Mr. Jong-Mook Choi Chief Researcher R&D center of Rotem Company

Relevant Education 1977~1981 Majored in Electrical Engineering and Graduated from

Yonsei University in Koera 1981~1983 Master of Engineering in Electrical Engineering and

Graduated from Yonsei University in Koera

Relevant Experience 2002~ Engaged in Athens Metro Line 2 Greece EMU, Daegu

Subway Line 2 EMU Project, SMSC Line 2 EMU Project, Pusan Line 3 EMU Project as a Chief Researcher

1995~2001 Engaged in Taiwan Railway Administration (TRA) EMU , Gwangju Subway Line 1 Project as a Principle Research Engineer

1990~1994 Engaged in Seoul Metropolitan Government Corporation (SMSC) Kwachun EMU Project as a Senior Research Engineer

1985~1989 Engaged in New Saemaul DHC (Diesel Hydraulic Coach) Project as a Junior Research Engineer

1982~1984 Engaged in Seoul Metropolitan Government Corporation (SMSC) Line #2,3,4 EMU Project as a Electrical Design Engineer

1981~ Entered the company

A study on the technical development of propulsion and control system for electric multiple unit

Jong-Mook Choi Chief Researcher R&D center of Rotem Company 462-18, Sam-Dong, Uiwang-Shi, Kyunggi-Do, 437-718, Korea Tel : +82-31-460-1240, E-mail : jmchoi@rotem.co.kr

Abstract

In recent, the rail road industry requires the high level of reliability and safety, the cost

reduction, the maximizing of operation efficiency, the various services and the

minimizing of maintenance time and cost.

For these requirements, the high reliable electric equipments (e.g. Inverter, Aux power

supply, traction motor etc.), fast network technique and the newest IT technology are

being applied to rail road industry. The various electric systems are developed and

applied to railroad industry such as propulsion system, braking system, train signal

system, HVAC system, passenger service system and train control/monitoring system.

This paper explains the new trend of rail road industry, especially the propulsion control

system, traction motor and train control communication. In order to achieve the new

trend, present the issues that have to be resolved.

A study on the technical development of propulsion and control system for electric multiple unit

Jong-Mook Choi Chief Researcher R&D center of Rotem Company

462-18, Sam-Dong, Uiwang-Shi, Kyunggi-Do, 437-718, Korea Tel : +82-31-460-1240, E-mail : jmchoi@rotem.co.kr

1. Introduction In recent, the rail road industry requires the high level of reliability and safety, the cost

reduction, the maximizing of operation efficiency, the various services and the

minimizing of maintenance time and cost.

For these requirements, the high reliable electric equipments (e.g. Inverter, Aux power

supply, traction motor etc.), fast network technique and the newest IT technology are

being applied to rail road industry. The various electric systems are developed and

applied to railroad industry such as propulsion system, braking system, train signal

system, HVAC system, passenger service system and train control/monitoring system.

This paper explains the new trend of rail road industry, especially the propulsion control

system, traction motor and train control communication. In order to achieve the new

trend, present the issues that have to be resolved.

2. Propulsion System 2.1. Development of propulsion system

Fig.1 Development of Propulsion system

DC motor has been widely used in electric traction for many years since it is

Thyristor GTO Thyristor

Op-amp & Discrete

Logic Device u-Processor DSP & FPGA

Chopper

CONTROL

SEMI-

CONDUCTOR

PROCESSOR

IGBT (IPM)

Rheostatic 4Q Chopper VVVF INV.

MOTOR DC

MOTOR

DC MOTOR Series Wound

DC MOTOR Separate Wound

Induction

MOTOR PMSM

relatively easy to control it in order to achieve the desired motion. It was seen earlier

that the DC motor can be controlled by varying the voltage and current applied to it.

The simplest way of doing this is by adding or removing resistance in the motor

circuit. A resistor in series with the motor will reduce the current flowing in the circuit

and reduce the voltage across the motor terminals, and so the motor can be

controlled by varying that resistance.

In practice, to achieve an acceptably smooth control of the motor, many different

resistors are progressively switched in or out. This is generally achieved by using a

rotating camshaft fitted with cams profiled to switch the various resistors in the

correct sequence determined by the camshaft position which is in response to the

driver’s operation of the controls.

