An Energy-Saving Method By Balancing The Load Of Operations For .

TMECH-04-2017-6553 The total power losses are divided into two parts, one is technical loss generated in energy form conversion such as l l l PE1-M (e), PM-H (e), and PH'-M' (e), which is essential to the hydraulic press system but can be reduced by improving the energy conversion efficiency. And the other one is redundant loss ( Prl (e) ) caused by adjusting the output power of drive l (e) , which system to match the load such as PE2(e) and PH-H' should be reduced and even eliminated. It can be expressed as: l Prl (e) PE2 (e) PH-H' (e) (2) l PH'-M' (e) Nw(e) PE(e) PE2(e) Pipe s PH'(e) Cylinde rs Nu(e) p PM'(e) l PH-H' (e) Fig. 3. The energy flow and conversion processes of a hydraulic press. (PE(e) is the electrical power inputted to the motor-pumps for operation e. It is consist of two parts: PE1(e) is the power of motor-pumps (Nw(e)) supplying energy for hydraulic press, whereas PE2(e) is consumed by the idling motor-pumps (Nu(e)) which stay unloading status. PM(e) is the mechanical power outputted by the motor, which is converted into hydraulic power (PH(e)) by pumps. PH'(e) is the hydraulic power inputted to the hydraulic cylinder and is converted to mechanical power (PM'(e)), part of PM'(e) is converted to forming power in l l l l forming operation. PE1-M (e), PH-H' (e), and PH'-M' (e), PM-H (e) are power losses in the corresponding conversion processes and eventually turn into thermal energy.) To deform workpiece successively, several operations for movement are included in a forming process. They are fast falling (FF) operation, pressing with slow falling and forming operation (PF), pressure maintaining operation (PM), unloading operation (UD), fast returning operation (FR), slow returning (SR) and waiting (WT) operation, as shown in Fig. 4. Motor Valves &pipes Flow Time Fig. 5. Demanded power and its implementation of operations (p is the system pressure and ȕ is the load rate which is the ratio of output power to rated power, PMO is the maximum output power of drive system with high energy efficiency). During the controlling process, system pressure also varies in different operations as shown in Fig. 4, leading to different energy efficiency of the drive system. The energy efficiency (ȘD(p)) of the drive system can be expressed as: N DS K D ( p ) K Di ( p )D i (3) i where ȘDi(p) is the energy efficiency of motor-pump i, Įi is the ratio of its rated power to installed power of the drive system. ȘDi(p) ȘPi(p)xȘMi(ȕ), ȘPi(p) is the energy efficiency of pump and ȘMi(ȕ) is the motor’s energy efficiency[2, 32]. NDS is a set of motor-pumps in the drive system, NDS Nw(e)ĤNu(e), and NDS is the number of motor-pumps in the drive system. Considering the energy efficiency of working motor-pumps supplying energy and others staying idle in the drive system, the output power of the drive system working in high efficiency is defined as the maximum output power (PMO), which can be expressed as: N DS § pq i Among these operations, the slider moves downward to approach workpiece quickly during FF. PF is used to form ˇ ȕ PMO Fig. 4. Displacement change of slider, system pressure profile, and operations performed in a forming process (MD is the maximum displacement of a hydraulic press, and MP is the maximum pressure). Pump Drive System PMO Demanded Power PE1(e) Motors PM(e) Pumps PH(e) Valves Pressure drop l PM-H (e) workpieces directly with a lower speed. Then the hydraulic system keeps a high pressure during PM and the pressure is released in UD. At last, the slider moves upward to the original position with a high speed in FR and a lower speed in SR. During WT, the slider stays at the original position until the workpiece is unloaded and the input material is loaded. Here we use e r(t) to describe the operations, and r(t) FF, SF, PR, PM, UD, FR, SR, WT. The power demand varies by operations but the installed power of the drive system has to meet the maximum power demand of any operations. Valves are usually adopted to control the output power of the drive system to align with the much lower demanded power, as shown in Fig. 5. Efficiency l PE1-M (e) 3 Pi ( p) · KDi ( p) p p , (dKD ( p) dp p m ¹ pm 0) (4) where pm is the system pressure when drive system works in high efficiency and qPi(p) is flow of pump i with pressure p. It is the gap between the input power and PMO that leads to the lower energy efficiency of the drive system and redundant loss for a hydraulic press, which is the excessive power (Pex(e)) of the hydraulic press system supplying for other system and can be expressed as: Pex (e) PMO PE (e) Prl (e) (5) Based on (1)-(5), improving the energy efficiency of working and idling motor-pumps in the drive system by approaching pressure to pm so as to reduce Pex(e), is effective for energy reduction, which can be achieved by sharing the idle motor-pumps by the additional hydraulic press and increasing

