIQ drive?

Float, integer, functions, global functions, operands (conditional, rational, mathematical)

What are the means/tools for user program debugging?

Step by step, break points

Explain the usage of the “if” command.

An expression or iteration that filters or executes a specified routine when the conditions are met.


How do I close an analog position loop without a digital drive?

Use a linear analog feedback (a potentiometer for example) and connect it to the feedback input of the amplifier. Connect your position command to the reference input. Then convert the amplifier to Current Mode and adjust the gain for best position accuracy.

What is the difference between PWM and Direction control, and Step and Direction control?

PWM and Direction is a digital method of transferring analog information. It is mainly useful when optical isolation is required between the source of the command and the amplifier. Step and Direction is a position control method and it requires a position controller on top of the analog amplifier.

We have an existing design that uses a linear amplifier with a bipolar power supply. Can we use the same power supply for a PWM amplifier?

The PWM amplifier requires a unipolar power supply. This means that the bipolar power supply is wasted when used with a PWM drive. If you insist on using your bipolar supply with a PWM amplifier, pay special attention to the circuit common connection and common mode issues. You may be required to isolate your reference command.

What is preferred for better speed control: a low-voltage tachgenerator or a high-voltage tachogenerator? a low-inductance tachogenerator or a high-inductance tachogenerator?

Higher voltage and lower inductance is preferred for servo performance with a tachogenerator.

I have a voice coil motor that has only two leads (like a brush motor) but has no brushes. Should I use a brush or a brushless drive for this type of motor?

Despite the fact that the voice coil motor is a “brushless drive”, you can use a standard brush drive with it.

Can we drive a brushless motor that has an optical encoder only but no Hall sensors?

There are many optical encoders in the market that provide the “Hall sensor signal” through optical channels. If your encoder is one of these, just treat these outputs as if they are Hall sensors and connect them to what is defined in the amplifier literature as Hall inputs.

Can we control the speed of a brushless motor with Hall sensors only?

Yes you can. Elmo has achieved 200:1 speed regulation with Hall sensor speed control using inexpensive analog drives.

What is better: a trapezoidal or sinusoidal brushless drive?

A good trapezoidal drive is a less expensive drive and it will satisfy most applications, especially the point-to-point ones. The sinusoidal drive is more expensive and is required for better low speed smoothness.

What is better: an optical encoder or a Resolver?

The resolver withstands a harsh environment better but its R/D circuit is expensive and it is also bandwidth limited. The optical encoder is more fragile than the Resolver but it performs better for servo applications and is also less expensive.

Why do we want to use digital drives?

Although digital drives are more expensive, they are friendlier than analog ones and have excellent long-term stability. Also, their serviceability is far easier than analog drives.