After the resistor control, a more advanced electronic device known as a thyristor

has been used for traction control. This effectively acts like a diode in that it only

permits conduction in one direction, however the point at which it starts conduction

can be controlled by application of a control pulse to a third terminal on the device

called the gate. The thyristors are also configured in a bridge circuit to provide

controlled rectification, in which the turn on point is often referred to as the phase

angle. Turn off comes automatically at the reversal of the incoming AC waveform, a

feature known as natural commutation.

A further advance in silicon semiconductor technology is a thyristor that can be

switched off as well as on. This is known as the Gate Turn Off Thyristor or GTO for

short. Here, the application of a positive control pulse to the gate will turn it on, while

a negative pulse turns it off again. This opens up further possibilities.

The most significant disadvantage of the DC motor is its cost which is largely

attributable to the work involved with the commutator, both to build and maintain.

AC motors provide a simpler alternative of which there are two main types -

synchronous and asynchronous of which the latter has been most widely adopted

for electric traction. Here 3-phase AC is applied to the stator, creating a rotating

magnetic field which draws the rotor round with it, without need for external

connections to the rotor.

A circuit which converts DC into AC is known as an inverter and is effectively the

reverse of a rectifier bridge. Here the DC input is chopped up for narrow time

intervals (pulse width) proportional to the magnitude of the AC waveform required at

that particular instant (i.e. the pulses are widest at the peak of the AC waveform).

The Insulated Gate Bipolar Transistor (IGBT) is the latest generation of power

electronic device to see use in traction drives. These devices can be switched much

faster than their GTO predecessors (in excess of 2000 operations per second) and

require simpler control electronics to switch them on and off.

2.2 Development of Power Semiconductors

The recent development of power electronics equipments are mostly owing to the

rapid development of power semiconductors. Therefore, development of traction

inverter must depend on the future development of power semiconductors of the

evolution of brand new power semiconductors.

Recently, the semiconductors for high power traction application are IGBTs(including

IPMs), Because the voltage rating of IGBTs are of 3300V or higher, current rating is

1200A or higher. most of the traction inverter for EMU(Electric Multiple Unit) are

using IGBTs as switching devices. For the ease of power stack design, and more

safe operation, IGBT and its gate drivers are combined, known as IPM, so that the

protection function is more easy to realized, and more compact design is possible in

traction inverter.

Also, the IGBTs are now developing in two directions. First one is developing IGBT

specialized in low on-state loss which is suitable for the low frequency switching

applications, and the other one is specialized in high switching frequency. It is

related with the base material doping and thickness of the device.

Fig.2 Development of high power IGBT

Though the IGBTs will have been the main component for the near future, one

possible semiconductor that can be used as a main switching device is SiC

semiconductor.

Presently, almost all of the power electronics converter systems use silicon- (Si-)

based power semiconductor switches. The performance of these systems is

approaching the theoretical limits of the Si fundamental material properties. The

emergence of silicon carbide- (SiC-) based power semiconductor switches likely will

result in substantial improvements in the performance of power electronics

converter systems in transportation applications. SiC is a wide-bandgap

semiconductor, and SiC-based power switches can be used in electric traction

drives and other automotive electrical subsystems with many benefits compared

with Si-based switches.

As mentioned earlier, SiC is a wide-band gap semiconductor, and this property of

SiC is expected to yield greatly superior power electronics devices once processing

and fabrication issues with this material are solved. Some of the advantages of SiC

compared with Si based power devices are as follows:

1. SiC-based power devices have higher breakdown voltages (5 to 30 times

higher than those of Si) because of their higher electric breakdown field.

2. SiC devices are thinner, and they have lower on-resistances. The substantially

higher breakdown voltage for SiC allows higher concentrations of doping and

consequently a lower series resistance. For low breakdown voltage devices (~50V),

SiC unipolar device on-resistances are around 100 times less; and at higher

breakdown voltages (~5000V), they are up to 300 times less. With lower Ron, SiC

unipolar power devices have lower conduction losses and therefore higher overall

efficiency.

3. SiC has a higher thermal conductivity and thus a lower junction-to-case thermal

resistance, Rth-jc. This means heat is more easily conducted away from the device

junction, and thus the device temperature increase is slower.

4. SiC can operate at high temperatures because of its wider bandgap. SiC

device operation at up to 600°C is mentioned in the literature. Most Si devices, on

the other hand, can operate at a maximum junction temperature of only 150°C.

