A die casting machine requires different flow and pressure at different stages of the casting processing. For the pump motor, the load of casting machine is always changing. In the fixed displacement pump system, the oil pump controls oil volume in constant speed, and redundant oil returns through the relief valve. This means, which is high-pressure throttling, is estimated to cause energy loss up to 36%~68%. The power allocation directly results in energy loss and affects the work condition of machine. Changing the voltage, current and frequency of the motor are the methods of adjusting the motor. However, changing the current will cause heating of resistor and a waste of energy; adjusting the voltage can limit the flow, but it weakens the transmission capacity. Lowering the speed under rated load will dramatically increase internal current and cause failure of the equipment. Therefore, adjusting the frequency and speed is the best choice. Vector frequency technology can also be applied to reduce the speed, acquire a larger torque output and achieve efficient conversion of mechanical energy. Because of customers’ higher demands on die casting machines, wider application and lower prices of servo systems, servo control technology gradually becomes preferable for the driving unit of die casting machines. Energy-saving servo technology is an important breakthrough of hydraulic driving technology of die casting machines. The servo system with a high price-performance ratio perfectly meets customers’ requirements in cost and efficiency, which is the development trend of die casting machines.

(Ⅰ) The concept of servo system

1.1. Generally speaking, a servo system is a device that controls the position, mode and condition of a driven mechanism according to the signal. The servo system referred to in this article is a mechanism that tracks and controls the flow and pressure of the hydraulic system of the die casting machine so that the values of these two parameters change with the CPU commands within certain range.

1.2. Vector control is the control of both magnitude of current of the motor rotor and the rotor current’s magnetomotive force phase generated in the air gap.

(Ⅱ) Main components of the servo system
Programmable logic controller (PLC), analog module, digital module, touch screen, servo drive, servo motor, filter reactor, regenerative braking resistor, pressure sensor and so on. The servo system is shown in the diagram below:  

2.1  Touch screen in this system is used to display and confirm the messages of setting, monitoring, alarm and warning. It also has the functions such as user group management, I / O information monitoring and user help. Users can easily set the system pressure and flow of any stage. 
2.2  PLC monitors and judges the motions of the die casting machine according to the program, issues corresponding pressure command and flow  command at different injection phases to the analog-to-digital conversion module, controls the servo drive on the functions of “enable” and alarm according to the condition of servo system and sends instructions to the servo for high-precision tracking and control.

2.3  Digital module is mainly related to the conversion of signal input and output of PLC and has the functions like circuit isolation and filtering.

2.4  Analog module includes analog input and output modules and is used to input and output the external analog signals such as signals from the transducer and pressure sensor, the pressure signal and flow signal for analog-to-digital A / D conversion or digital-to-analog D / A conversion.

2.5  The servo drive is used for controlling the servo motor. The mainstream servo drive applying digital signal processor (DSP) as the key of controlling can implement complex control algorithms, which is digital, networked and intelligent. Most of the power devices now adopt intelligent power module (IPM)-oriented drive circuit. The IPM integrates a driver circuit, detecting and protective circuits in case of over voltage, over current, overheating, and undervoltage, as well as the soft start circuit in the main circuit to reduce impact on the servo drive during startup. With the three-phase full-bridge rectifier circuit, the power drive unit rectifies the input three-phase power or electric supply to direct current. The rectified and filtered direct current has to undergo frequency conversion finished by the PWM-based voltage inverter before driving the AC servo motor. Inversion is a process that several switching devices V1-V6 as shown below, with given firing angle and frequency, keep alternately turning on and off. Sinusoidal pulse width modulation (SPWM) is that the pulse width and duty cycle change in accordance with a sinusoidal rule. Though the voltage is constituted by a series of rectangular pulses, the input current of the motor is very close to sinusoidal wave because the motor windings are inductive. The right figure below shows the relationship among a phase of frequency, current and voltage:

To put it simply, the whole working process of the power drive unit is a AC-DC-AC process. The main circuit topology of the rectifying unit (AC-DC) is a three-phase full-bridge non controlled rectifier circuit. Generally speaking, servo drives have three controlling methods: position, velocity and torque. They are cutting edge technology that is mainly applied in high-precision positioning systems.