TMECH-04-2017-6553 As the great difference of energy demands exists in different operations, the total installed power of motor-pumps in drive system can be decreased by staggering operations with large energy demand in different hydraulic presses. Based on this concept, considering the convenience of realization, the drive systems of two hydraulic presses are combined into one (Combined Drive System, CDS) to supply energy for the both presses (Hydraulic Press 1 and 2), and each motor-pump in the combined drive system is controlled individually and supplies energy to a press at a particular time, as shown in Fig. 6. The motor-pumps in idle state of one drive system are used to supply energy for the other hydraulic press after staggering operations, which will reduce the energy loss generated by unloading motor-pumps. Drive System I HP a Flow a ma NDS2 Combined Drive System (CDS) Supplying energy Motor-Pumps: Working Idling Hydraulic Press 2 Fig. 6. Sharing of the drive system in two hydraulic presses. Since the idling motor-pumps of one hydraulic press can be utilized by the other one, the working motor-pumps in the CDS are the total motor-pumps used to supply energy for hydraulic press a performing operation ea' and for hydraulic press b performing operation eb'. And hydraulic press unit can work in order only when the installed power of the CDS meets their maximum demanded power simultaneously. Therefore, the configuration of the CDS (NCDS) is obtained as: NCDS max{N w (ea' ) N w (eb' )} (6) where ea' and eb' (a, b 1, 2 and a b) are operations changing of corresponding hydraulic press in a uniform time scale after being staggered, which can be expressed as: e1' r1' (t ) r1 (t ), e2' r2' (t ) r2 (t 't ) (7) where e1 r1(t), e2 r2(t) are operations changing in each time scale, including all operations in r(t). ǻt is adjusting time used to balance the energy demands of two hydraulic presses. To make the amount of the reduced installed power clear, the ratio of installed power between CDS and two drive systems is defined as the overlap degree (İD), and can be expressed as: HD N CDS N CDa P ri i a i Pri Flow b Control Signal mb Fig. 7. Scheme of two hydraulic presses after combining the load. Substituting Hydraulic Press 1 Controller Flow Adjustment Drive System II NDS1 HP b CDS Auxiliary device B. Sharing drive system and combining load of hydraulic presses where Pri is the rated power of motor-pump i. The range of İD is from 0.5 to 1, and the less İD is, the less installed power will be. On the other side, improving the energy efficiency of working motor-pumps for reducing energy consumption is as important as decreasing the number of idling ones, which can be achieved by increasing the system pressure derived from the load. Thus, the energy efficiency could be increased if the load of the other hydraulic press is added to the current hydraulic press system to make the load pressure to approach pm. Meanwhile, the mechanical energy of the hydraulic press, which serves as the input of the other one, can be utilized directly, as shown in Fig. 7. Auxiliary device the pressure of the lower load operation through adding useful load. 4 (8) In this system, flow adjustment is achieved by control valves which are used to balance the output flow of one hydraulic press and the needed flow of the other one when the velocity or chamber areas of cylinders in two presses vary from each other. Through these valves, redundant flow is released and inadequate flow is complemented from CDS to make sure that cylinders in each press are supplied with proper hydraulic flow and pressure. The auxiliary device consists of direction control valves and pressure control valves to ensure operations with different directions and system pressures in two presses are performed in order. After adding load to the hydraulic system, the forming force in PF operation is less when the system pressure is the same as original one. To meet the demands, the maximum forming force of each hydraulic press needs to keep the same by enhancing the system working pressure. The constraint can be expressed as: f (ea' ) p(ea' ) Aau - mb gAad Abd , ea' PF (9) where f(ea') and p(ea') are the maximum forming force and working pressure, mb is total mass of movable components in hydraulic press b, including the mold, the slider and rods fixed on it, Aad and Abd are the sum ram areas of all cylinder lower chamber in hydraulic press a and b respectively, and Aau is the sum ram areas of all cylinder upper chamber in press a. The output flow of pump in the CDS, which is related to the system pressure, affects the velocity of the slider in each operation, especially for FF (in this operation, the gravity energy of movable components is used to drive the other press, instead of driving itself). The slider velocity in the drive of working motor-pumps is obtained as: N w ( ea' ) i 1 § m gAd Ad ma g · u ' qPi b a ub Aa v(ea ), Aa ¹ ea' FF (10)