5. Forward and reverse characteristics of SiC power devices vary only slightly

with temperature and time; therefore, SiC devices are more reliable.

6. SiC-based devices have excellent reverse recovery characteristics. With less

reverse recovery current, the switching losses and electromagnetic interference

(EMI) are reduced and there is less or no need for snubber.

Fig.3 600V/75A SiC Shottky Diode

The SiC switching device jointly developed by KEPCO and Cree for the new 110

kVA inverter is a 4.5kV, 100A design called the SiC Commutated Gate Turn-off

Thyristor (SiCGT). This switch, which can turn off or on in less than 2 microseconds,

has a switching speed 10 times faster than that of an equivalently rated silicon Gate

Turn-off Thyristor (GTO). This device does not require a snubber circuit, a

commonly used protective circuit for GTOs, thus reducing the part count and heat

dissipation.

A SiCGT module was then developed that utilizes one SiCGT and one 6mm x 6mm

SiC PiN diode in a metal can package. The module can operate at higher

temperatures (300°C) than conventional silicon modules (125°C), by utilizing a new

high-temperature resin for dielectric insulation. Using six of these modules, the

three-phase Pulse Width Modulation (PWM) inverter demonstrated an output power

of about 110 kVA. The PWM frequency was 2 kHz.

2.3. Hybrid Traction System

Fig. 4 Series Hybrid Configuration

Conventional Diesel Train

Gear Hydraulic

Transmission

Hybrid Train

Wheel

Axle

G Converter Inverter M Gear

Wheel

Axle

Battery (Energy Storage)

ENGINE

ENGINE

This system uses a series-hybrid configuration (Fig.4) that first converts the engine

output into electrical power and uses only motors for propulsion. The AC output

generated by the engine is converted to a VVVF AC supply by the main converter to

drive the induction motors. Storage batteries are located on the intermediate DC

section of the main converter, and the charging and discharging of the storage

batteries is controlled using output adjustment of the converter and inverter.

The series-hybrid system allows the engine speed to be set irrespective of the

vehicle speed, thereby permitting high-efficiency power generation by operating

predominantly in the low fuel consumption engine speed range. This also reduces

exhaust gases. The use of electric train inverter control technology allows the use of

regenerative braking, and using regenerated energy temporarily stored in the

batteries as auxiliary power for acceleration is expected to give fuel savings of

approximately 20% compared with conventional diesel trains. An engine cut-out

control is also employed to reduce noise when stopped at stations.

The series-hybrid system eliminates the need for equipment such as hydraulic

transmissions, which entail high maintenance costs on conventional diesel trains.

Similarly, commonality of equipment with electric trains saves maintenance labor

and allows more efficient utilization of existing inspection equipment.

The output from the storage batteries and engine are controlled as follows

according to the running conditions

1) Accelerating : The storage batteries alone are used for acceleration at low

speeds, and additional power is provided by the engine generator from the mid-

speed range.

2) Braking : The engine is shut down and regenerated power is stored in the

batteries.

3) Constant-speed braking : Regenerated power is absorbed using engine braking

to prevent overcharging on continuous downhill gradients.

4) Stationary : The engine is shut down to reduce noise in stations and improve fuel

consumption.

Stored energy must be available to the full extent possible from the start of

acceleration in order to accelerate up to high speeds using storage-battery power.

Similarly, reducing stored energy at the start of deceleration from high speeds using

the train brakes enables more regenerated energy to be absorbed.

The hybrid propulsion system uses energy management controls to maintain the

optimum level of stored energy to suit the speed range. This is achieved by

adjusting the engine generator output so as to ensure a constant total of vehicle

kinetic energy and battery stored energy.

The hybrid propulsion system utilizes constant-power converter control using

electrical power in order to manage the energy balance of the DC section. This is

because charge/discharge control is not possible using conventional constant-

voltage converter control, as the voltage at the DC section between the converter

and inverter varies depending on the storage-battery energy. New constant-power

converter control has been developed to manage the energy balance for the DC

section using electrical power, allowing optimum charge/discharge control.