2.6  The servo motor in this system is a three-phase permanent magnet synchronous motor (PMSM) which is equipped with three-phase parallel armature windings on the stator and permanent physical on the rotor surface generating a constant magnetic field. The encoder is a rotating transformer that output the angle between the encoder rotor and stator, as well as the rotor speed. In this way, we can get the angle between the rotor and the winding axis, as well as the rotating speed. The servo drive can control the time phase and value of the stator current according to the spatial position of the magnetic pole of the rotor to realize vector control.

2.7  Filter reactor is to limit the charging and short circuit current, suppress the surge voltage and current generated by the servo drive and minimize the higher harmonic and distorted harmonic.

2.8  During motor braking, with the regenerative braking resistor, the regenerative energy will be introduced to the resistor so that the excessively high DC bus voltage will be consumed in the form of heat, reducing the heating of motor.

2.9  The pressure sensor sends the feedback to the controller with the real-time pressure of 0-10V proportional to 0-500bar.

(Ⅲ) The control principle and method of energy-saving servo system

3.1  Diagram of the control principle:

3.2  The working principle of servo system
Under normal operation, the PLC output corresponding pressure analog signal (0-10V proportional to 0-160Bar)and flow analog signal (0-10V proportional to 0-100% the product of rated motor speed and the pump displacement ) to the servo drive at different injection phases according to the program and user settings. The servo drive receives these signals, as well as the real system pressure analog signal from the pressure sensor. The control unit then works out the rotating speed analog signal within certain pressure range. This given signal and the motor’s real speed signal control the rotary loop of the servo drive. The servo motor works (with speed closed loop) following the speed command from the servo drive, and the pressure closed loop is formed. When the actual pressure is higher than the set pressure and pressure relief is required, the control unit output the analog that controls the torque of servo drive to the current loop. The pump discharges with the counter force of the oil pressure.

The efficiency of servo motor adopted by the servo system is over 92%, while that of the asynchronous motor applied in ordinary die casting machine (including dosing pumps or variable displacement pump machine) is about 80%. Under no-load and low-load conditions, the output efficiency of asynchronous motor is even lower. In other words, for the same work, the servo system consumes much less power.

The key of the distinctive controlling mode of the servo system is that the flow and pressure feedback signals synchronously control the pressure and flow of the machine. The adoption of feedback closed-loop control greatly reduces the consumption due to the motor’s useless work, which maximum the energy savings. The three-phase permanent magnet servo motor and the closed loop vector technology make sure the servo system gets high torque output even under rather low speed, and the motor has rapid response. In this way, the work efficiency of the machine is improved without influencing the work performance.  When the system pressure reaches the set value, the servo system will not continue oil supply like the power frequency or variable frequency control system at maximum speed or set speed. Therefore the oil temperature is considerably reduced and the system performance can be improved.

(Ⅳ) Analysis of the performance of servo system applied in die casting machine
Take Yizumi DM400 die casting machine as an example. The power-frequency asynchronous motor and servo motor are tested separately.

Parameter settings of DM die casting machine

Injection options
Machine model: DM400				2nd phase	deselect	3rd phase	deselect
Intensification triggering:	Select position	Auto triggering of cycle (s)	3	Core pulling	deselect	tracking	deselect
General parameters
Cooling time (s)	2	Plunger back delay (s)	15	Shot time (s)	5	Charging time (s)	5
System pressure (Bar)	160	System flow (%)	100	Accumulator pressure (Bar)	160	Charging time (s)	3
Clamping parameters				Mold opening parameters
Name	Pressure (Bar)	Velocity (%)	Position (MM)	Name	Pressure (Bar)	Velocity (%)	Position (MM)
Slow	70	60	550	High pressure	160	20	600
Fast	80	100	530	Fast	80	100	420
Low pressure	30	80	220		70
High pressure	160	60	200
Injection parameters				Plunger backward parameters
Name	Pressure (Bar)	Velocity (%)	Position (MM)	Name	Pressure (Bar)	Velocity (%)	Position (MM)
3rd phase	Accumulator pressure	100	305	Plunger backward	100	100
Intensification	Accumulator pressure	100	585
Ejector forward parameters				Ejector backward parameters
Name	Pressure (Bar)	Velocity (%)	Delay (s)	Name	Pressure (Bar)	Velocity (%)	Delay (s)
Ejector forward	100	100	1	Ejector backward	100	100	1