TMECH-04-2017-6553 where ma is total mass of movable components in hydraulic press a. When hydraulic press a works in FR operation, the movable component of hydraulic press b moves downward. As the flow to the lower chamber of cylinder in hydraulic press a consists of flow from hydraulic press b and the flow from CDS to complement the inadequate flow, the velocity can be expressed as: § m gAd Ad mb g · Aad § ma g · d ' ' qPi a b ua u qPi d Aa v(ea ) , ea Ab k l ¹ Ab Aa ¹ H H (e ) T T ' a ea' a ea' a (12) ea' where Tae is the duration of operation ea' of hydraulic press a, ' a H (ea' ) is the overlap degree of each operation and expressed as: H (e ) ' a N w ( ea' ) i N CDa Pr i P ri a shorten waiting time and can be completed when the slider reaches haFF in downward operation, as shown in Fig. 8. hbM (13) i Thus, the installed power of CDS will be determined by the maximum comprehensive overlap degree. After the load of the presses combined and the drive system shared, the efficiency of the drive system is improved and more excessive energy is utilized. Moreover, shortening total duration without increasing the installed power of CDS from the perspective of operations coordinating is an effective way to increase the comprehensive overlap degree for further reduction of energy consumption in the combined system, which will be solved in the following section. C. Optimization of operations for hydraulic presses Since one of the two sliders moves downward using the drive system and the other one moves upward simultaneously, the time of returning operation overlaps that of FF and PF operation. As a result, there is no returning (FR and SR) time in both forming processes competed on two hydraulic presses with identical cylinders. The following problem is that both of the presses will be standby once one of them needs waiting, which is essential for successive forming processes but consumes large energy. So unloading workpiece for press a can begin when its slider reaches haFF (height where the slider has no influence on unloading and loading process exactly) in upward operation to TaWT TbWT haM hbFF haFF FR (11) u where Ab is the sum ram areas of all cylinder and plunger upper chamber in hydraulic press b, k is the number of the sharing motor-pumps, and l is the number of the motor-pumps which is working to complement the inadequate flow, k Nw(ea')ģ Nw(eb') , l Nw(ea')-Nw(ea')ģNw(eb') . When the working efficiency affected by the velocity obtained by (10) and (11) is reduced, which can be improved by increasing the installed power of CDS, the energy consumption will also be increased. So that a comprehensive overlap degree ( H ) in a working process is introduced to balance them, which is obtained as: 5 haPF I TaFF TaPF TaPMTaUD TaFR II hbPF TbFF TbPF TbPMTbUD TbFR Fig. 8. Displacement change of two sliders in hydraulic press unit after combining load ( haPF and hbPF are the height of operation PF for hydraulic press a and b; TaFF , TaPF , TaPM , TaUD , TaFR and TaWT is the duration of corresponding operation for hydraulic press a respectively; TbFF , TbPF , TbPM , TbUD , TbFR and TbWT is the duration of corresponding operation for hydraulic press b respectively). Between haFF and haM (the maximum height of the slider), some operations are performed for hydraulic press b (part of FF operation, PF operation, PM operation, UD operation and part of FR operation are performed in a certain condition, as shown in Fig. 8.). To ensure WT operation of hydraulic press a can be completed when its slider reaches haFF in the downward operation, the duration of UD operation of hydraulic press b will be prolonged under the circumstance that the sum of its duration and the duration of operations performed between haFF and haM is shorter than that of WT operation of hydraulic press a. During prolonged duration of UD operation, more energy loss will occur by idling all motor-pumps in CDS. From the displacement of the two sliders, it is noticeable the overlap between one press's WT time and the other's forming time increases with the increase of haM .To eliminate the duration of extra UD, haM can be obtained as: FF WT ha , Ta d T1 -1 FF § TbPF · M WT WT 1 ha ha PF (Ta T1 ), T1 d Ta d T2 (14) v h C b ¹ FF v PF WT WT C ha hb (Ta T2 ), Ta t T2 2 where vC is the FF velocity of the hydraulic press unit with identical cylinders. T1 and T2 are the sum duration of different operations. They can be obtained as: T1 Tbm ; T2 Tbn hbPF vC ; m vC hbFF -hbPF ) n FF b (haFF haPF ) (15) FF a T T where m UD, PM; n UD, PM, PF. As can be seen from the expression, the maximum of height of sliders will be obtained according to the duration and velocity of operations. And when