The hybrid propulsion system uses holding-speed engine brake control that

augments conventional braking so as to maintain a constant speed (holding speed)

on downhill gradients. The series-hybrid system achieves engine braking by using

the braking power generated by the traction motors and inverter to drive the

converter and generator in reverse and absorb energy with the engine load. This

has the advantage of allowing the engine braking power to be set irrespective of the

vehicle speed, which enables the hybrid propulsion system to provide stable,

constant-speed running under any running conditions.

2.4. Traction System with Energy Storage Device

Fig.5 Energy storage of traction Inverter

Recent development of energy storage devices such as super-capacitor(Ultra-

capacitor) make s it possible that no pantograph or current collecting equipment on

the train. Practically, this system can reduce the construction cost of urban subway

system because the tunnel space can be reduced drastically.

On the station, the train stops and the necessary energy to run to next station shall

be stored on energy storage device. Of course, the regenerated energy during the

braking may reduce the required amount of charging energy in station area. The key

technology of this scheme is the energy density of storage device. Supercapacitor is

now believed to be able to meet this requirement and the energy density will be

ChargingM

SuperCap

Powering

Regenerating

M

SuperCap

ChargingM

SuperCap

Station (Charging)

Powering & Regenerating Braking

Station (Charging)

improved continuously.

2.5. Fuel Cell Propulsion System

Fig.6 Operating principle of fuel cell propulsion

Recently, some of buses equipped the fuel cell as a energy source for driving are

running in United States. In train system, the first test train with the fuel cell has

been developed in Japan. Because it has pollution-free fuel cell as an energy

source, it is environment friendly, and because it doesn’t need catenary line, it has

space saving effect for construction.

The safety of hydrogen tank has been tested in vehicle applications, but in train

application it needs more test till now. Also, the running distance of train with limited

hydrogen and infrastructure for supplying the hydrogen will be the remaining

problem to solve for this train system.

3. Traction Motor 3.1 Developments of traction motor

With the progress in power electronics and advanced control technologies, the

rolling stock drive system has changed from DC motor to the induction motor

system. This has brought about a compact size, light weight and maintenance-free

and there has been a growing need for further advances in this direction. However,

the conventional main circuit system has almost reached the stage maturity, and it

is difficult to expect further improvement from the conventional system. To solve this

problem, some kind of new drive systems like as Direct Drive Motor, Linear

MOTOR

FCC

Battery

DC/DC Converter Controller

BMS MCU

Gear

PCU

3f

Inverter

DC/DC Converter

(V)

(A)

Power Line

Fuel Cell

Control Line

Induction Motor are developed and introduced to meet the requirements of

reduction of maintenance cost, increasing of reliability, easy to maintenance.

The development of the new technology for the traction motor is as follows.

One is to reduce the energy cost. High quality magnetic material and high

conductivity materials are developed in conventional induction motor. But nowadays,

permanent magnet synchronous motor (PMSM) is applied in traction drive system

based on the high efficiency characteristics.

The other is to reduce maintenance cost, totally enclosed motor, gearless motor,

application of long life bearing and ductless motor are studied and developed to

solve these requirements.

3.2 Application of PMSM Generally, permanent magnet synchronous motors are high efficient because of no

heat generation in its rotation parts, it is compared with the induction motor that are

used rotor bars and end-rings. This means that PMSM has characteristics of less

energy consumption and less effort to cool the traction motor. So it is possible to

apply totally enclosed type cooling method. This gives additional merits like as low

noise and maintenance free. If the motor has a totally enclosed structure, the noise

is shielded and dust is not penetrate to inside motor. Generally, conventional

traction motors are required periodical maintenance to clean dust and bearing

pollution inside motor. If there is no penetration of dust, it is possible to extend

maintenance period as long as bearing life.

PMSM can be divided by two types of driving system. One is Direct Drive Motor to

eliminate the transmission loss. The other is indirect drive system to be changed

conventional induction motor to PMSM.

Direct Drive Motor is possible to maximize energy efficiency by eliminating reduction

gear and by application of permanent magnet motor, to minimize noise by low

revolution of motor. But DDM system has problems like as high un-sprung mass

and difficulty of maintenance in case of DDM failed. Though indirect drive system

has also some disadvantage, it will be applied some period.

The requirement technologies of PMSM are development of High-quality magnet

material, demagnetization at high temperature and Reduction of Production cost.