The power-frequency asynchronous motor and servo motor are tested separately while the data above remain the same. The test results are as follows:

Data on Energy Consumptio		Mains-frequency motor	Servo motor
	Energy consumption of the first hour of auto operation (kW·h)	16	10.4
	Energy consumption of the second hour of auto operation (kW·h)	16	10.4
	Total energy consumption of two-hour auto operation (kW·h)	32	20.8
	Energy consumption of a cycle (kW·h)	0.122	0.067
	Max. load current (A)	50.7	85.5
	No-load current (A)	19.7	1.5
	Time of an oil temperature increase from 43.6℃to57.8℃(H), with the environment temperature of29.6 ℃and no cooling water.		4
	Time of an oil temperature increase from29.4℃to50℃(H), with the environment temperature of29.4℃and no cooling water.	1
Power Parameters	Cycles in the first hour of auto operation (times)	131	155
	Cycles in the second hour of auto operation (times)	131	155
	Cycles in two-hour auto operation (times)	262	310
	Average cycle time (s)	27.48	23.2
	Interference of the transducer (Yes/ No)	No	No
	Interference of the pressure sensor (Yes/ No)	No	No

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Statistics show that the servo system
1) saves electricity of 5.6 kW•h per hour on average, with an energy-saving rate of 45% per mold use.
2) has few oil temperature increase.
3) improves the production efficiency by 18.3% per hour on average
4) has very low no-load current.
5) does not interfere other control, without any failure. 

(Ⅴ) Features and advantages of servo die casting machine
1) Saving more energy per cycle: eliminating high-pressure throttling, saving energy by 30%-60% compared with fixed displacement pump.
2) Fast response of servo system: Pressure increasing from 0 to 100% takes 30ms at most, improving productivity.
3) Lower temperature of hydraulic oil: Cooling water is reduced by over 30% and in certain conditions it is not even needed.
4) Improvement of Working Environment: reducing the noise from machine.
5) Annual Increase in Production
For Yizumi DM400 Die Casting Machine, output per shift is 420 pieces, and daily output (3 shifts per day) is 3*420=1,260 pieces. Assume that productivity is increased by 18% after the servo system is used and there are 26 workdays per month, annual increase in production will be: 1,260*26*12*0.18=70,761 pieces. 
6) Annual Savings on Electric Bill
Take Yizumi DM400 Die Casting Machine (motor power: 22kW) that is equipped with servo system, for example. Assume that the machine works 22 hours per day and 26 days per month. The actual power consumption of the load is 40% of power rating and actual electricity consumption is approximately saved by 45%. Annual electricity saving is: 22*40%*45%*22*26*12=271,180kWh/year. Assume that the electricity rate is 0.8 yuan/kWh, and annual savings on electricity bill will be: 27,180*0.8=21,744yuan.

(Ⅵ) Conclusion
We can draw the following conclusion from the analysis on the composition, work principle and trial running of servo system:
1) The servo system is notably energy-efficient. Under certain conditions, the servo system not only considerably improves productivity, creating more economic benefits, but also effectively restrains the increase of oil temperature so that hydraulic parts are less likely to have problems and the machine has better performance and longer service life.
2) The motions of servo die casting machine such as clamping, mold opening, injection and ejection. There is no impact on the normal automatic cycling of the machine. By comparing the injection process curves, it is found that no matter in slow or fast injection phase, the injection speed and pressure buildup time of servo die casting machine have no obvious regular difference from that of the general die casting machine. 
3) 50-hour continuous no-load working of servo system causes no alarms or faults.
4) The impact of the servo system which is a little bit noisy on the power system is slightly higher than that of the power-frequency system.