TMECH-04-2017-6553 maximum of height is more than its stroke, prolonging duration of UD operation will become the compromise way. RESULTS AND DISCUSSIONS III. Tandem hydraulic press lines for draw-forming of automobile body panels are widely used in industrial applications to perform a series of pressing processes (blanking, stretching and punching, etc.), as shown in Fig. 9 (a). Two identical hydraulic presses with the nominal force of 20 MN in the line, which perform two forming processes by employing hydraulic system as shown in Fig. 9 (b), are selected to evaluate the energy-saving potential. 6 reaches a certain value, the flow will decrease with the increase of the pressure in the variable interval. These controls are beneficial for the hydraulic press with the application power demand for both high flow/low pressure (i.e., FF and FR ) and low flow/high pressure (i.e., PF) conditions in a working cycle. The flow curves of pumps in the variable interval and the fitted function (qP1(p), qP2(p), and qP3(p)) in the entire interval are shown in TABLE I(a). The energy efficiency characteristics (ȘP1(p), ȘP2(p), and ȘP3(p)) of the pumps are shown in TABLE I(b). Then, the energy efficiency characteristic (ȘM1(ȕ) and ȘM2(ȕ)) changing with the load rate of each motor can be obtained as shown in TABLE I(c). 1 (a) First Step: Blanking Second Step: Stretching Data Acquisition Fourth Step: Forming Third Step: Punching U/q 2 U/p Valve C A Block P P Power Source (Hydraulic B U/h 3 Circuit) 4 A1 A2 A3 V1 V2 V3 3P3W(2VT2CT) 1. Pressure transducer, 2. Flow transducer, 3. Displacement transducer, 4. Power meter (AWS2103), three phase power acquisition of motors. Each hydraulic circuit (b) Fig. 10. Schematic diagram of the experimental system. CU PF Cylinder Valves&Pipes CL FF P6 P7 N6 M6 P5 M5 N5 P4 M4 P3 M3 N4 N3 Drive System P2 M2 N2 P1 M1 N1 Fig. 9. Connection of tandem press lines with hydraulic presses (a) and configuration of the hydraulic system (b) (CU is the inlet of upper chamber of the cylinder, and CL is the inlet of lower chamber of the cylinder, ‘FF’ and ‘PF’ indicate different valves employed in FF and PF operations). A. Testing and quantification of energy conversion units In order to obtain the energy consumption of the hydraulic press unit, precise energy dissipation characteristics of all the energy conversion units are quantified, including variable displacement pumps (PV270 5 (P1, P2, , P5), PV180 (P6), PV092 (P7)) with power control type ‘LC’ in this series, three-phase asynchronous motors (Y280S-4 5 (M1, M2, , M5), their rated power are 75kW; Y280M-4 1 (M6), it rated power is 90kW), valves and pipes, as shown in Fig. 9(b). The functions that relate the energy characteristic under different loads are fitted according to the experimental tests (the schematic diagram is shown in Fig. 10) we have carried out, as shown in TABLE I. In this power control type, the output flow of this kind of variable displacement pump keeps unchanged basically with the increase of the outlet pressure in a range. When the pressure Owning the similar principle in pressure dropping, the valves and pipes in the hydraulic circuit before and after hydraulic cylinders in the corresponding operation are equivalent to a component with identical energy characteristic as shown in TABLE I(d). As circuits employed in different operations are different (Fig. 9 (b)), the variable of the energy characteristic in each operation is identified, (i.e., kFF-lower 1.322 10-6 MPa/(L/min)2, kPF-lower 2.396 10-4 MPa/(L/min)2, and kFF-upper kPF-upper 1.1958 10-7 MPa/(L/min) 2). The system pressure of pump outlet in each operation (ps) can be obtained as: ps 'pvp pF (16) where 'pvp is corresponding dropping pressure ('pFF-upper and 'pPF-upper are the dropping pressure from the outlet of pumps to the upper chamber of cylinders in FF and PF operations, 'pFF-lower and 'pPF-lower are the dropping pressure from the lower chamber of cylinders to the tank), and pF is the pressure generated by forming force. After getting the system pressure curve during each operation, the input mechanical energy to the pumps can be calculated by employing energy efficiency characteristic of pumps shown in TABLE I(b). Then the electrical energy consumption for each operation (EC(e)) will be calculated as: 7 EC (e) ³ ( p q s Pi ( ps ) KP-1i ( ps ) KM-1i ( Esi ))dt (17) e i 1 where ȕsi is the load rate of motor i, and qP1 ( ps ),(i 1,.,5) KP1 ( ps ),(i 1,.,5) qPi ( ps ) qP2 ( ps ),(i 6) ;KPi ( ps ) KP2 ( ps ),(i 6) ; q ( p ),(i 7) K ( p ),(i 7) P3 s P3 s Esi KM1 ( Esi ),(i 1,.,5) ps qPi ( ps ) ;KMi ( Esi ) . KPi ( ps ) Pr i KM 2 ( Esi ),(i 6,7) (18)

TMECH-04-2017-6553 7 TABLE I DISSIPATION CHARACTERISTICS OF ALL CONVERSION UNITS (a) Flow characteristic curves of pumps (b) Energy efficiency characte

A. Energy flow of the hydraulic press The hydraulic press converts the electrical energy into forming energy acting on the workpiece through several forms of energy in turn. They are electrical energy (E), mechanical energy (M and M'), hydraulic energy (H&H'), and forming energy (F), as shown in Fig. 3.