Table 1. Compare indirect drive motor with DDM

Index Advantage Disadvantage

Indirect drive motor

(Conventional System) � Light size & weight

� Complex Construction

� High Noise

Direct drive motor

(DDM System)

� Simple Construction

� High Efficiency

� Low Noise

� High un-sprung Mass

� Difficulty of maintenance

� High impact on DDM

Fig.7 Totally-Enclosed type PMSM Fig. 8 Direct drive motor

Fig. 9 compares indirect drive motor with DDM

3.3 Application of totally-enclosed type of traction motor

(1) Conventional self ventilation type traction motor

Fig 10. Conventional self ventilation type traction motor

Conventional self ventilation type traction motor is lighter and smaller than same

capacity enclosed traction motor (without forced cooling systems) and shorter

maintenance interval, but practical maintenance interval is 3 year overhaul and 6

year heavy overhaul.

(2) Totally-Enclosed Type Traction Motor

Fig 11. Two-circuit cooling system for an enclosed traction motor

Totally-enclosed type traction motor is heavier and larger than same capacity self-

ventilation type traction motor (without forced cooling systems) and longer

maintenance interval and higher maintenance cost reflecting that the structure of

motor is some complicated.

Air flow of fresh air

Air flow of internal

circular air

Air flow of fresh air

3.3 Development of LIM (Linear Induction Motor) The LIM is similar to the rotary motors which is split and spread. The primary coil of

the linear induction motor is mounted on the bogie, while the secondary reaction

plate is installed on the rail.

The LIM can be operated on steep gradient easily, does not depend on adhesion;

no sliding, no slip and can run through tight curves smoothly.

And the LIM has no friction, so can be lower noise and comfortable ride. Further

more, there are no rotation parts, so can be free maintenance.

Fig.11 Linear induction motor

To achieve the linear induction motor, followings have to be resolved.

- Improvement of efficiency and power factor

- Reduction of the weight and size

- Adoption of high conductivity material for reaction plate

4. Communication Network

4.1. Development of communication network

In recent, the rail road industry area requires the high safety level, the cost reduction,

the maximizing of operation efficiency, the various services and the minimizing of

maintenance time. Also, the service providers produce the new business model

such as making the new profits through the various advertisements except the

original purpose of huge transportation. For this requirement, the high reliable and

fast network technique and the newest IT technology are being applied to railroad

industry. The various electric systems are developed and applied to railroad industry

Electro-

such as propulsion system, braking system, train signal system, heating ventilation

and air conditioning system, passenger service system and train control/monitoring

system. These discrete electronic systems are connected through the on-board train

network. For the train networks, the international standards such as IEEE 1473-L,

IEEE 1473-T and IEC 61375 are established in the end of 1900s.

4.2. IEEE 1473-T/IEC61375 Network Application

Fig.12 The system configuration of metro adopting IEEE 1473-T/IEC61375

The figure 12 shows the application of IEEE1473-T network (same as IEC61375) on

the late electrical urban train which can run without the driver using auto control

function. In the figure 1, the connection of electronic systems through the network

can be seen. The IEC61375 network is divided into WTB(Wire Train Bus) and

MVB(Multifunction Vehicle Bus). The WTB is used as the train main network and

supports the long distance communication. So, it can be called as a back-born

network of train formation. The MVB is used to connect several sub-systems

installed in the same vehicle or neighbor-vehicle. The MVB is divided into three

types such as ESD(Electrical Short Distance, 20m), EMD(Electrical Middle Distance,

200m) and OGF(Optical Glass Fiber).

Because the most of train operation functions are controlled with the electronic

systems, those systems should have the high reliability. So, the train networks have

the redundancy concept for their high reliability. The amount of hardwired train

cables can be reduced by applying the network. And the amount of failure points

can be reduced by adopting the electric sub systems. The figure 13 shows the

system configuration of high speed train which was developed by Korea itself.

Fig.13 The system configuration of high speed train adopting IEC61375

4.3. IEEE 1473-L Network Application

Fig.14 The system configuration of metro adopting IEEE 1473-L Network

The figure 14 shows the application of IEEE1473-L network on the late electrical

urban train which can run without the driver using auto control function. In the figure,

the train control and monitoring system is connected with IEEE1473-L network and

the train sub electronic systems are connected to monitoring system through the car

network. The train network is connected to TLC/TMX(Train Line Controller/Train

Multiplexer) and the TLC/TMS can connected to other train formation for the multiple

operation. Information Indication System, Train-number Indication System,

Destination Indication System and Indoor Passenger Address System are

connected to audio train communication line.

