There are many different product models to be pressed; each model requires a different pressure, and the holding (dwell) time also varies, from a few seconds to several minutes.<\/p>\n
The scheme I proposed at the time was a constant-pressure variable-displacement piston pump + proportional relief valve + pressure sensor. During holding, the pump replenishes oil and the PLC controls the proportional valve to keep the pressure constant.<\/p>\n
1. The constant-pressure variable pump was set to a maximum pressure of 200 bar. When the holding pressure was set to around 200 bar from the touch screen alone, the pressure fluctuation increased, and the pressure oscillated like a sine wave, going from small to large and then from large to small.<\/p>\n
2. When the holding pressure was set to 160 bar on the touch screen, the pressure could be stabilized at 160 bar, but the oil temperature rose rapidly, the cooler could not dissipate the heat fast enough, and the high oil-temperature alarm was frequently triggered. When the holding pressure was set to 100 bar on the touch screen, the behavior was the same as at 160 bar.<\/p>\n
1. Cause of the sine-wave fluctuation of the PID-controlled pressure at 200 bar<\/p>\n
The pump selected was the PCY constant-pressure variable-displacement piston pump. The characteristic of this pump is that once the pressure reaches the pump's set pressure value, the pump's flow is cut off automatically. Owing to this characteristic, it satisfies the holding requirement while also saving energy during the holding stage of the press, which is why it is widely used in ordinary presses. In this scheme a proportional relief valve was used to regulate the pump's pressure; however, besides the 20% electrical dead zone that has to be avoided, a proportional relief valve also has a low cracking pressure and a minimum stable flow. Therefore, when the holding pressure was set to 200 bar, the PCY pump's set pressure was reached and the pump cut off its flow; at that moment the flow through the proportional relief valve was smaller than the valve's minimum stable flow, so pressure regulation by the proportional relief valve failed and the pressure dropped. When the pressure fell below the 200 bar set on the PCY, the PCY pump opened again and built up pressure. The pressure therefore fluctuated like a sine wave.<\/p>\n
2. Cause of severe heating of the system below 200 bar.<\/p>\n
Another characteristic of the constant-pressure variable-displacement piston pump is that when the pressure has not reached the pump's set pressure, it is equivalent to a fixed-displacement pump and delivers full flow. At this time, while the press is holding, the oil flowing into the holding cylinder is close to zero, so the pump's entire flow returns to the oil tank through the proportional relief valve; that is, the motor's power is entirely converted into heat through the relief valve, causing the oil temperature to rise rapidly.<\/p>\n
The problem this time was caused by an unreasonable design scheme. Changing the scheme was unavoidable, but changing the scheme means that a profitable project will ultimately turn into a loss. After communicating with the customer, the PID control during holding was removed and the proportional relief valve served only to limit pressure. The proportional relief valve was used only for multi-stage pressure regulation, and holding was maintained by the PCY pump itself. Not long after the customer started using it, they reported that the hydraulic oil temperature rose too quickly, and that once the oil temperature came up it affected product quality.<\/p>\n
First, increase the heat dissipation capacity by replacing the water cooler with one of greater capacity; this requires work at the customer's site. The main concern is that, judging from the customer's current pressure-accuracy requirements, replacing the water cooler alone may create new problems. Another option is to install an additional temperature-control (chiller) unit. The customer did not agree.<\/p>\n
Second, add a small fixed-displacement pump group to be used only for holding. This is equivalent to redoing the work, and the workload is too great.<\/p>\n
Third, replace the PCY pump with the BCY electro-hydraulic proportional pump. The BCY pump can adjust its output flow according to a given analog signal, giving a small flow output during the holding stage. However, this requires adding an amplifier board, which occupies one analog output channel of the PLC. After communicating with the customer, it turned out that the customer's PLC had one spare analog output channel.<\/p>\n
After comprehensive consideration, the constant-pressure PCY pump was replaced with a BCY electro-hydraulic proportional pump of the same displacement. After the replacement, the control method for the holding stage is as follows: the holding pressure is held at the set value by the proportional relief valve under PLC PID control. At the same time, the PLC outputs a signal to the BCY electro-hydraulic proportional pump for a flow of about 5 L\/min (which must also be greater than the BCY's dead zone), so that the flow output by the proportional pump is greater than the minimum stable flow of the proportional relief valve. This both ensures that the proportional relief valve works stably and minimizes the heat generated by the proportional relief valve, so that the water cooler can carry all the heat away. The PID control accuracy will then no longer be affected by excessive changes in oil temperature.<\/p>\n
Although after the change to the electro-hydraulic proportional pump the customer's requirements were perfectly met, the change of scheme increased the hardware cost, and the back-and-forth struggle over nearly half a year turned a profitable project into a loss-making one. The characteristic of constant-pressure control with a proportional relief valve is that stable pressure is obtained at the cost of energy loss. To have stable pressure and save energy at the same time, the reasonable solution is a high\/low-pressure combined fixed-displacement pump group, or a proportional pump.<\/p>\n","protected":false},"excerpt":{"rendered":"
A real-world failure analysis: why a constant-pressure variable-displacement piston pump combined with a proportional relief valve for press holding-pressure control caused sine-wave pressure oscillation and severe oil heating, and how the scheme was finally fixed with an electro-hydraulic proportional pump.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-292","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/posts\/292","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/comments?post=292"}],"version-history":[{"count":1,"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/posts\/292\/revisions"}],"predecessor-version":[{"id":293,"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/posts\/292\/revisions\/293"}],"wp:attachment":[{"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/media?parent=292"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/categories?post=292"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.wmdtech.cn\/index.php\/wp-json\/wp\/v2\/tags?post=292"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}