In recent, the railroad service providers require the train manufacturer to design the

train which can support the multiple operating and the easy changeable formation

for the efficiency of revenue service. The high lever train network can support the

easy multiple operating and easy changeable formation. The figure 15 shows the

multiple operating concept of the Irish DMU.

Fig.15 The multiple operating concept of the Irish DMU

4.4. System Redundancy Scheme

Fig.16 The redundancy concept of TCMS

CCU1CBTC

CCU2

VCU1Master Controller

Driver’s Console

DoorControl

ECU ECU

VVVF

DCU DCU

ZVR

DoorControl

(Vita

l)

(Vita

l)

WTB, Redundancy

MV

B R

edun

danc

y

PWM Generator

Powering/Braking CommandPWM

Powering ModeBraking Mode

Tc1 WayTc2 Way

Back-up Mode

Door Open/Close Command

PW

M

Analog

Ana

log

RS-485 I/F

DI/DO/Analog I/F

EBR

(Vital)

DI/DO I/F

MV

B

Red

unda

ncy

DI/DO/Analog I/F

FDU FDU

DIDI

Fire Detection

Driv

ing

Mod

e S

elec

tion

As explained above, many electronic systems are applied on train. And, an

electronic system failure can be a cause of the revenue service stopping. So, in

recent, the system design concept is to go to reduce the impact of electronic

system’s failure to the service. In the figure 16, the redundancy concept of

TCMS(Train Control and Monitoring System) is shown. In the figure, TCMS is

composed of CCU(Central Control Unit) and VCU(Vehicle Control Unit), which are

connected each other through the WTB network. As the core device of TCMS, CCU

monitors the train status using most of all train status data collected from VCUs and

train relay logic. Also, CCU makes the train control command and conducts that

command. According to these CCU roles, CCU has to be designed using the

redundancy concept.

In recent, the train status information displayed as analog gages or lamps such as

line voltage, main air pressure, motor current and other train main status are

displayed on driver’s screen device. So, the driver’s display unit should be designed

according to the redundancy concept, also. The figure 17 shows the two driver’s

display units installed on driver’s desk and the backup concept of those display units.

Fig.17 The picture of driver’s display units and backup concept

4.5. Wireless LAN Application

Fig.18 The wireless LAN application concept

In the rate train, the failure data and operation history data recorded in memory

device of monitoring system are moved to depot system installed in train depot

using IC memory card or PCMCIA card after the revenue service. The depot system

manages and analyzes those moved data. But, the wireless LAN system is to be

applied on train and on the track side or on the stations progressively. The data

moved to depot system using PCMCIA card is transferred to depot system using the

wireless LAN network as real time. So, the maintenance staff members can know

the train failure status with so fast. This means the fast maintenance action, the

safety revenue services and the growing up of profits. The figure 18 shows the

wireless LAN application concept.

Laser Printer Laser Printer

Depot System Depot System

AP(Access Point)

RTD

TCMS

Antenna

RS-485

RTD

TCMS

Antenna

RS-485

NMIS Network

AP(Access Point)

HUB HUB

PTU(Portable Test Unit)

RS-232

Jeong-ja Station Bun-dang Depot

USB Memory

Stick

4.6. Ethernet Application

Fig.19 The Ethernet application concept

Figure 19 shows the Ethernet application on the AC train of JR, Japan. In figure, the

Ethernet network is used as a main train network and the LonWorks which follows

the IEEE1473-L specification is used as a local car network. The communication

speed of the Ethernet is 10Mbps and control/status data, video/audio data are

flowing through the Ethernet network. Thus, because the Ethernet is so much

important as the main train network, it is designed using redundancy concept.

5. Summary

As explained already, the new trend of traction system, traction motor and

communication are achieved in near future.

The future version of traction system is to become a hybrid system and fuel cell system.

The types of PMSM (Permanent Magnet Synchronous Motor), DDM (Direct Drive

Motor) and LIM (Linear Induction Motor) will be widely applied.

The new trends of control and communication are the applying of faster and lower cost

network, redundancy design concept and wireless network. And to achieve the new

trend of control and communication, the electromagnetic noise, heating, impact of

vibration and fast recover from error have to be resolved.